Discriminator circuits

One of the classic FM discriminator circuits is shown in Fig. 27-21. This circuit uses a special transformer that has two secondary windings. One secondary winding is turied slightly above the IF frequency while the other is tuned the same amount below the IF frequency. The two frequencies are spaced slightly more than the expected transmitter swing. Their outputs are combined in a differentia] pair of diodes (Py and The outputs of the diodes are connected to load resistors R\ and Normally, when the signal is unmodulated, the algebraic sum of the two diodes outputs is zero, resulting in zero output. When the frequency or phase swings above the quiescent value, one diode will conduct higher than the other, resulting in an imbalance across R\ and That produces an output. Similarly, when the frequency or phase drops below the quiescent value, then the opposite situation exists. The other diode will conduct harder and produce an output of the opposite polarity.

MICROPHONE

REACTANCE

OSCILLATOR

STAGES

MODULATOR

FOLLOWING

FM OUTPUT

OSCILLATOR

REACTANCE

STAGES

MODULATOR

FOLLOWING

MICROPHONE

output

27-21 Two forms of angular modulation (A) FM, (B) PM.

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