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Explain frequency demodulation system using phase discriminator. Draw the circuit diagram and phasor diagram.

Subject : Principle of Communication Engineering

Topic : Angle modulation and demodulation

Difficulty : High

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• The FOSTER-SEELEY DISCRIMINATOR is also known as the PHASE-SHIFT DISCRIMINATOR.
• It uses a double-tuned RF transformer to convert frequency variations in the received fm signal to amplitude variations.
• These amplitude variations are then rectified and filtered to provide a dc output voltage. This voltage varies in both amplitude and polarity as the input signal varies in frequency.
• A typical discriminator response curve is shown in fig.1.
• The output voltage is 0 when the input frequency is equal to the carrier frequency (FR).
• When the input frequency rises above the center frequency, the output increases in the positive direction. When the input frequency drops below the center frequency, the output increases in the negative direction.

Fig.1 Discrimination response curve

• The output of the Foster-Seeley discriminator is affected not only by the input frequency, but also to a certain extent by the input amplitude. Therefore, using limiter stages before the detector is necessary.

Circuit Operation of a Foster-Seeley Discriminator

• Fig.2. shows a typical Foster-Seeley discriminator. The primary tank circuit consists of C1 and L1. C2 and L2 form the secondary tank circuit. Both tank circuits are tuned to the center frequency of the incoming fm signal.
• Choke L3 is the dc return path for diode rectifiers D1 and D2. Resistors R3 and R4 are the load resistors and are bypassed by C3 and C4 to remove rf.

Fig.2 Fooster seeley discriminator

• To obtain the different phased signals a connection is made to the primary side of the transformer using a capacitor, and this is taken to the centre tap of the transformer. This gives a signal that is 90° out of phase.
• When an un-modulated carrier is applied at the centre frequency, both diodes conduct, to produce equal and opposite voltages across their respective load resistors. These voltages cancel each one another out at the output so that no voltage is present.
• As the carrier moves off to one side of the centre frequency the balance condition is destroyed, and one diode conducts more than the other. This results in the voltage across one of the resistors being larger than the other, and a resulting voltage at the output corresponding to the modulation on the incoming signal.
• The choke is required in the circuit to ensure that no RF signals appear at the output. The capacitors C1 and C2 provide a similar filtering function.
• The operation of the Foster-Seeley discriminator can best be explained using vector diagrams fig. 3 that show phase relationships between the voltages and currents in the circuit. Let's look at the phase relationships when the input frequency is equal to the center frequency of the resonant tank circuit.

Fig.3 Phasor diagram of fooster seeley discriminator

• Offers good level of performance and reasonable linearity.

• Simple to construct using discrete components.

• Provides higher output than the ratio detector

• Provides a more linear output, i.e. lower distortion than the ratio detector

• Does not easily lend itself to being incorporated within an integrated circuit.

• High cost of transformer.

• Narrower bandwidth than the ratio detector

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• The FOSTER-SEELEY DISCRIMINATOR is also known as the PHASE-SHIFT DISCRIMINATOR.
• It uses a double-tuned RF transformer to convert frequency variations in the received fm signal to amplitude variations.
• These amplitude variations are then rectified and filtered to provide a dc output voltage. This voltage varies in both amplitude and polarity as the input signal varies in frequency.
• A typical discriminator response curve is shown in fig.1.
• The output voltage is 0 when the input frequency is equal to the carrier frequency (FR).
• When the input frequency rises above the center frequency, the output increases in the positive direction. When the input frequency drops below the center frequency, the output increases in the negative direction.

Fig.1 Discrimination response curve

• The output of the Foster-Seeley discriminator is affected not only by the input frequency, but also to a certain extent by the input amplitude. Therefore, using limiter stages before the detector is necessary.

Circuit Operation of a Foster-Seeley Discriminator

• Fig.2. shows a typical Foster-Seeley discriminator. The primary tank circuit consists of C1 and L1. C2 and L2 form the secondary tank circuit. Both tank circuits are tuned to the center frequency of the incoming fm signal.
• Choke L3 is the dc return path for diode rectifiers D1 and D2. Resistors R3 and R4 are the load resistors and are bypassed by C3 and C4 to remove rf.

Fig.2 Fosster seely discriminator