FREQUENCY MODULATION CIRCUIT

ENEE 417

Lawrence HanAfrouz Azari

MONDAY, DECEMBER 17, 2007

1

INTRODUCTION..............................................................................................3PARTS LIST...................................................................................................................................................4

FM TRANSMITTER..........................................................................................5DESCRIPTION................................................................................................................................................5DESIGN........................................................................................................................................................6

Amplifier...............................................................................................................................................6Voltage Controlled Oscillator (VCO)......................................................................................................6

PSPICE DESIGN: AMPLIFIER AND VCO...............................................................................................................8Simulation of FM transmitter...............................................................................................................9Simulation of FM transmitter amplifier input and output...................................................................10

PRINTED CIRCUIT BOARD..............................................................................................................................11TRANSMITTER EXPERIMENTAL RESULT..............................................................................................................12

Amplifier output.................................................................................................................................13VCO Output........................................................................................................................................15

FM RECEIVER................................................................................................18DESCRIPTION..............................................................................................................................................18DESIGN......................................................................................................................................................18

Phase locked Loop..............................................................................................................................18PSPICE DESIGN............................................................................................................................................20

FM Receiver Circuit (Not including photodiode circuit).......................................................................20PRINTED CIRCUIT BOARD..............................................................................................................................21RECEIVER EXPERIMENTAL RESULTS..................................................................................................................22

PLL input/Photodiode circuit’s amplifier output using Sinusoidal Audio input....................................22Receiver output using Sinusoidal Audio input.....................................................................................24Receiver output using Square wave Audio input.................................................................................26Frequency Response...........................................................................................................................27

CONCLUSION................................................................................................29

APPENDIX.....................................................................................................30DATA SHEET FOR MC14046B.......................................................................................................................30

References...................................................................................................36

2

Introduction

The basic Frequency Modulation communications system has:

1) Transmitter, which is the sub-system that takes the information signal and

processes it prior to transmission. The transmitter modulates the information

onto a carrier signal and broadcasts it over the channel

2) Channel: The medium which transports the modulated signal to the receiver. In

this project, LED will be connected to FM transmitter end. Signals then will be

picked up by the Photo Detector diode right before the FM Receiver

3) Receiver that is the sub-system that takes in the transmitted signal from the

channel and processes it to retrieve the information signal. The receiver must be

able to discriminate the signal from other signals which may use the same

channel medium, amplify the signal for processing and demodulate (remove the

carrier) to retrieve the information.

Figure 1) simple drawing for FM system

3

ReceiverTransmitter

VCOAudio Amplifier

PLL

Channel

Parts List

FM Transmitter and Receiver Circuits Parts List

Description ID ValueTransmitter

QuantityReceiver Quantity

Total Quantity Unit Price * Total Cost

Resistor R4, R6, R7, R8, R9 5.1k 3 2 5 $0.01 $0.05Resistor R2 18k 1 1 $0.01 $0.01Resistor R1, R3, R5 1k 3 3 $0.01 $0.03Resistor R10 22k 1 1 $0.01 $0.01

Capacitor C2, C3 62pF 1 1 2 $0.10 $0.20Capacitor C1 470uF 1 1 $0.20 $0.20Capacitor C4 1.8nF 1 1 $0.10 $0.10Capacitor C5 220uF 1 1 $0.20 $0.20

PLL chip, MC14046B U2, U3 1 1 2 $0.53 $1.06Op-Amp chip, LM741 U1 1 1 $0.84 $0.84

9V battery 9V 2 1 3 $4.29 $12.87

XLR3 connector, female 1 1 $4.00 $4.00

Total: 14 8 22 $19.57

*Note - only the MC14046B was purchased while all other components were provided by the school, therefore unit prices were estimated.

Figure 2) Parts list

To design the FM transmitter and receiver from the beginning we estimate that 25 man-hours would be needed. The majority of those man-hours would be devoted to discovering the frequency characteristics of the transmitter’s VCO (VCO voltage input vs. VCO frequency output) by changing the resistor and capacitor components that determine it. However, we estimate that only 5 hours would be needed to produce the printed circuit boards and install all the components of the transmitter and receiver.

4

FM Transmitter

Description

The FM Transmitter uses FM waves (frequency modulated waves) to send sound. Frequency modulation transmits data (in our case an audio signal) over a carrier wave by changing the frequency of the carrier wave, where the frequency of the carrier wave corresponds to the voltage level of the audio signal. In order to use electromagnetic transmission, the audio signal must first be converted into an electric signal. The conversion is accomplished by a transducer, in our case the microphone. After conversion, the audio signal is used to modulate a carrier signal.

Figure 3) An example of frequency modulation. The top diagram shows the modulating signal superimposed on the carrier wave. The bottom diagram shows the resulting frequency-modulated

signal. Ref (1)

The process of modulation means to systematically use the information signal (what you want to transmit) to vary some parameter of the carrier signal. The carrier signal can be a sinusoidal, as shown above, but in our case the carrier signal will be a square-wave that is generated by a voltage-controlled oscillator (VCO).

5

Design

Amplifier

In our circuit we used an amplifier in order to amplify the audio signal and increase the amplitude of the VCO input. We chose an LM741 operational amplifier that was readily available in our lab, and configured it for use as a non-inverting amplifier. Our amplifier was designed to have gain of 20, therefore if the input signal is 100mV the output of amplifier will be 2V.

Voltage Controlled Oscillator (VCO)

A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied voltage, while in our case signals will be fed into the VCO to cause frequency modulation (FM).

A voltage-controlled oscillator (VCO) provides a periodic signal where the frequency of the signal is related to the level of an input voltage control signal supplied to the VCO.

We collected data to plot the voltage-frequency characteristics of the VCO of the MC14046B.

-

200,000

400,000

600,000

800,000

1,000,000

1,200,000

1,400,000

1,600,000

1,800,000

2,000,000

0 1 2 3 4 5 6 7 8 9 10

VCO Input (V)

Freq

uenc

y (H

z)

Figure 4) voltage-frequency characteristics of the VCO

6

Due to limitation of the frequency response of the LED and photodiode, we limited our frequency range to a maximum of approximately 1MHz. Therefore we chose the characteristics of the pink line, which reaches 1MHz at approximately 5.9V.

For the choice of the center frequency, we considered the capture frequency range of the receiver. We believed that the PLL would safely be able to lock onto a frequency range of 300kHz. This meant a frequency range of the VCO between approximately 700kHz and 1MHz. We chose a center frequency of 860kHz which corresponds to a VCO input voltage of 4.9V.

7

Pspice Design: Amplifier and VCO

Input audio signal is reproduced by V4 voltage source. Output signal from pin 4 of U2 (MC14046B PLL IC not found in PSpice, therefore

CD4046 IC used. Both IC’s have same pin configurations) will be passed to LED circuit.

R 2

1 8k

1 k

V

V

0

C 26 2p

R 7

5 . 1k

C O M P _ I N3

I N H5

C 16

C 27

ZE

NE

R15

S IG _ IN1 4

R 11 1

R 21 2

V C O _ I N9P H _ P U L S E1

C O M P 1 _ O U T2

C O M P 2 _ O U T1 3

V C O _ O U T4

D E M O _ O U T1 0

VD

D*

16

VS

S*

8

U 2

C D 4 0 4 6

V 29 V d c

+3

-2

V+

7V

-4

O U T6

O S 25

O S 11

U 1 L M 7 4 1 / N S

V 19 V d c

R 1

1 k

R 3

1 k R 65 . 1k

R 45 . 1k

C 1

4 70 uV 4

F R E Q = 1kV A M P L = 5 0 m VV O F F = 00

V 59 V d c

Figure 5) Pspice design for transmitter; the circuit on the left is the amplifier and the one on the right is the VCO circuit.

8

Simulation of FM transmitter

Time

0s 0.1ms 0.2ms 0.3ms 0.4ms 0.5ms 0.6ms 0.7ms 0.8ms 0.9ms 1.0msV(V4:+) V(R3:2)

-1.2V

-0.8V

-0.4V

0V

0.4V

0.8V

1.2V

Figure 6) Input signal (green): from V4 voltage source (audio signal from XLR connector)Output signal (red): amplifier output before C1 capacitor

Time

0s 0.1ms 0.2ms 0.3ms 0.4ms 0.5ms 0.6ms 0.7ms 0.8ms 0.9ms 1.0msV(C1:2) V(R3:2)

-2.0V

0V

2.0V

4.0V

6.0V

Figure 7) Input signal (green): amplifier output before C1 capacitorOutput signal (red): amplifier output after C1 capacitor

9

Simulation of FM transmitter amplifier input and output

Input signal (red): zoomed in view of amplifier output/VCO inputOutput signal (green): VCO output (digital signal with 9V amplitude) (shows center frequency of approximately 1.3MHz but our circuit will use the MC14046B, not the CD4046)

Time

480us 485us 490us 495us 500us 505us 510us 515usV(C1:2)

4.80V

4.90V

4.75V

5.00V

480us 485us 490us 495us 500us 505us 510us 515us

:VCO_OUT

Figure 8) Transmitter amplifier input and output

10

Printed Circuit Board

Figure 9) Transmitter PCB layout

Figure 10) Transmitter Circuit (top view)

Figure 11) Transmitter Circuit (back view)

11

VCO chip (Transmitter Side)

Amplifier (Transmitter Side)

MC14046B)

LM741

Audio input

+9VGround

-9V

Transmitter Output

Transmitter Experimental result

For our transmitter side, we use the VCO circuit that is integrated into the MC14046B chip (phase locked loop chip).

The following shows the VCO output and the center frequency. The actual measured frequency is slightly higher than 860kHz due to the actual resistance of R7 resistor being less than the desired 5.1kΩ (being 5.04kΩ). This led to a DC voltage level at our amplifier output of 5V.

Figure 13) VCO output and the center frequency

12

Input to VCO (using pin 9 only) VCO output M put)B usedO. Oute distorted when input frequncy ll be distorted.(using pin 4 only)

PLL CHIP

Amplifier Chip

Figure 12) Amplifier and VCO circuit (Source: LM741 and MC14046B datasheets)

Amplifier output

Figure 14) Output of amplifier for transmitter with Input Audio Signal 100Hz (p-p 100 mV))

Figure 15) Output of amplifier for transmitter with Input Audio Signal 1kHz (p-p 100 mV))

13

Figure 16) Output of amplifier for transmitter with Input Audio Signal 10kHz (p-p 100 mV))

Figure 17 ) Output of amplifier for transmitter with Input Audio Signal 20kHz (p-p 100 mV))

14

Figure 18) FM transmitter amplifier output using square-wave audio input of 20kHz (100mV pk-pk amplitude) (signal before the C1 capacitor)

VCO Output

Figure 19) Output of transmitter’s VCO with Audio Signal 100Hz (p-p 100 mV)

15

Figure 20) Output of transmitter’s VCO with Audio Signal 1kHz (p-p 100 mV)

Figure 21) Output of transmitter’s VCO with Audio Signal 10kHz (p-p 100 mV)

16

Figure 22) Output of transmitter’s VCO with Audio Signal 20kHz (p-p 100 mV)

17

FM Receiver

Description

For the FM receiver our team is using the Phase-Locked Loop (PLL) chip which receives an amplified signal from the photodiode circuit and is able to demodulate the signal. The demodulated signal is the reproduced audio signal.

Design

Phase locked Loop

A PLL is a circuit that causes a particular system to track with another one. PLL synchronizes its output signal (generated by oscillator (VCO)) with a reference or input signal in frequency as well as phase. In the synchronized, often called locked, state the phase error between the oscillator’s output signal and the reference signal is zero, or remain constant.

If a phase error builds up, a control mechanism acts on the oscillator in such a way that the phase error is again reduced to minimum. In such a control system the phase of the output signal is actually locked to the phase of the reference signal. This is why it is referred to as a phase locked loop.

The PLL consists of three basic functional blocks:1) A Voltage-Controlled Oscillator (VCO)2) A Phase Detector 3) A Low Pass Filter

Figure 23) PLL diagram Ref (2)

18

If the phase error (error signal) is not zero, the phase detector develops a nonzero output signal. After some delay the loop filter would also produce the signal. The signal would cause the VCO to change its operating frequency in such a way that the phase error finally vanishes.

For PLL, we use the MC14046B chip, which contains two phase comparators, a voltage−controlled oscillator (VCO), and a source follower. The VCO has been already described. For our design purpose we use Phase Comparator 1. Phase comparator 1 (an exclusive OR gate) provides a digital error signal PC1out, and maintains 90° phase shift at the center frequency between PCAin and PCBin signals. The following figure shows the inside structure of the MC24046B chip, and input and output signals to the phase comparator. (for more information please refer to Appendix)

19

Figure 24) PLL Circuit design and Phase comparator (Source: MC14046B datasheet from ON Semiconductor)

Pspice Design

FM Receiver Circuit (Not including photodiode circuit)

Input signal from photodiode circuit received at pin 14 of U3 Output signal at pin 10 of U3 (passed to speaker amplifier circuit)

0

C O M P _ I N3

I N H5

C 16

C 27

ZE

NE

R15

S I G _ I N1 4

R 11 1

R 21 2

V C O _ I N9P H _ P U L S E1

C O M P 1 _ O U T2

C O M P 2 _ O U T1 3

V C O _ O U T4

D E M O _ O U T1 0

VD

D*

16

VS

S*

8

U 3

C D 4 0 4 6

0

R 8

5 . 1 kC 36 2 p

C 41 . 8 n F

R 9

5 . 1 k

0 0

R 1 05 1 0

V 29 V d c

0

Figure 25) Pspice design for receiver

20

Printed Circuit Board

Figure 26) Receiver PCB layout

Figure 27) Receiver circuit (top view)

Figure 28) Receiver circuit (back view)

21

PLL Chip: MC14046B)

Receiver unit+9V Ground

Receiver output

Receiver Input

Receiver Experimental Results

PLL input/Photodiode circuit’s amplifier output using Sinusoidal Audio input

Figure 29) Receiver’s PLL input using Sinusoidal Audio input at 100Hz (100mV pk-pk amplitude)

Figure 30) Receiver’s PLL input using Sinusoidal Audio input at 1kHz (100mV pk-pk amplitude)

22

Figure 31) Receiver’s PLL input using Sinusoidal Audio input at 10kHz (100mV pk-pk amplitude)

Figure 32) Receiver’s PLL input using Sinusoidal Audio input at 20kHz (100mV pk-pk amplitude)

23

Receiver output using Sinusoidal Audio input

The measured DC value at the output of the PLL is 0.9V.

Figure 33) Receiver output (PLL output) using Sinusoidal Audio input at 100Hz (100mV pk-pk amplitude)

Figure 34) Receiver output (PLL output) using Audio input at 1kHz (100mV pk-pk amplitude)

24

Figure 35) Receiver output (PLL output) using Audio input at 10kHz (100mV pk-pk amplitude)

Figure 36) Receiver output (PLL output) using Audio input at 20kHz (100mV pk-pk amplitude)

25

Receiver output using Square wave Audio input

Figure 37) Dark Blue Signal (Channel 1): Input square wave at 7kHzLight blue Signal (Channel 2): output of receiver’s PLL

Figure 38) Audio input, 20kHz square-wave (100mV pk-pk amplitude)

26

Figure 39) FM receiver’s output using square-wave Audio input of 20kHz, (100mV pk-pk amplitude)

Frequency Response

The combination of the FM transmitter and FM receiver was found to have a constant gain up to a frequency of about 56kHz. After this frequency, the output signal of the receiver becomes noise because the PLL is no longer locked onto its input signal.

Figure 40) Dark Blue Signal (Channel 1): Audio input 1kHz (100mV pk-pk amplitude)Light Blue Signal (Channel 2): receiver PLL output

27

Figure 41) Dark Blue Signal (Channel 1): Audio input 55kHz (100mV)Light Blue Signal (Channel 2): receiver PLL output (circuit is still reproducing audio signal)

Figure 42) Dark Blue Signal (Channel 1): Audio input 56kHz (100mV)Light Blue Signal (Channel 2): receiver PLL output (no longer locked)

28

Conclusion

Possible Improvements

Increase the input impedance of the speaker’s power amplifier stage to reduce the current sinked by that circuit (whose input is from the MC14046 PLL IC), and therefore requiring less power from the MC14046.

Lower the center frequency and utilize the VCO configured with a less steep frequency-voltage input curve. This change could enable the PLL IC to stay locked at higher frequencies of the audio signal (our limit was an audio signal of approximately 56kHz).

Improve techniques on soldering wires. For instance, wires soldered onto the PC board have a tendency to detach if the soldering was not sufficient.

Lessons Learned From The Course

Initially, for our FM transmitter design we considered using some type of analog oscillator circuit (i.e. using an RLC circuit). However we learned that simply utilizing a VCO already built on an IC was much easier and quicker to implement.

Additionally, in this course we learned practical design and implementation issues that were not explicitly taught in previous lab courses such as the effects of ground loop. Also, since we worked more in-depth with AC circuits versus previous courses, we discovered the ill effects of impedance mismatching. This is because this course required the connections of separately designed circuits. These are issues to keep in mind when doing future circuit designs.

29

Appendix

Data Sheet for MC14046B

30

31

32

33

34

35

References

1) http://www.bpm-music.com/SiteFiles/Image/Grafica/Frequency-modulation.png 2) http://www.ami.ac.uk/courses/ami4428_ahdl/u03/images/ad_ma_img03.gif 3) www.onsemi.com/pub/Collateral/MC14046B-D.PDF

36