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AM demodulator diode


Kevin Weddle
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Hi Kevin,
AM is Amplitude Modulation, which is a fixed carrier (RF) frequency whose amplitude (AC voltage) is changed at an audio rate. 100% modulation occurs when the modulation causes the carrier's amplitude to change from zero for half the audio wave to double on the other half of the audio wave. The frequency of the carrier doesn't change but the modulation of it causes sum and difference frequencies to occur. It is easy to AM modulate the output of a crystal oscillator.

An AM detector diode rectifies the carrier frequency but its filtering is small so that its DC output changes at the audio modulation's rate. Another part of the detector diode's DC is heavily filtered so that even its audio modulation is gone and the resulting DC is used for RF automatic-gain-control.

FM is Frequency Modulation, which is a fixed amplitude carrier (RF) whose frequency is changed at an audio rate. 100% modulation occurs when the carrier's frequency changes plus and minus 75KHz which is the standard for the FM broadcast band.
It is difficult to change the frequency of an RF crystal oscillator so transmitters modulate the frequency of a low-RF frequency crystal oscillator a very small amount and use frequency multipliers to reach the desired RF carrier frequency. The frequency multipliers also multiply the frequency change that is caused by the modulation. An FM detector is a discriminator coil or quadrature coil, phase-locked-loop or pulse-counter.
Since an FM detector doesn't respond to amplitude changes, "crackling and popping" and even atmospheric noise interference is minimised.

Maybe you are confusing an AM detector diode with an FM modulator. It is possible to directly change (frequency modulate) the RF frequency of an unstable RF oscillator by changing the capacitance of a diode (or oscillator transistor) with a changing DC voltage. The diode's capacitance becomes part of the oscillator's tuned coil and capacitor. The oscillator is unstable because its frequency drifts with changes in supply voltage, ambient temperature and proximity to other objects.

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Hi Kevin,
As I pointed-out in you other post about AM demodulation, a modulated AM carrier has equal positive-going parts and negative ones. Therefore the modulation cancels itself. Rectification of the carrier will produce an amplitude-changing single-polarity DC voltage that is the modulation recovered. The varying DC voltage is filtered to make it easier for the audio stages to amplify.

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