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SUMMARY

Now that you have completed this chapter, a short review of what you have learned is in order. The following summary will refresh your memory of demodulation, its basic principles, and typical circuitry required to accomplish this task.

DEMODULATION, also called DETECTION, is the process of re-creating original modulating frequencies (intelligence) from radio frequencies.

The DEMODULATOR, or DETECTOR, is the circuit in which the original modulating frequencies are restored.

A CW DEMODULATOR is a circuit that is capable of detecting the presence of rf energy.

HETERODYNE DETECTION uses a locally generated frequency to beat with the cw carrier frequency to provide an audio output.

The REGENERATIVE DETECTOR produces its own oscillations, heterodynes them with an incoming signal, and detects them.

The SERIES- (VOLTAGE-) DIODE DETECTOR has a rectifier diode that is in series with the input voltage and the load impedance.

SHUNT- (CURRENT-) DIODE DETECTOR is characterized by a rectifier diode in parallel with the input and load impedance.

The COMMON-EMITTER DETECTOR is usually used in receivers to supply a detected and amplified output.

The COMMON-BASE DETECTOR is an amplifying detector that is used in portable receivers.

The SLOPE DETECTOR is the simplest form of frequency detector. It is essentially a tank circuit tuned slightly away from the desired fm carrier.

The FOSTER-SEELEY DISCRIMINATOR uses a double tuned rf transformer to convert frequency changes of the received fm signal into amplitude variations of the rf wave.

The RATIO DETECTOR uses a double-tuned transformer connected so that the instantaneous frequency variations of the fm input signal are converted into instantaneous amplitude variations.

The GATED-BEAM DETECTOR uses a specially-designed tube to limit, detect, and amplify the received fm signal.

PHASE DEMODULATION may be accomplished using a frequency discriminator or a quadrature detector.

PEAK DETECTION uses the amplitude, or duration, of a pulse to charge a holding capacitor and restore the modulating waveform.

A LOW-PASS FILTER is used to demodulate pdm by averaging the pulse amplitude over the entire period between pulses.

PULSE CONVERSION is used to convert ppm, pdm, or pcm to pdm or pam for demodulation.




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