Discussion
Which modulator would a designer choose to obtain crystal-stable wideband FM?
Answer: D
The Armstrong arrangement is the answer: integrate the audio, phase modulate a crystal oscillator at low level, then multiply up to reach the deviation wanted, so the crystal is never pulled and the centre frequency is as stable as the crystal itself. A reactance modulator gives large deviation directly but drifts. A balanced modulator with a filter produces single sideband. And collector modulation is high-level AM. This is the one place phase modulation earns its keep in analogue transmitters.
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