FM Detectors

Balanced Slope DetectorRatio Detector

Demodulation of FM Waves

In frequency modulation, basically the frequency of the carrrier signal is changing with the instantaneous value of the baseband signal.

When such an frequency modulated wave is transmitted at the transmitter, it has to be received at the receiver. This purpose is fulfilled by the FM detector.

So, the FM detector accepts the FM wave at its input and produces the original modulating signal at its output. So, the process of frequency demodulation is exactly opposite to the frequency modulation.

The FM demodulator is basically a frequency to amplitude converter. It converts the frequency variations in FM wave into amplitude variations at its output to recover the original modulating signal.

Requirements of FM detector:

It must convert frequency variations into amplitude variations.

This conversion must be linear and efficient. The demodulator circuit must be insensitive to

amplitude changes. It should not be too critical in its adjustments and

operation.

Classification of FM Demodulators:

Direct Type Indirect Type

FM Demodulators

Frequency Discriminators

Zero CrossingDetector

Phase LockedLoop

Balanced Frequency DiscriminatorThe frequency discriminator operates on the principle of the slope detection.

Principle of Slope Detection: To understand the principle of operation of fm detectors, we need to first study

the simplest form of FM detector, the SLOPE DETECTOR. The slope detector is essentially a tank circuit which is tuned to a frequency

either slightly above or below the fm carrier frequency. The FM signal, with center frequency fc and frequency deviation δf is applied to

the input of this tuned circuit. The amplitude of the output voltage of the tank circuit depends on the freq.

deviation of the input FM signal

Simple Slope Detector:The circuit of a simple slope detector is as follows:

The output of the tank circuit is then applied to simple diode detector with a low pass filter of proper time constant (similar to AM diode detector).

Drawbacks of Simple Slope Detector: It is inefficient. It’s linear only over a limited frequency range. It’s difficult to adjust as primary and secondary windings of the transformer must be

tuned to slightly different frequencies.

So, to overcome the drawbacks of the simple slope detector , Balanced Slope Detector is used.

Balanced Slope Detector:

Construction: The balanced slope detector consists of two slope detector circuits. The input transformer has a center tapped secondary. So, the input

voltages of the two slope detectors are 180o out of phase. There are 3 tuned circuits. The primary is tuned to fC , the upper tuned

circuit of the secondary(T1) is tuned above fC by δf i.e., its resonant freq. is ( fC + δf ), the lower tuned circuit of the secondary(T2) is tuned to resonant freq. ( fC - δf ).

Balanced Slope Detector (contd..)

V01 and V02 are the output voltages of the two slope detectors.

The final output voltage V0 is taken by subtracting V01 and V02 i.e.,

V0 = V01 – V02

Working of the circuit:The working of the circuit can be understood by dividing the input frequency

into three ranges as follows: f in = fC :

When input freq. is equal to fC , the induced voltage in T1 and T2 windings are exactly equal. Hence, input voltages to D and D will be same. Thus,

V01 = V02 and so, V0 = 0

fC < fin < ( fC + δf ) :

In this case, the induced voltage in winding T1 is greater than that induced in T2 . Thus, the input voltages to D1 is higher than D2 . Thus,

V01 > V02 and so, V0 = +ve

Balanced Slope Detector (Contd..)

(fC – δf ) < fin < fC :

In this case, the induced voltage in winding T1 is less than that induced in T2 . Thus, the input voltages to D1 is less than D2 . Thus,

V01 < V02 and so, V0 = -ve

If the input frequency goes outside the range of (fC – δf ) to (fC + δf ), the output voltage will fall due to the reduction in tuned circuit response.

Characteristics of the Balanced Slope Detector :

Due to the typical shape , its

called the S-shape characteristics

Balanced Slope Detector (Contd..)

Drawbacks of Balanced Slope Detector:

Even though linearity is good, it is not good enough. This circuit is difficult to tune since the three tuning circuits are to be tuned at

different frequencies , i.e., (fC – δf ) , fC , (fC + δf ) . Amplitude limiting is not provided.

So, the ratio detector is used which provides amplitude limiting also.

Ratio Detector:

C5

The primary and secondary windings are tuned to the same freq fC of the incoming signal.

Its similar to the Foster Seeley Discriminator with the following differences:

1. Direction of D2 is reversed.

2. A large value capacitor C5 included in the circuit.

3. The output is taken differently.

In this circuit the regular conversion from frequency to phase shift and phase shift to amplitude takes place.

Ratio Detector (Contd..)

Working of the circuit: The phase shift between the primary and secondary voltages of the tuned

transformer is a function of the frequency. Secondary voltage lags primary voltage by 90o at fC .

The polarity of the voltage in the lower capacitor is reversed as connections of D2 is reversed. Hence, the voltages in the two capacitors Vo1 and Vo2 add.

When Vo1 increases, Vo2 decreases and vice versa, and since V’o is the sum of Vo1

and Vo2 , it remains constant. So, from the circuit,

Vo = (½)V’o - Vo2

Ratio Detector (Contd..) Hence, output voltage is proportional to the difference b/w the individual

output voltages. Hence, at:

1) f in = fC :

The individual output voltages of the two diodes will be equal and opposite, i.e., V01 = V02 and so, V0 = 0

2) f in > fC :

The phase shift b/w the primary and secondary winding is such that the output of D1 is higher than D2 . Hence,

V01 > V02 and so, V0 = +ve

3) f in < fC :

The phase shift b/w the primary and secondary winding is such that the output of D2 is higher than D1. Hence,

V01 < V02 and so, V0 = -ve

Ratio Detector (Contd..)Phasor Diagrams:

V1

Ratio Detector (Contd..)Characteristics of the ratio detector: