Detection of PM signals

Chapter Detection of PM signals

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • negative) is directly proportional to the instantaneous frequency. Therefore, the outputvoltage is a duplicate of the modulating waveform.A discriminator is sensitive to amplitude variations in the signal, but this problemcan be overcome by the use of a limiter.The ratio detectorA discriminator with a built-in limiting effect is known as a ratio detector. This type ofFM detector was developed by RCA and is used in high-fidefity receivers and in the au-dio portions of TV receivers.A simple ratio detector circuit is shown in Fig. 27-7. A transformer splits the signalinto two components.508 Data reception27-7A ratio detector for demodulating FM.A change in signal amplitude causes equal level changes in both halves of the cir-cuit. These effects cancel because they are always 180 degrees out of phase. This nulli-fies amplitude variations on the signal.A change in signal frequency causes a phase shift in the circuit. This unbalances it,so that the outputs in the two halves of the circuit become different, which produces anoutput in direct proportion to the instantaneous phase shift. The output signal is a du-plication of the modulating waveform on the FM signal.Detection of PM signalsPulse modulation (PM) operates at a low duty cycle. The pulses are far shorter in du-ration than the intervals between them. A PM signal is “mostly empty space.” The ratioof average signal power to peak signal power is low, often much less than 1 percent.When the amplitude or duration of the pulses changes, the average transmitterpower also changes. Stronger or longer pulses increase the effective signal amplitude;