fm crystal radio receivers

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FM Crystal Radio Receivers

The notion of "crystal radio" is strongly associated with huge antennas and radio broadcasting on long

and medium bands, in this article, the author describes the experimentally tested detector circuits of

VHF receivers designed to listening to a FM stations.

The very possibility of receiving VHF FM detector was discovered accidentally. One day I was walking in

the Terletskiypark in Moscow, Novogireevo, I decided to listen to the broadcast - I had a simple crystal

set without resonant tank (this circuit is described in the "Radio", 2001, 1, Fig. 3). The receiver had a

telescopic antenna with length of about 1.4 m. Wonder whether it is possible to receive radio broadcast

with this short antenna? It was possible to hear, but weakly, simultaneous operation of two stations. But

what is surprised me is the volume of receiving was rise and fall periodically almost to zero after every

5...7 m, and it was different for each radio station!

It is known that in the LW and MW bands, where the wavelengths are hundreds of meters, it is

impossible. I had to stop at the point of receive with maximum volume of one of the stations and listen

attentively. It turned out - this is "Radio Nostalgie", 100.5 MHz, broadcasting from the near city

Balashikha. There were no line of sight between antennas. How does the FM transmission could be

received by using the AM detector? Further calculations and experiments shows that it is quite possible

and is not depends on the receiver.

A simple portable FM crystal receiver is made exactly the same way as an indicator of the electric field,

but instead of measuring device it is necessary to connect a high-impedance headphones. It makes

sense to add an adjustment of coupling between the detector circuit and the resonant tank to adjust the

maximum volume and quality of the receiving signal.

The simplest Crystal radio

The circuit diagram of the receiver suitable for these requirements is shown in Fig. 1. This circuit is very

close to the circuit of the receivermentioned above.Only the VHF resonant tank has been added to the

circuit.

Fig. 1.

VD1, VD2 - GD507A - an old USSR Germanium high-frequency diodes with the capacitance of 0.8 pF (at

the reverse voltage of 5V), the recovery time of reverse resistance is no more than 0.1 uS (at the Idirect

pulse=10 mA, Ureverse pulse=20 V, Icutoff=1 mA)
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The device contains a telescopic antenna WA1, directly connected to the resonant tank L1C1. The

antenna is also an element of the resonant tank, so to get the maximum power of the signal it must be

adjust both the length of the antenna and the frequency of the tank circuit. In some cases, especially

when the length of the antenna is about 1/4 of the wavelength, it is useful to connect the antenna to a

tap of the tuning coil L1 (find the suitable tap of the coil by finding the maximum volume of the signal).

The coupling with the detector can be adjust by trimmer C2. Actually the detector is made of two high-

frequency germanium diodes VD1 and VD2. The circuit is completely identical to the voltage doubling

rectifier circuit, but the detected voltage would be doubled if only the trimmer capacitor C2 value is

high, but then the load of the resonant circuit L1C1 would be excessive, and its quality factor Q will be

low. As a result, the signal voltage in the circuit tank L1C1 will be lower and the audio volume will be

lower too.

In our case, the capacitance of the coupling capacitor C2 is small enough and voltage doubling does not

occur. For optimal matching the detector circuit with the tank circuit the impedance of the coupling

capacitor must be equal to the geometric mean between the input resistance of the detector and theresonant resistance of the tank circuit L1C1. Under this condition, the detector is getting the maximum

power of the high-frequency signal, and this is corresponding to the maximum audio volume.

The capacitor C3 is shunting the higher frequencies at the output of the detector. The load of the

detector is headphones with the dc resistance of not less than 4K ohms. The whole unit is assembled in

a small metal or plastic housing. The telescopic antenna with the length not less then 1m is attached to

the upper part of the housing, and the connector or the jack for the phones is attached th the bottom of

the housing. Note that the phone cord is the second half of the dipole antenna (a counterweight).

The coil L1 is frameless, it contains 5 turns of enameled copper wire with diameter of 0.6...1 mm wound

on a mandrel with diameter of 7...8 mm. You can adjust the necessary inductance by stretching orcompressing the turns of the coil L1. It's better use the variable capacitor C1 with an air dielectric, for

example, type 1KPVM with two or three movable and one or two fixed plates. Its maximum capacity is

small and can be in range of 7...15 pF. If the variable capacitor has more plates (the capacitance is

higher), it is advisable to remove any of the plates, or connect the variable capacitor in series with a

constant capacitor or a trimmer, it will reduce the maximum capacity.

The capacitor C2 is ceramic trimmer capacitor, such as a KPK or KPK-M with the capacity of 2...7 pF.

Other trimmers capacitors could be used too. The trimmer capacitor C2 can be replaced with a variable

capacitor, similar to C1, and it could be used to adjust the coupling "on the fly" to optimize radio

receiving capabilities.

Diodes VD1 and VD2, can be GD507B, D18, D20 (it is old USSR Germanium high-frequency diodes. This

diodes can be replaced with modern Schottky diodes). The shunting capacitor C3 is ceramic, its capacity

is not critical and can have a value in range from 100 to 4700 pF.

Adjustment of the receiver is simple. Tune the radio by turning the knob on the variable capacitor C1

and adjust the capacitor C2 to get the maximum audio volume. The tune of the resonant tank L1C1 will


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be changed, so all operations must be repeated a few more times, and at the same time find the best

place for the radio receiving. It is doesn't necessarily the same place where the electric field has

maximum strength. This should be discussed in more detail and explain why this receiver can receive FM

signals.

Interference and conversion of FM into AM

If the tank circuit L1C1 of our receiver (Fig. 1) will be set up so that the carrier frequency of FM signal

falls on the slope of the resonance curve, the FM can be converted into AM. Let's find the value of Q of

the tank circuit. Assuming that the bandwidth of the tank circuit L1C1 is equal to twice the frequency

deviation, we obtain Q = F0/2f= 700 for both the upper and the lower VHF band.

The actual Q of the tank circuit in a crystal radio probably will be less than 700 because of the low Q-

factor of its own Q (About 150...200) and because the resonant tank is shunted by the antenna and by

the input impedance of the detector. Nevertheless, a weak transformation of FM into AM is possible,

thus, the receiver will barely work if its tank circuit detune a little up or down in frequency.

However, there is much more powerful factor contributing to the transformation of FM into AM, - it is

an interference. It's very rarely when the receiver is in the line of sight of radio station, in most cases the

line of sight is obscured by buildings, hills, trees and other reflective objects. A few radio beams

scattered by these objects comes to the antenna of the receiver. Even in the line of sight to the antenna

comes some reflected signals (and of course, direct signal comes too). The total signal depends on both

the amplitudes and phases of summing components.

The two signals are summed if they are in phase, i.e., the difference of their ways is multiple of an

integer of the wavelength, and the two signals are subtracted if they are in opposite phase, when the

difference of their ways is the same number of wavelengths plus half wavelength. But the wavelength,as well as the frequency varies at FM! The difference of the beams and their relative phase shift will

vary. If the difference of ways is large, then even a small change in frequency leads to significant shifts in

the phases. An elementary geometric calculation leads to the relation: f/f0= /4C, or C = f0//4f,

where C - the difference of the ways of the , it's required for the phase shift /2, to get the full sum of

AM signal, f - frequency deviation. The full AM is the total variation of the amplitude signal from the

sum of the amplitudes of the two signals to their difference. The formula can be further simplified if we

consider that the multiply of frequency by the wave length f0 is equal to the speed of light c: C = c/4f.

Now it is easy to calculate that to get a full AM of the two-beam FM signal, the sufficient difference

between the ways of beams is about a kilometer. If the difference of ways is smaller, the depth of AM

proportionally decreases. Well, but if the difference of ways is more? Then, during one period of the

modulating audio signal the total amplitude of the interfering signal will pass several times through the

highs and lows, and distortion will be very strong when converting FM into AM, up to complete

indistinct of the sound when you receive the FM by an AM detector.

Interference with FM broadcast reception is an extremely harmful phenomenon. It is not only produces

a concomitant parasitic AM of a signal, as it is described above, but it is produces the parasitic phase


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modulation, what leads to distortion even if we got a good FM receiver. That's why it is so important to

place the antenna in the right location, where the only one signal prevails. It is always better to use a

directional antenna, because it increases the magnitude of the direct signal and reduces reflections

coming from other directions.

Only in this case with a very simple detector radio receiver the interference played a useful role andallowed us to listen to the radio broadcast, but the radio broadcast can be heard weakly or with

significant distortions, and the radio broadcast can't be heard everywhere, but only in certain places.

This explains the periodic changes in the volume of the radio broadcast in the Terletskiypark.

Crystal Detector Radio Receiver with a frequency detector

A radical way to improve reception is to use a frequency detector instead of an amplitude detector. In

Figure 2is shown a circuit of a portable detector radio receiver with a simple frequency detector, based

on a single high-frequency germanium transistor VT1. The germanium transistors is used because it's

junctions works at a low voltage about 0.15 Volts, this allows to detect very weak signals. The junctions

of silicon transitions works at a voltage approximately 0.5 V, and the sensitivity of the receiver with a

silicon transistor is much lower.

Fig. 2.

VT1 - GT313A - an old USSR Germanium high-frequency transistor with h fe=10...230 (at DC: Uke=3 V,

Ie=15 mA), hfe=3..10 (at f=100 mHz, Ukb=5 V, Ie=5 mA)

As in the previous design, the antenna is connected to the input tank circuit L1C1, the variable capacitor

C1 is used for the tuning function. The signal from the input tank circuit goes to the base of the

transistor VT1. The other tank circuit, L2C2, is inductively coupled with the input tank circuit L1C1. The

tank circuit L2C2 is tuneble with the variable capacitor C2. Because of the inductive coupling between

this two tanks the oscillation in the resonant tank L2C2 is phase shifted by 90 relative to the signalacross the input tnak circuit L1C1. From the tap of the coil L2 the signal goes to the emitter of the

transistor VT1. A bypass capacitor C3 and high impedance headphones BF1 is connected to the collector

of the transistor VT1.

The transistor begins to turn on when its base and emitter has the positive half-wave of the signal, and

the instantaneous voltage on the emitter is greater then its base voltage. At the same time the
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smoothed detected current passes through the headphone in the collector network. But the positive

half-wave of the signal is only partially overlapping when the phase shift of the signal is 90 in the

resonant tanks, so the detected current reaches the maximum value determined by the signal level.

With frequency modulation, depending on the frequency deviation, the phase shift is also changing,

corresponding to the phase-frequency response of the tank circuit L2C2. When the frequency deviatesin one direction then the phase shift decreases and the half-waves of the signal at the base and emitter

is overlapped more, as a result, the detected current increases. When the frequency deviation goes in

the opposite direction, its decreases the overlap of half-waves of the signal and the current decreases.

So the frequency detection of the signal occurs.

The gain of the detector depends directly on the quality factor Q of the resonant tank L2C2, the quality

factor Q should be as high as possible (in the limit of 700, as we calculated earlier), therefore the

coupling with the emitter of the transistor is weak. Of course, such a simple detector does not suppress

the AM of the received signal. In fact, its detected current is proportional to the signal level at the input,

this is an obvious disadvantage. But anyway it's the very simple circuit.

Just like the previous circuit, the receiver is built in a small housing, on the top of the housing a

telescoping antenna is mounted, and the headphone socket in the bottom the housing. The knobs of the

variable capacitors is located on the front panel. These variable capacitors should not be combined into

one unit, because a louder volume and a better quality of reception can be obtained with separate

tuning.

The coils L1, L2 if frameless, they wound with the copper wire 0.7 mm (AWG 21) in diameter on the

mandrel of diameter 8 mm. L1 contains 5 turns, L2 - 5+2 turns. If possible, the coil L2 wound with silver

plated wire to improve the quality factor Q, the diameter of the wires is not critical. The inductance of

the coils is adjusted by compressing or stretching of the coils L1 and L2 to get the FM radio stations inthe middle of the variable capacitors tuning range. The distance between the coils L1 and L2 is in the

range of 15...20 mm (the axis of the coils is parallel), the distance is adjusted by bending their terminals,

soldered to the variable capacitors.

With this receiver can be done a lot of interesting experiments, exploring the possibility of reception of

VHF radio broadcasts with the detector receiver, exploring the propagation of radio waves in urban

areas, etc.Can be done experiments to further improve the receiver. However, the sound quality in a

high-impedance headphones with membranes is poor. Because of it a better receiver was developed,

which provides better sound quality and allows you to use a different external antennas, connected to

the receiver by feedline.

Radio receiver powered by the energy of radio waves

Experimenting with a simple crystal radio set, repeatedly had to make sure that the power of the

detected signal is sufficiently enough (tens or hundreds of microwatts) to provide a very loud sound in

the headphones. But the quality of reception is not good because there is no frequency detector. This

problem is partially solved in the second receiver (Fig. 2), but the signal strength is also used inefficiently


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because the transistor is powered by quadrature high-frequency signal. Therefore it was decided to use

two detectors in the receiver: the envelope detector - to power the transistor, and the frequency

detector - to improve signal detection.

Fig. 3. C1, C2 - 2.2...15 pF, C3 - 0.15 uF, C4 - 1 uF, C5 - 1 nF, R1 - 130 k

The circuit diagram of the receiver is shown in Fig. 3. An external antenna (dipoles) connected to the

receiver by a two-wire line, made of ribbon VHF cable with the impedance of 240...300 ohms. The

impedance matching between the cable and the antenna is performed automatically, and the

impedance matching of the input tank circuit L1C1 is performed by selecting a suitable tap of the coil L1.

Generally speaking, unbalanced connection of the feeder to the input tank circuit reduces the

noiseproofing of the antenna feeder system, but because the low sensitivity of the receiver, it doesn't

matter. There is a well-known methods of balanced connections for a feeder with the use of a coupling

coil or a balun.

The author's folded dipole was made of a conventional isolated connecting wire, the dipole was placed

on the balcony, in a place with a maximum field strength. The length of the feeder does not exceed 5 m.

With such a small length the losses in the feeder is negligible, and therefore, the balanced line can be

successfully used.

The input tank circuit L1C1 is tuned to a frequency of a signal, and a high frequency voltage across L1C1

is rectified by an amplitude detector, based on the high-frequency diode VD1. Since the amplitude of FM

signal has a constant value, there is practically no requirements for smoothing the rectified DC voltage.

However, to remove possible parasitic amplitude modulation in case of multipath propagation of radiosignals (see above story about the interference), the capacitance of the smoothing capacitor C4 is

selected sufficiently large. A rectified DC voltage is used to power transistor VT1. For the control of the

current consumption and for a signal level indication is used an analog current meter PA1.

A quadrature frequency demodulator of the receiver is implemented with the transistor VT1 and phase

shifter tank circuit L2C2. The high-frequency signal from the tap of the coil L1 is applied to the base of

the transistor VT1 through the coupling capacitor C3, and it's signal is applied to the emitter of the
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transistor VT1 from the tap of the coil L2 of the phase-shifting tank circuit L2C2. The work of the

detector is exactly the same as in the previous design. To increase the gain of the frequency

demodulator, on the base of the transistor VT1 is applied an offset voltage through the resistor R1, and

because of it the coupling capacitor C3 is used. Note that the capacitor C3 has sufficient capacitance

(0.15 uF) - this capacitance is chosen to shunt the low-frequency currents, i.e., for grounding the base of

the transistor VT1 for the sound frequencies. This increases the gain of the transistor and increases the

volume of reception.

The primary winding of the output transformer T1 in the collector circuit of the transistor VT1 is used to

match the high output impedance of the transistor to the low impedance of the headphones. A stereo

headphones TDS-1 (8..16 ohms) or TDS-6 (8 ohms) can be used with this radio. Both the earpieces (left

and right channels) are connected in parallel. The bypass capacitor C5 is used to filter the high-

frequency currents in the collector circuit. The button SB1 is used to short the collector circuit of the

transistor VT1 while tuning the input tank circuit and the search for a signal. The sound in the

headphones at the same time disappears, but the sensitivity of the indicator PA1 is significantly

increased.

The design of the receiver can be very different, but anyway it needs the front panel with the knobs of

the two variable capacitors C1 and C2 (each capacitor has individual knob) and the button SB1. To

reduce hand effect on the tuning, it is desirable to make the front panel of a metal plate or a copper clad

laminates. It can work also as a common wire of the receiver. Rotors of the variable capacitors should

have good electrical contact with the panel. The antenna socket X1 and the phone jack X2 can be placed

either on the front panel or on the side or back of the receiver. Its dimensions are dependent on the

available components. So let's say a few words about them.

The capacitors C1 and C2 is KPV type with a maximum capacity of 15...25 pF. The capacitors C3-C5 are

ceramic.

The coils L1 and L2 are frameless (see Figure 4), wound on a mandrel of diameter 8 mm, L1 contain 5, L2

contains 7 turns. The length of the winding is 10...15 mm (do some tuning by adjusting the length). The

enameled copper wire of 0.6...0.8 mm (AWG 20..23) is used, but it is better to use a silver-plated wire,

especially for the coil L2. The taps are made from 1 and 1.5 turns (L1) and from 1 turn (L2). The coils can

be arranged coaxially or axis parallel to each other. The distance between the coils (10...20 mm) is

adjusted. The receiver will work even in the absence of inductive coupling between the coils - the

capacitive coupling through the junction capacitance of the transistor is enough. The audio transformer

T1 is TAG-3, it has a winding ratio of 10:1 or 20:1.


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Fig. 4.

The transistor VT1 can be replaced by any germanium transistor with maximum operating frequency ft

not lower than 400 MHz. A p-n-p transistor can be used too, for example, GT313A, in this case the

polarity of the indicator PA1 and the diode VD1 should be reversed. The diode can be any germanium

type, a high-frequency. As the indicator PA1 any ammeter with a current range of 50..150 mA can be

used.

Tune the tank circuits to the frequency of a radio station, adjust the taps of the coils and the distance

between the coils to get the best result (maximum volume and best quality of the reception). It is useful

to adjust the value of the resistor R1 for maximum volume.

On the balcony the receiver with the antenna described above provided high quality reception of two

stations with the strongest signal from the radio center at the distance not less than 4 km and with no

direct line of sight (obscured by buildings). Collector current of the transistor was 30...50 mA.

Of course, the possible design of VHF crystal radios is not limited to described above. On the contrary,

this circuit should be considered only as the first experiments in this interesting field. When using an

efficient antenna, placed on a roof and targeted at a radio station, it is possible to obtain sufficient signal

strength, even at a considerable distance from the station. This provides a high-quality reception on a

headphones, and in some cases, you can get loudspeaking reception. It is possible to improving this

receivers by using a more efficient detection circuit and using a high-quality resonant tanks, in

particular, spiral resonators as resonant circuits.


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Power supply

1. NiCd battery charger for flashlight.

2. FET based voltage regulator.

3. Constant current sources.

4. Constant current source.


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5. Op-amp as voltage regulator circuit.

6. Constant current source with cascode of n-p-n transistors.

7. Two-terminal constant current source.


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8. Constant current sources.

9. Battery charger circuit diagram.


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10. DC-DC voltage converter circuit schematic.

11. Very good battery charger circuit.

12. Replacement for high voltage zener diode.

13. Simple Geiger counter circuit diagram.
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14. Battery charger circuit.

15. DC-DC converter based on CD4000.

16. Constant current source.


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17. Replacement for zenerdiod.

18. Voltage regulator with current limiter - constant current source circuit diagram.


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19. Voltage regulator with solar battery for charger.

20. Power supply with 50/60 Hz noise suppression circuit.

21. Voltage regulator with suppression for main harmonic circuit.

22. Charger powered by free energy.


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23. Logic gate (7400) based voltage regulator.

24. Current source controlled by voltage.

25. Constant current source based on voltage regulator IC.


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26. Switch Mode Power Supply.

27. Isolated Power supply for digital clock.

28. DC-DC converter with current multiplier.


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29. DC-DC voltage converter based on Voltage Quadrupler circuit with IC CD4093.

30. Constant current source based on L431 IC.

31. Voltage to current convertrt.


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32. Shunt voltage regulator circuit.

33. Unusual rectifier circuit diagram.

34. Powerful shunt voltage regulator.


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35. Thyristor-based switching power supply.

36. Voltage regulator with current source.

37. Voltage regulator - charger.


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38. DC-DC voltage converter.

39. Voltage-to-current converter drives LED.

40. DC to DC converter drives blue LED.


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41. Current source, controlled by voltage.

42. Led step-up converter for flashlight.

43. IC voltage regulators connected in parallel.


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44. Voltage regulator with improved stability.

45. Voltage regulator with double pulse frequency.

46. Current source.


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47. Circuit with negative resistance.

48. Voltage converters with current coupling feedback.


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1. Schmitt trigger circuit diagram.

2. Audio compressor.

3. Comparator with level-dependent hysteresis.


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4. Capacitive sensor.

5. Voltage to current converter.


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6. Bridge with current stabilisation circuit.

7. Two-terminal circuit with negative resistance.

8. Window comparator circuit diagram.


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9. Phase detector circui.

10. Switch mode phase detectors.

11. Balanced phase detector circuit diagram.


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12. High Input Impedance AC Amplifier.

13. Trigger based on opto-isolator circuit schematic.

14. Frequency doubler.


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15. Stepper motor controller based on IC 7474 circuit schematic.

16. Stepper motor working in synchronous mode.

17. Triangular wave to sine wave converter.


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18. Bidirectional intercom circuit.


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19. Frequency divider with variable division ratio.

20. Sawtooth wave to sine wave converter circuit.


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21. Changing band of variable capacitor by transformer.

22. Welding transformer circuit diagram.

23. Metal detector circuit.


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24. Superregenerative metal detector circuit.

25. Sensitive capacitive sensor circuit.


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26. Voltage to frequency converter.

27. Regenerative capacitance multiplier.

28. Equalization of output resistance.


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29. Wien bridge notch filter circuit schematic.

30. Compensate capacity load to avoid self-excitation.

31. Notch filter circuit.


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32. Notch filter based double T-shaped bridge circuit.

33. Adjustable notch filter circuit based on bridge differential unit.



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35. Active notch filter circuit.

36. Notch filter circuit with Wien-Robinson bridge.



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38. Adjustable notch filter circuit.

39. Adjustable notch filter circuit.

40. Variable capacitor based on Op-Am.


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41. Phase modulator based on op-amp.

42. Increasing amplitude of single pulse.

43. Equivalent of resistor with high resistance.


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44. Resonant filter based on rejection filter.

45. Resistance to period converter circuit.

46. Sine wave to sawtooth wave converter.


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47. Frequency divider based on DIAC.

48. Nonlinear sawtooth wave to sine wave converter.


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49. Sine wave former circuit diagram.


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50. Regenerative notch filter circuit.

51. Replacement of the high-resistance feedback resistor on a low resistance.


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52. Sawtooth wave to sine wave converter circuit.

53. Voltage to current converter.

54. Neutralization feedthrough capacitance.


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55. Regenerative frequency divider circuit.

1. Duty-cycle to dc converter.


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2. Frequency divider (Fin


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5. Frequency doubler circuit diagram.

6. Triangle-wave Generator.

7. Frequency comparator circuit diagram.(with IC's 74121 and 7474)
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8. Analog of DIAC circuit schematic.

9. Comparator with two edges.

10. Frequency divider.


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11. Buffer/inverter gate made of the trigger CD4013.

12. Circuit finding difference of two frequencies and phase detector circuit.

13. Contact bounce eliminator circuit.


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14. Narrow bandpassfilte.

15. Current stabilizer for Zener diode.

16. Restore signal by DC.


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17. Phase changer with constant amplitude of signal at output.

18. Phase filter circuit.

19. Edge detection circuit diagram.

20. Frequency comparator circuit diagram.


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21. Comparator-monostablemultivibrator based on LM139 voltage comparator .

22. Monostablemultivibrator with wide range of pulses (CD4000).

23. Monostablemultivibrator doubles number of pulses (CD4000).


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24. Monostablemultivibrator based on inductor (CD4000).

25. Monostablemultivibrator based on inductor (CD4000).

26. Monostablemultivibrator based on inductor and trigger (CD4000).

27. Duty-cycle indicator circuit.

If Duty-cycle=50% then U1=U2.


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28. Analog of zener diode with low operating voltage.

29. Synchronous detector.

30. Output stage of phase detecto.


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31. Gyrator circuit.

32. Detector of the frequency. (CD4000)


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33. Regenerative filter circuit.

34. Comparator-monostablemultivibrator.

35. Filter for carrier frequency.

36. Voice frequency doubler circuit.


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37. Notch filter with Q multiplier circuit.

38. Flashes lights based on neon lamps circuit diagram.

39. Output stage of a DC to DC converter circuit.


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40. Simple ADC with potential divider circuit based on CD4000 series.See details

41. Simple ADC with current summator circuit based on CD4000 series.See details
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42. 3D image on the screen of the oscilloscope.

How to calculate coil inductance - Coil Inductance Calculator

The inductance of a coil depends on its geometrical characteristics, the number of turns and the method

of winding the coil. The larger the diameter, length, and the larger number of winding turns, the greater

its inductance.


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If the coil is tightly wound, turn to turn, then it will have more inductance than a not tightly wound coil,

with gaps between the turns. Sometimes you need to wind a coil with a given geometry, and you don't

have a wire with required diameter, then if use a thicker wire you should increase slightly number of

turns, and if use a thinner wire it takes to reduce the number of turns of the coil to get the required

inductance.

All of the above considerations are related to winding coils without ferrite cores.

Inductance of single-layer coils on cylindrical winding forms can be calculated by the formula:

L=(D/10)2*n2/(4.5*D+10*l) (1)

D= 18

n= 20

l= 20

L= H

Where

L- inductance of the coil, H;

D- diameter of the coil (diameter of the former), mm;

l- length of the coil, mm;

n- number of turns of windings.

There is could be two tasks in the calculation:

A. The geometry of the coil is given, find the inductance;

B. The inductance of the coil is given, calculate the number of turns and the diameter of the wire.

In the case "A" all data are given, it is easy to find the inductance.

Example 1. Let's calculate the inductance of the coil shown in the figure above. Put the values in the

formula 1:

L=(18/10)2*202/(4.5*18+10*20) = 4.6 H

In the second case the coil diameter and the length of the wound are known. The length of the wound

depends on the number of turns and the wire diameter. Therefore, it is recommended to calculate in

this order. Based on geometric considerations, determine the size of the coil, the diameter and the

length of the wound, and then counting the number of turns by the formula:
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n=10*(5*L*(0.9*D+2*l))1/2/D (2)

D= 10

L= 0.8

l= 20

n= 0

turns.

After you have found the number of turns, determine the diameter of the wire with insulation according

to the formula:

d=l/n (3)

l= 20

n= 14

d= 0 mm.

Where

d- diameter of the wire, mm;

l- winding length, mm;

n- number of turns.

Example 2. We need to make a coil with a diameter of 10 mm and with a length the winding of 20 mm,

the coil should have an inductance of 0,8 H. The winding has one layer, turn to turn.

Put the values in theformula 2,we get:

n= 10*(5*0.8*(0.9*10+2*20))1/2/10 = 14

The diameter of the wire: d= 20/14 = 1.43 mm

To wind the coil with a wire of smaller diameter, it is necessary to place obtained by calculation 14 turns

across the entire length of the coil (20 mm) with equal intervals between the turns (the step of winding).

The inductance of the coil will be 1-2% less than the nominal value, it should be considered in the

manufacture of these coils. To wind the coil with a thicker wire than 1.43 mm, the new calculation

should be done with the increased diameter or length of the coil winding. You may also need to increase

both the diameter and the length at the same time, until get the desired dimensions of the coil for a

given inductance.

It should be noted that the above formulas is intended to calculate the coils with the length of winding l

equal to or more than half of the diameter. If the length of winding is less than half the diameter of the

winding D/2, the more accurate results can be obtained by using the formulas below:

L= (D/10)2*n2/((4D+11l)) (4)
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D= 18

n= l= 20

L= 0

H

and

n= (10L*(4D+11l))1/2/D (5)

D= 10

L= 0.8

l= 20

n= 0

turns.

Hidden wire detector

If you want to drill a hole in the wall then you must be sure that there's no electrical wiring. This simple

device.shown on the figure 1, can detect electrical wiring in the walls or ceiling. Resistor R1 protects IC

CD4011 against electrostatic. A rigid copper piece of wire (~ 18 AWG, with length of 5..15 cm) works as

antenna. Sensitivity of the circuit depends on the length of the antenna. When the antenna is placed

near the electrical wiring, then the circuit produces a sound with 50 or 60 Hz frequency.

Fig. 1. Simple Non-Contact AC Mains Voltage Detector

This device can detect broken wires in cables - near the broken wire the sound is off. The piezoelectric

speaker HA1 is connected to the bridge circuit to achieve higher loudness.

On the figure 2 is shown more complicated circuit, except audio it has visual indication based on LED

VD1. Resistance of the resistor R1 should be more than 50 Megohms.


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Fig. 2.

LED VD1 doesn't have a resistor in series with it, because IC CD4011 can limit the current.

Radioamateur, 1998, N9,

Non-Contact AC Mains Voltage Detector

Radio, 1997, N 3

The circuit diagram of the non-contact voltage detector is shown on the figure 1. It consist of two parts -

the AC amplifier and the audio oscillator, based on the Schmitt trigger DD1.1 of IC CD4093 with the

network R7C2, which determines the frequency of the audio signal, generated by piezoelectric buzzer

BF1.

Fig. 1.Circuit diagram of non-contact voltage detector.

DA1 - UA776; DD1 - CD4093; C1 - 47 mF; C2 - 33nF;

Op-amp UA776 can be replaced with a 741 (then you don't need R5), but it may reduce the sensitivity.


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When the antenna WA1 is located near a power supply, the interference of 50 or 60 Hz is amplified by IC

DA1 and as a result the LED HL1 will illuminate. The same output voltage of the op amp starts audio

generator.

The current consumption of the device is no more than 2 mA when using 9V battery, and current

consumption goes up to 6..7 mA when the LED lights up. If disconnect the LED and use only audiogenerator then it reduces current consumption.

The PCB is shown on the figure 2. Antenna WA1 is made of a foil strip with size of about 55x12 mm.

Fig. 2.PCB of the non-contact voltage detector.

The PCB is enclosed in a suitable plastic box, the antenna must be as far as possible from a hands. The

switch SA1, LED HL1 and piezoelectric buzzer BF1 is mounted on the front panel.

The sensitivity of the device is adjusted by the potentiometer R2 To make it smoother the

potentiometer R2 220K can be replaced with another one of 22K and a resistor of 200K, connected

between the lower pin and the ground.

Correctly assembled device doesn't any special adjustment.

The modernization of a crystal radio

Radio, 2001, 1

A crystal radio... For many decades, it is one of the first designs built by novice amateurs. The crystal

radio is an interesting introduction to the world of radio receivers. It allows the young enthusiasts of

Radio Engineering to carry out a variety of exciting experiments with the radio receiving the local radio

stations. However, what can be improved in this long-known device? But, as the author of this articles

says, the potential for growth has not yet been exhausted.


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In the simplest receivers (Fig. 1 a) the resonant tank is overloaded by the detector impedance. Although

the volume and the sensitivity are quite acceptable, the selectivity is insufficient. Because of the low

quality factor Q of the tank circuit, it is often to listen simultaneously to two or three radio stations.

Assume that the receiver is tuned to the middle of MW frequency range (1 MHz). The inductance of the

coil L1 is 200 uH, the capacitance of the capacitor C1 is 120 pF (typical values). Its reactive resistance isabout 1.2 Kilohms and the impedance of the resonant circuit in Q times more. With the quality factor of

the coil (with no load) Q = 200 we get 240 Kilohms. For the frequency range of LW the resonant

impedance of the circuit is close to 1 Megohm!

At the same time, the input impedance of the detector is considered to be equal to half the load

resistance, the load is a high-impedance headphones with an impedance at audio frequencies is only

10...15 Kilohms (the full impedance of the headphones is more than the value shown on their case

because of the inductance of the headphones capsules).

It is easy to see that the tank circuit L1C1 is shunted too much, and its real Q is less than 10 (the ratio of

the load resistance to the reactance of the tank circuit). Making the coupling with the detector circuit

weaker, you can improve the quality factor Q, and hence the selectivity increases. The volume will

almost not change because a voltage of a signal across a resonant tank circuit with a higher quality

factor Q is higher, this will compensate the decrease of the signal across the detector. The coupling is

usually adjusted by connecting the detector to a selected tap of the coil (Fig. 1b).

Fig. 1.

If we adjust the coupling, it is useful to optimize the resonant tank. In [1-3] it was shown that the

maximum efficiency of the antenna circuit is achieved when the antenna circuit is directly connected to

the upper end of the resonant coil L1 without a coupling capacitor. Tuning are provided by changing the


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inductance of the coil, as the capacity of the resonant tank is used the capacity of the antenna. If the

antenna is large and its capacity is significant, then it is necessary to include a tuning capacitor in series

with the antenna (Fig. 1b).

This receiver works better than the previous one and has a higher selectivity, but it isn't convenient to

regulate the coupling between the detector circuit and the resonant tank, because it would require amulti-tap coil. Therefore, the process of adjustment is not smooth.

There is a method of impedance matching using a capacitive coupling, where the capacitive resistance of

the capacitor is equal to the geometric mean of both impedances. In our example (the impedances of

240 Kilohms and 6 Kilohms is matching), it will be about 40 Kilohms( R=(R1*R2)0.5), and the

corresponding capacity is only 4 pF! (C=1/(2**F*Rc)). It turns out that the coupling can be adjusted by

an ordinary trimmer of KPK or KPM type.

Fig. 2.

VD1, VD2 - D18 (an old USSR Germanium diode); C1 - 5..180 pF; C2 - 8..30 pF; C3 - 680 pF

But the coupling capacitor breaks the DC current path of the detector circuit. To avoid this problem it is

possible to add the second diode to the circuit (Fig. 2). It seems we get a detector with a voltage

doubler. In fact, because of the small capacitance of the capacitor C2 there is no voltage doubling effect.

During the negative half-cycle of the signal across the tank circuit L1C1, the capacitor C2 is charged

through the diode VD1, and during the positive half-cycle the capacitor C2 discharges through the diode

VD2 and the load. The headphones BF1, shunted by the bypass capacitor C3 to smooth out ripple, is the

load of the detector.

The smaller the capacity of the capacitor C2, the less the charge and the energy, respectively, taken

from the tank circuit. The coupling network is adding to the tank circuit a small reactive (capacitive)

resistance, which is automatically compensated while tuning of the tank circuit in resonance with theoscillations of the input signals.

In this experimental design the coil L1 is wound on a 12 mm in diameter plastic pipe with one layer of

0.2 mm (AWG 32) copper enameled wire, the coil has 240 turns. The ferrite rod of 10 mm in diameter

made of ferrite 400NN (beg=400, max=800) is used for adjustment. The tuning range is from 200 kHz

(when the capacitance of C1 is maximum and the ferrite rod is fully retracted) to 1400 kHz (with the

removal of the rod and decreasing the capacitance of the C1).


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At the apartment with a small antenna (about 7 m) and a ground (a central heating system) the receiver

showed excellent results, received all Moscow LW and MW radio stations. By adjusting the coupling

with the trimmer C2, it was able to get sufficient selectivity at the normal volume level.

There is another advantage of the receiver - because the detector is powered by a current going through

a high impedance of the coupling capacitor C2, the "step" on the current-voltage characteristics ofdiodes is smoothed out. By the way, the usefulness of the detector powered by the current has been

reported in [4]. In our receiver a silicon diodes (with a threshold of 0.5 V) works almost as well as

germanium diodes (with a threshold of 0.15 V). Moreover, it was possible to connect to the receiver a

low-resistance (50-70 ohms) headphones, it is absolutely unacceptable in the traditional version. But in

this case the bigger capacitance of the coupling capacitor is required - up to 40...50 pF. The sound

volume will be less because of the significant losses in the direct resistance of the diodes.

Fig. 3

The high sensitivity of the detector described above to weak signals came to the idea to try a simple

resonant tank-free version of the receiver (Fig. 3). It was easy to build - all components have been

soldered to the terminals of the headphones, and a 1.5 meter of insulated hookup wire with the clamp

"Crocodile" at the end worked as antenna. With the "Crocodile" the antenna can be attached to the

trees or other high objects. The headphones cord has some stray capacity C strayto the operator and

further to the ground was used as the counterweight (instead of ground). Even with such a primitive

version it was able to listen to some of the most powerful radio stations.

This receiver almost does not perceive low frequency interferences, for example, from the mains power

line because the small capacitance of the coupling capacitor C1 prevents it. The audio frequency currentis completely shorted in the isolated network of the headphones BF1 and the diodes VD1, VD2.

I cannot say that the circuit diagram of this receiver is something new. Half-bridge rectifier that's used in

it, was known long ago - it was used in the indicator of electrical field [5]. By the way, nothing prevents

to use a full-bridge circuit based on four diodes, connect it to the tank circuit or antenna by a capacitor

of small capacity.


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Fig. 4

A similar circuit has been described in [6], but, unfortunately, the author incorrectly interpreted the

principle of operation of the receiver. The correct receiver circuit is shown in Figure 4. It differs from the

author's circuit only in the presence of a stray capacitance Cstray between the headphones and the the

earth, the stray capacitance acts as a coupling capacitor and matches the tank circuit with the detectorcircuit. By a happy coincidence, the capacitance Cstray was close to optimal. But the author didn't took

it into account! As experimental results, it is proved to be excellent, as it follows from the publication

[6].

At the end, let's go back to the circuit, shown in Figure 2 and bring it to the attention of the radio

amateurs. This crystal radio set has shown excellent results. Experiments with it not less interesting and

attractive than with the more complex electronic devices.

Loudspeaking radio receiver with a bridge amplifier powered by "free energy"

Radio 2001, 12

Receivers without power supply are interested for radio amateurs. This article describes an improved

radio receiver powered by radio waves.

While experimenting with different receivers and amplifiers powered by "free energy", it was found that

it is more convenient to connect the audio amplifier to the receiver by using only two wires for audio

signals and supply voltage. This would allow to use the radio receiver with no switches, just connecting

headphones to the output of the receiver.

In general, this receiver reminds thepreviously described version of "crystal" radio receiver,but it has

some interesting features.
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Fig. 1

VT1, VT3 - MP37 (Germainum, hFE= 15...30, ft= 1MHz); VT2, VT4 - MP41 (Germainum, hFE= 30...60, ft=

1MHz);

VD1-VD4 - D18 (Germainum); T1 - transformer with ratio 30:1;

L1 - LW loopstick ferrite antenna.;

* - tweak the value (see text).

The schematic diagram of the receiver is shown in Figure 1. From the detector bridge the circuit is

completely symmetric, the detector is connected to the amplifier by two wires (the terminals Aand B)

and the output of the amplifier is connected to the loudspeaker (the terminals Cand D) by two wires.

The resonant circuit of the receiver comprised the antenna capacitance and inductance of the coil L1.

This solution provides a maximum power of the signal in the resonant tank circuit. The switch SA1 and

the neon lamp HL1 are used to protect the receiver during thunderstorms. The static charge doesn'tbuild up in the antenna because the antenna connected to the ground through the coil L1.

A bridge detector circuit (VD1 - VD4) is used in this receiver, it works very well for the inductive load.

The detector connected to the antenna through the capacitor C1, this capacitor is matching impedances

between them. Once adjusted for maximum voltage across the amplifier, the capacitor C1 may be

replaced with a constant capacitor with proper value. The optimal capacitance of the capacitor C1 is

about 47 pF for LW band.

The output voltage of the detector is symmetric with respect to ground. Through the wires Aand Bthe

voltage passes from the detector to the input of the audio amplifier. At the input of the amplifier the

voltage decomposes into AC and DC parts. The AC part feeds through the coupling capacitors C3 and C4to the transistor bases of the bridge amplifier. The DC part charges through the low-frequency chokes

the capacitor C6. The DC part is used for power supply. The receiver doesn't have a common wire. The

arms of the amplifier balances automatically, because the bases of the complementary transistors are

connected together.

But transistors in this type of amplifiers don't have a bias, they does not work in the class "B"but rather

in the class "C". This leads to crossover distortion of the signal waveform, as shown in Figure 2(A).


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Fig. 2

The graph shows the dependence of the output current in one arm of the amplifier (for example, VT1,

VT2) on the input voltage. We see a distorted output current for a sinusoidal input voltage. Thesedistortions are especially noticeable with silicon transistors that have higher junction drop voltage of

about 0.5 V. Germanium transistors has lower junction drop voltage of about 0.15 V, so they are used in

the audio amplifier.

Crossover distortion is related to the moments when voltage crosses zero point, that is very unpleasant

to the ear. Crossover distortions can be reduced by using a slight forward bias Ubias, as shown in Fig. 2(B).

The distortions disappear but some initial current i0appears, it makes the amplifier less efficient.

The same result can be obtained by other means. If mixing the audio signal with a high frequency signal,

as shown in Fig. 2(C). This method is used in tape recorders with AC bias, because the magnetization

curve of the type is very similar to the amplifier transfer characteristic of a push-pull stage without bias.

By adjusting the amplitude of the "high frequency bias" the desired initial current (quiescent current)

can be set, this current should not be too high, but sufficient to eliminate the distortion.

But we already have high frequency bias, we got the detected RF voltage ripple. In the bridge detector

circuit the ripple has twice the frequency of the carrier signal. We just need to tweak the value of the

smoothing capacitor C2 (Fig. 1) to obtain the desired quiescent current. It's better tweak the capacitor

C2 when there is no audio transmission (but there is a carrier frequency of the radio station) because if

there is an audio signal then the current of the audio amplifier increases. At the output of the audio

amplifier the ripple don't need anymore, so there is the smoothed capacitor C5.

The coil L1 is wound with litzwire 7 x 0.07mm (7 wires x AWG 41) on a cardboard pipe with a ferrite slug

of 8 mm in diameter and 160 mm long (the permeability of the slug is about 1000). The coil has about

200 turns. Actually, any other litzendraht may be used or any copper wire with silk insulation of

0.15...0,25 mm in diameter (AWG 35...AWG 30). A standard loopstick antenna with the LW band can be

used as the coil L1. C1 is a ceramic or air trimmer capacitor.


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In the detector circuit the best result was obtained by using diodes D18, diodes GD507 works not too

bad, and the worst result was obtained by using diodes D311 (D18, GD507, D311 are germanium

diodes). In the detector circuit may be used any germanium diodes.

The transformer T1 has ratio 30:1. The primary winding has 2700 turns of the wire with diameter 0.12

mm (AWG 37) and the secondary winding has 90 turns of the wire with diameter 0.5 mm (AWG 24)wound on a former which is mounted on a core made of permalloy E-shape plates of 15 mm2. Any

suitable output audio transformer can be used here. A primary winding of the same transformers can be

used as chokes L2 and L3. The inductance of this chokes should be not less than 6..7 H. Any low

frequency germanium transistors may be used in this circuit. If possible, match the transistors with

similar hFE.

The receiver can be adjusted in a few minutes. Disconnect the audio amplifier from the detector and

connect a high-impedance headphones to the terminals Aand B, check the detector part of the receiver,

try to tune to a powerful radio station, if necessary change the number of turns of the coil L1. The tuning

is performed by moving the ferrite rod in and out the coil L1. Next, connect the amplifier to the receiverand connect a high-impedance DC voltmeter across the capacitor C6 to monitor the voltage, tune the

receiver to the frequency of a powerful radio station and adjust the capacitor C1 for the maximum

reading of the voltmeter. Keep in mind that the voltage across C6 increases slowly because of the large

capacitance of the capacitor C6. Connect across the capacitor C2 another capacitor with a value of a few

thousand picofarads and wait for some seconds, read the voltmeter. Then tweak the capacitor C2 to get

the voltage 20...30 % below the nominal value. In the author's version of this receiver the voltage was

5.5 V and 4 V. There is nothing more to adjust in this circuit.

The receiver was tested in the city apartment located in the eastern outskirts of Moscow. An external

antenna was used. The antenna has length 30 meters of copper enameled wire with a diameter of 0.7

mm (AWG 21). The maximum height of the antenna above the roof does not exceed 7 meters. Metal

pipes of a central heating system was used for grounding.

Even with this antenna it was possible to receive signals of a five radio stations with loud speaker

volume. The loud speaker volume means that the volume is sufficient for normal listening in a small

room when there is no ambient noise. The values of the detected voltages, currents and power,

extracted from the air by the receiver of the above mentioned radio stations are shown in Table 1. The

voltage was measured across the capacitor C6, and the current was measured in series with any of the

wires Aor B, while the receiver is working.

Frequency, kHz Voltage, V Current, mA Power, W

198 4,2 0,3 1,25

261 3,5 0,25 0,9

549 2,5 0,17 0,42


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873 3 0,2 0,6

918 1,2 0,1 0,12

It should be noted that the audio amplifier loads sufficiently the detector, because the value of the

capacitor C2 that was chosen provides the best quality of the sound, so with this value the quiescent

current of the amplifier is sufficient.

The widespread opinion that the quality reception of long and middle wave signals is impossible

especially in the night time are wrong, and this receiver disproved this mistaken opinion. This receiver

does not suffer from interference because of its low sensitivity. The quality of the sound cannot even be

compared to the sound quality of conventional portable receivers.

Loudspeaking "crystal" radio receivers

Radio 2000, 7

A lots of radio amateurs have an interest to power a simplest radio receivers with the "free energy", i.e.

the energy, taken by the receiver antenna directly from the air. The circuits described here can provide a

radio reception using a loudspeaker.

The question of how much power can get out of a signal from an antenna, and how to build a

loudspeaking crystal set, was already discussed in the author's articles[1,2].However the questions

"how much power we need for loudspeaking reception?" and "how to better use the power from the

antenna?", still remain.

According to the old reference books, to listening a voice of a broadcaster from the distance of 1 meter

it takes the sound level of the loudspeaker about 60 dB. In this case, the radiated acoustic power is 12.6

W. The necessary electrical power can be calculated by dividing the radiated acoustic power by the

energy conversion efficiency of the loudspeaker. For the common loudspeakers the energy conversion

efficiency is about 1%. Thus we get the electrical power about 1mW. It is interesting to calculate the

required power for loudspeakers to get the sound level of 60 dB. The calculation results for the different

loudspeakers are presented in table 1.

Model Power, mW

0,025GD-2 3,6

0,05GD-1 1,8

1GD-5, 1GD-28, 2GD-7 1

5GD-1, 6GD-1PP3, 6GD-30 0,25
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8GD-1PP3 0,2

Table 1.

From the table 1 we see that we need to use the high-efficiency loudspeakers. The acoustic design of

the loudspeaker systems is very important, the bigger speaker cabinets is better. In the experiments the

author used two loudspeakers type 4GT-2 in wooden cabinets with the enclosure volume of 50 liters.

Horn loudspeakers has three times better efficiency because of the improved coupling efficiency

between the speaker driver and the air and because of the directional characteristics of the produced

sound waves. The simple and effective loudspeakers was built by radioamateurs, they used paper,

cardboard and plywood[3].Horn loudspeakers with a bass reflex system with U-shaped design provides

with the loudspeaker 6GD-1 efficiency of about 2.3%, and at the low frequencies about 3.4%. So, we

found that the audio signal of 0.2 mW is sufficient for the sensitive acoustic system.

The second part of this "research" is related to electrical circuits of the loudspeaking detector radio.

Analysis of the detector circuit leads to the conclusion that the current should be amplified, but not the

voltage, because the voltage amplification could limit the peaks of the signal. Because of this it is wise to

use in this circuit the push-pull emitter follower, based on the complementary pair of transistors

working in class AB. This amplifier has good efficiency and low current consumption while the quiet

sounds and pauses of the signal, this allows to store the energy of the carrier and use the energy at the

peaks of the audio signals.

The circuit diagram of the receiver with the amplifier based on the push-pull emitter follower is shown

in Fig. 1. The AC component of the detected signal passes through the coupling capacitors C3, C4 to the

bases of transistor amplifier, and the DC component passes through the choke L2 to the storage

capacitor C5. This capacitor cannot be directly connected to the detector because in this case the audio

signal will be smoothed and suppressed. The parameters of the choke are not critical, so you can use

any choke or any transformer with a winding, containing not less than 2000 turns wound on the

magnetic core with cross section not less than 1 cm2.
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Fig. 1.

VD1 - D311 (Germanium diode); VT1 - MP37 (Ge, hFE= 15..30, Ft= 1 MHz); VT2 - MP39 (Ge, hFE 12, Ft=

0.5 MHz);

R1, R2 - 560K; C1 - 17..500 pF; C2 - 680 pF; C3, C4 - 0.68 F; C5 - 68F x 6.3 V; C6 - 22F.

L1 - magnetic loop antenna for MW with a moveable ferrite core; L2 - an audio choke; T1 - transformer

with ratio 30:1;

Loudspeaker BA1 - 4 ohms.

The optimal transformation ratio of the transformer T1 is about 30 for the load of 4 ohms. It is

convenient to use a small power supply transformers from transistor radios with the voltage ratio 220 V

to 6.5..9 V. A suitable output audio transformer can be used too.

The large size of the device (due to the heavy transformer and choke) is not an issue, because it uses the

large antenna and a floor standing speaker system, so this is not a portable radio!

The use of a voltage doubling rectifier allows to increase the supply voltage of the circuit. Distortion on

peaks of signal will be decreased. A bridge amplifier loads the voltage doubling rectifier symmetrically

and furthermore decreases the distortion. This allows to get rid of the capacitive coupling at the output.

The circuit schematic of the receiver with the voltage doubling detector and the bridge power amplifier

is shown in Fig. 2. The positive half-wave of the signal detected by the diode VD1, smoothed by

capacitor C2 and filtered by low-frequency choke L2 and the capacitor C8, so it creates a positive supply

voltage. Similarly, the components VD2, L3, C3 and C9 produce a negative supply voltage. The emitter

followers based on the VT1, VT2 and VT3, VT4 are working in opposite phases, the signals to this emitter

followers feeds from the different detectors. The emitter followers are loaded with the transformer T1.

Just like in the previous circuit, the transformer ratio is about 30, but due to the bridge circuit the output

power of the amplifier is higher than in the previous circuit. The purpose for the other components ofthe circuit shown in fig.2 is the same as the circuit shown in fig. 1, and the recommendation about the

chokes is the same.


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Fig. 2.

Adjustment of the receivers powered by the "free" energy has some features. This receivers will not

work until they are tuned to a powerful radio station, because there is no power supply. But after tuning

it will take some time to charge the capacitors (C5 - in Fig. 1 and C8, C9 - in Fig. 2). Charge time is directly

proportional to the capacity of this capacitors, so in the first experiments the capacity should not be too

large. But in this case while receiving a long loud sounds (especially in the musical passages), the power

supply voltage and the detected voltage drops significantly due to increasing current of the audio

amplifier, the result of it is the limitation of dynamic range. This does not lead to issues, but even

improves speech perception.

When the receiver will be completely adjusted, the capacity of the smoothing capacitors can be

increased even up to several thousand microfarads, it will improve the dynamics of the receiver and the

audio amplifier would work out the peaks of the signals. In any case, all the capacitors should have a

small leakage current (check it with an ohmmeter), to avoid the necessary load of the power supply.

Tweaking of the bias resistors in the receivers are based on the next reasons: the greater the resistance,

the less the current consumption (the current when there is no signal in the receivers - see fig. 1 and 2),

and the less the gain of the transistors but higher the supply voltage! A compromise can only be found

empirically for a particular antenna, by getting the maximum sound volume and quality of the radio

reception. The bias resistors for the circuits shown in Fig. 1 and Fig. 2 may have different values, itdepends on the parameters of the transistors. The voltage at the emitters of the transistors is half of the

power supply voltage (Fig. 1) and zero (Fig. 2).


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It's better to start the experiments without bias resistors, and then try to use this resistors with value

from 2.7 M to 1 M; if there is a "powerful" antenna than use the bias resistors with value of hundreds

of kilohms, because the power supply voltage can be dropped. If a pair of complementary transistors

have a sufficient initial current, it can be reduced by inserting a resistor between the bases of the

transistors, or the bases could be connected together, and one of the base capacitors could be removed

from the circuit. It has not mush sense to use in this circuits a bias thermal stabilization networks due to

the low power of the amplifiers (some milliwatts).

This radio receivers has been tested in a country house (33 km to south-east from Moscow). The audio

volume level has been enough for a small quiet room. The second circuit has been tested with especially

good results. The antenna type the end fed half wave with the length of 12 m was used. The antenna

was stretched from the window to the tree outside. The receivers was grounded to the water pipes of

the well. The receivers was tuned to "Radio Rossii" 873 kHz, the radio stations "Radio 1" and "Radio

Mayak" was received with loud volume too. The quality of the sound was excellent.

FM crystal radio receivers

Optimization of a radio receiver powered by the energy of radio waves

7/8/2013

Dante Bianconi

Vinci (Florence), ITALY

This report shows the results of some experiments carried out on the basis of Mr. Vladimir Polyakov

(RA3AAE) researches. The original circuit was modified by the use of a more efficient detection circuit

and by the use of a simple amplifier, self-biased by the radio itself. An efficient antenna (5 elements FM

Yagi) was used also to permit to explore overall the weak signals close to the range between 98 - 103

MHz. Earlier it was proposed by Mr. Polyakov, to use a simple dipole as an antenna. According to the

original circuit, the high frequency germanium transistor GT311A with Ft= 300 MHz is used there, it

provides high impedance at its output (the transistor was grounded to provide the matching with the

low impedance of the resonant tank), so it was possible only to use ear speakers with at least 600 ohm

impedance to listening the radio.

A further analysis of the circuit came to the idea to use a simple amplifier based on a silicon transistor

(BC109C with hFE= 700), it's finally allowed to get acceptable loudspeaking reception. With the use of a

more efficient detection circuit, the voltage in unloaded condition reached 2.2 volt across the capacitor

C8. With two loudspeakers (are both connected in parallel), the current in the high impedance circuit

was also measured by a micro ammeter and sometimes the value of this current reaches more than 100

A. The transistor of the simple amplifier was used in common emitter configuration to lower the

impedance of the output of the high frequency transistor (AF239). The impedance transformer that gave


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the best results had a 14 k in input and 4 in output. An active low pass filter was used to reduce the

noise coming from the first stage of RF amplifier to the low frequency stage.

With the directional antenna it was possible to receive three FM radio broadcast stations, two of them

are located at 15 km, and the third - more than 30 km from the reception antenna. The current

researches are focused on the circuit with a coaxial resonator, it would allow to reach high Q as well asthe selectivity of the tuning.

Fig. 1. The circuit diagram of the FM crystal radio receiver

T: Zin 14 k Zout 4,8 (K 60:1)

R1: 70+200 k

R2: 30

Tr1: AF239

Tr2: BC109C


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D1,D2: 1N82A

L1: 5 turns (silver wire 1 mm, coil with internal diameter of 8 mm)

L2: 7 turns (silver wire 1 mm, coil with internal diameter of 8 mm)

C1: 8.5 pF (ceramic NP0 type)

C2: 5-25 pF (KPV type)

C3, C4: 4n7 (ceramic type)

C5: 0.15 F

C6: 3-28 pF (KPV type)

C7: 0.01 F

C8: 1 F

WA1: 5 elements YAGI antenna

LS1: 3.5 loudspeaker (diameter 200 mm)

LS2: 3.5 loudspeaker (diameter 100 mm)


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Fig. 2.


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Fig. 3.


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Fig. 4.

The FM crystal radio, called BIDA 1: the dimensions of this design are only 80x35x80. In the circuit is

used the variable capacitors with the silver plated coils, it allows to obtain a better quality factor Q and

the high frequency germanium transistor AF239. The taps on both coils are visible on the Fig. 3, it allows

to reach the better matching between the antenna impedance and the main resonant tank L1 and the

second coil L2. The signal amplified by the transistor AF239. The detection circuit was composed by

using a Villard voltage doubler with the use of two very sensitive germanium diodes 1N82A. This

sensitive diodes has been used for radar applications after the Second World War.


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Fig. 5.


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Fig. 6.


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Fig. 7.


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Fig. 8.

5 Elements Yagi Antenna


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Fig. 9.

5 Elements Yagi Antenna - view from the side

It's clearly visible on the circuit diagram (see Fig. 1) that the HF radio amplifier stage is self biased by the

main radio circuit. The output AF stage is loaded with the high impedance transformer. In the Fig 8 and 9

you can see the 5 elements Yagi antenna, the heart of the FM crystal set (the central frequency is 100

MHz). See the folded dipoles and the indicated dimensions of the RG8 cable. The impedance of this

antenna is close to 52 as well as the impedance of RG8 cable. The cable's length was optimized, so it is

only 6 meters long.

While testing, the 5 elements Yagi antenna was pointed to the north-east direction (in the opposite site

the field intensity is higher, but there is a medium voltage power line that probably interfered withreceiving - unfortunately the most of the radio broadcasts are can be received in this south-east

direction). The centre frequency of the Yagi antenna is 100 MHz, it allows to receive three radio stations:

Radio Lady--> 98.2 MHz (20 km from the receiving antenna);

Radio SeiSei--> 101.5 MHz (20 km from receiving antenna);

RTL102.5--> 101.2 MHz (35 km from receiving antenna).

FM Crystal Radio Receivers
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Radio, 2002, 7

The notion of "crystal radio" is strongly associated with huge antennas and radio broadcasting on long

and medium bands, in this article, the author describes the experimentally tested detector circuits of

VHF receivers designed to listening to a FM stations.

The very possibility of receiving VHF FM detector was discovered accidentally. One day I was walking in

the Terletskiypark in Moscow, Novogireevo, I decided to listen to the broadcast - I had a simple crystal

set without resonant tank (this circuit is described in the "Radio", 2001, 1, Fig. 3). The receiver had a

telescopic antenna with length of about 1.4 m. Wonder whether it is possible to receive radio broadcast

with this short antenna? It was possible to hear, but weakly, simultaneous operation of two stations. But

what is surprised me is the volume of receiving was rise and fall periodically almost to zero after every

5...7 m, and it was different for each radio station!

It is known that in the LW and MW bands, where the wavelengths are hundreds of meters, it is

impossible. I had to stop at the point of receive with maximum volume of one of the stations and listen

attentively. It turned out - this is "Radio Nostalgie", 100.5 MHz, broadcasting from the near city

Balashikha. There were no line of sight between antennas. How does the FM transmission could be

received by using the AM detector? Further calculations and experiments shows that it is quite possible

and is not depends on the receiver.

A simple portable FM crystal receiver is made exactly the same way as an indicator of the electric field,

but instead of measuring device it is necessary to connect a high-impedance headphones. It makes

sense to add an adjustment of coupling between the detector circuit and the resonant tank to adjust the

maximum volume and quality of the receiving signal.

The simplest Crystal radio

The circuit diagram of the receiver suitable for these requirements is shown in Fig. 1. This circuit is very

close to the circuit of the receivermentioned above.Only the VHF resonant tank has been added to the

circuit.

Fig. 1.

VD1, VD2 - GD507A - an old USSR Germanium high-frequency diodes with the capacitance of 0.8 pF (at

the reverse voltage of 5V), the recovery time of reverse resistance is no more than 0.1 uS (at the Idirect

pulse=10 mA, Ureverse pulse=20 V, Icutoff=1 mA)
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The device contains a telescopic antenna WA1, directly connected to the resonant tank L1C1. The

antenna is also an element of the resonant tank, so to get the maximum power of the signal it must be

adjust both the length of the antenna and the frequency of the tank circuit. In some cases, especially

when the length of the antenna is about 1/4 of the wavelength, it is useful to connect the antenna to a

tap of the tuning coil L1 (find the suitable tap of the coil by finding the maximum volume of the signal).

The coupling with the detector can be adjust by trimmer C2. Actually the detector is made of two high-

frequency germanium diodes VD1 and VD2. The circuit is completely identical to the voltage doubling

rectifier circuit, but the detected voltage would be doubled if only the trimmer capacitor C2 value is

high, but then the load of the resonant circuit L1C1 would be excessive, and its quality factor Q will be

low. As a result, the signal voltage in the circuit tank L1C1 will be lower and the audio volume will be

lower too.

In our case, the capacitance of the coupling capacitor C2 is small enough and voltage doubling does not

occur. For optimal matching the detector circuit with the tank circuit the impedance of the coupling

capacitor must be equal to the geometric mean between the input resistance of the detector and theresonant resistance of the tank circuit L1C1. Under this condition, the detector is getting the maximum

power of the high-frequency signal, and this is corresponding to the maximum audio volume.

The capacitor C3 is shunting the higher frequencies at the output of the detector. The load of the

detector is headphones with the dc resistance of not less than 4K ohms. The whole unit is assembled in

a small metal or plastic housing. The telescopic antenna with the length not less then 1m is attached to

the upper part of the housing, and the connector or the jack for the phones is attached th the bottom of

the housing. Note that the phone cord is the second half of the dipole antenna (a counterweight).

The coil L1 is frameless, it contains 5 turns of enameled copper wire with diameter of 0.6...1 mm wound

on a mandrel with diameter of 7...8 mm. You can adjust the necessary inductance by stretching orcompressing the turns of the coil L1. It's better use the variable capacitor C1 with an air dielectric, for

example, type 1KPVM with two or three movable and one or two fixed plates. Its maximum capacity is

small and can be in range of 7...15 pF. If the variable capacitor has more plates (the capacitance is

higher), it is advisable to remove any of the plates, or connect the variable capacitor in series with a

constant capacitor or a trimmer, it will reduce the maximum capacity.

The capacitor C2 is ceramic trimmer capacitor, such as a KPK or KPK-M with the capacity of 2...7 pF.

Other trimmers capacitors could be used too. The trimmer capacitor C2 can be replaced with a variable

capacitor, similar to C1, and it could be used to adjust the coupling "on the fly" to optimize radio

receiving capabilities.

Diodes VD1 and VD2, can be GD507B, D18, D20 (it is old USSR Germanium high-frequency diodes. This

diodes can be replaced with modern Schottky diodes). The shunting capacitor C3 is ceramic, its capacity

is not critical and can have a value in range from 100 to 4700 pF.

Adjustment of the receiver is simple. Tune the radio by turning the knob on the variable capacitor C1

and adjust the capacitor C2 to get the maximum audio volume. The tune of the resonant tank L1C1 will


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be changed, so all operations must be repeated a few more times, and at the same time find the best

place for the radio receiving. It is doesn't necessarily the same place where the electric field has

maximum strength. This should be discussed in more detail and explain why this receiver can receive FM

signals.

Interference and conversion of FM into AM

If the tank circuit L1C1 of our receiver (Fig. 1) will be set up so that the carrier frequency of FM signal

falls on the slope of the resonance curve, the FM can be converted into AM. Let's find the value of Q of

the tank circuit. Assuming that the bandwidth of the tank circuit L1C1 is equal to twice the frequency

deviation, we obtain Q = F0/2f= 700 for both the upper and the lower VHF band.

The actual Q of the tank circuit in a crystal radio probably will be less than 700 because of the low Q-

factor of its own Q (About 150...200) and because the resonant tank is shunted by the antenna and by

the input impedance of the detector. Nevertheless, a weak transformation of FM into AM is possible,

thus, the receiver will barely work if its tank circuit detune a little up or down in frequency.

However, there is much more powerful factor contributing to the transformation of FM into AM, - it is

an interference. It's very rarely when the receiver is in the line of sight of radio station, in most cases the

line of sight is obscured by buildings, hills, trees and other reflective objects. A few radio beams

scattered by these objects comes to the antenna of the receiver. Even in the line of sight to the antenna

comes some reflected signals (and of course, direct signal comes too). The total signal depends on both

the amplitudes and phases of summing components.

The two signals are summed if they are in phase, i.e., the difference of their ways is multiple of an

integer of the wavelength, and the two signals are subtracted if they are in opposite phase, when the

difference of their ways is the same number of wavelengths plus half wavelength. But the wavelength,as well as the frequency varies at FM! The difference of the beams and their relative phase shift will

vary. If the difference of ways is large, then even a small change in frequency leads to significant shifts in

the phases. An elementary geometric calculation leads to the relation: f/f0= /4C, or C = f0//4f,

where C - the difference of the ways of the , it's required for the phase shift /2, to get the full sum of

AM signal, f - frequency deviation. The full AM is the total variation of the amplitude signal from the

sum of the amplitudes of the two signals to their difference. The formula can be further simplified if we

consider that the multiply of frequency by the wave length f0 is equal to the speed of light c: C = c/4f.

Now it is easy to calculate that to get a full AM of the two-beam FM signal, the sufficient difference

between the ways of beams is about a kilometer. If the difference of ways is smaller, the depth of AM

proportionally decreases. Well, but if the difference of ways is more? Then, during one period of the

modulating audio signal the total amplitude of the interfering signal will pass several times through the

highs and lows, and distortion will be very strong when converting FM into AM, up to complete

indistinct of the sound when you receive the FM by an AM detector.

Interference with FM broadcast reception is an extremely harmful pheno

fm crystal radio receivers

Documents

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