The Foster-Seeley discriminator circuit is shown in Fig. 27-22, while the waveforms are shown in Fig, 27-23. This circuit requires only two tuned circuits rather than the three required by the previous circuit. The output voltage is the algebraic sum of the voltages developed across the R, and R$ load resistances. Figure 27-23A shows the relationship of the output voltage and the frequency.
The primary tuned circuit is in series with both halves of the secondary winding. When the signal is unmodulated, the IF voltage across the secondary is out of phase with the primary voltage. This makes the voltages applied to each diode equal but out of phase (Fig. 27-23B), resulting in zero output. But consider what happens when the frequency deviates (Fig, 27-23C). The voltages applied to the diodes are
C3 0-01 uF
C3 0-01 uF
FROM LAST LHWTER STAGE
C3 0.01 uF
no longer equal but opposite, and that creates an output from the detector that is frequency- or phase-sensitive. As the input frequency deviates back and forth across the frequency of the tuned circuit an audio signal is created equal to the modulated frequency.
In order for the FM/PM transmitter to be received ilnoise-freelh it is necessary to precede the discriminator circuit with a timiier circuit. This circuit limits the positive and negative voltage excursions of the IF signal, thus clipping off AM noise
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