Modulation vs. Beating Confusion - Comments

[nsaspook]

The BFO generates a replacement for a suppressed or missing carrier. The frequency of the BFO is on one shoulder of the IF channel. The IF signal is usually multiplied by the BFO, but in a simple system the BFO is linearly added to the IF signal prior to a diode detector. A capacitor to ground forms the LPF needed to remove the IFs from the audio difference frequency before the audio amplifier input. The diode in that situation is not just an envelope detector, it is also a non-linear mixer.

I think you have an important point on what functions are called vs what that do.

Is it (the simple diode detector circuit) also a non-linear mixer when it demodulates the exact same signal (as on a spectrum analyzer) that's received as modulated AM when it's normally called and explained as an Envelope detector?

 

[Averagesupernova]

Is it (the simple diode detector circuit) also a non-linear mixer when it demodulates the exact same signal (as on a spectrum analyzer) that's received as modulated AM when it's normally called and explained as an Envelope detector?

Yes it is. You cannot demodulate without a carrier. Envelope detector is a bit of a misnomer. The audio signal follows exactly what the envelope is but the carrier is required. Think of it this way. A detector used in this manner is a device that intentionally generates intermodulation distortion. Several signals come into the detector (sidebands and carrier) and they intermodulate to form the audio.

 

[nsaspook]

Yes it is. You cannot demodulate without a carrier. Envelope detector is a bit of a misnomer. The audio signal follows exactly what the envelope is but the carrier is required. Think of it this way. A detector used in this manner is a device that intentionally generates intermodulation distortion. Several signals come into the detector (sidebands and carrier) and they intermodulate to form the audio.

A envelope detector will generate audio from an voice SSB signal RF envelope, it just won't be intelligible as human speech until there is a carrier added to modify the envelope.

"Intermodulation distortion" Yes it does but that's not the same as saying it's a mixer.

https://en.wikipedia.org/wiki/Intermodulation

IMD is also distinct from intentional modulation (such as a frequency mixer in superheterodyne receivers) where signals to be modulated are presented to an intentional nonlinear element (multiplied). See non-linearmixers such as mixer diodes and even single-transistor oscillator-mixer circuits. However, while the intermodulation products of the received signal with the local oscillator signal are intended, superheterodyne mixers can, at the same time, also produce unwanted intermodulation effects from strong signals near in frequency to the desired signal that fall within the passband of the receiver.

 

[Averagesupernova]

Not sure what your last post is getting at nsaspook. I find you contradict yourself.

 

[nsaspook]

Not sure what your last post is getting at nsaspook. I find you contradict yourself.

Sorry if that's happening.

I'm saying, just like the entry on Wiki that the generated intermodulation distortion from the diode envelope detector is not the same as heterodyne mixing.

 

[Averagesupernova]

I read the whole article and I have come to the exact opposite conclusion you have. Granted when we speak of intermod we generally are referring to UNWANTED signals being generated. But I think that should be obvious in this discussion.

 

[nsaspook]

I read the whole article and I have come to the exact opposite conclusion you have. Granted when we speak of intermod we generally are referring to UNWANTED signals being generated. But I think that should be obvious in this discussion.

Granted on Distortion being unimportant.

To me it's obvious the diode rectifies an AC signal creating a pulsed DC signal that's filtered (to remove the RF frequencies) to smooth the final resulting audio signal.