Brenda Ann wrote:


"craigm" wrote in message
...
The detector in your basic AM radio is much the same as it has been for
nearly 100 years now. It rectifies one half of the envelope and filters
out
the remaining RF to leave an audio waveform. It does not detect both
halves (both sidebands) of the waveform. This is why such things as
selectable
sideband make high end radios better able to pick out a signal. The
signal with both sidebands may be applied to the detector, but it's not
what comes out. I stand by my question. If only one sideband is actually
detected, there can be no phase cancellation.


You clearly don't understand how an AM detector works.


But I do. The basic AM detector works exactly as any half-wave rectifier.

That is correct.

If you used a full-wave rectification, then you could detect both sides of
the RF waveform,

So far, correct.

but logic would seem to dictate that this would cancel
the audio waveform,

No cancellation.

since the two sidebands are mirror images of one
another.

In the frequency domain, yes, that is true. But an envelope detector works
in the time domain. The two are not interchangeable.

You seem to believe that the positive peaks of the AM waveform represent one
sideband and the negative peaks, the other. That is grossly incorrect.

If all you need to add is a diode to detect the opposite sideband, it would
be cheap and easy for all radios to be sideband selectable with the flip of
a switch. But you don't see that, ever wonder why?

If the diode based envelope detector only detected one sideband, then half
the radios in the world would have trouble receiving CHU as it does not
broadcast one sideband. (Adding a balun and getting the wires reversed
would invert the signal and you could lose reception.) This follows from
what you have suggested, yet it doesn't happen.

Look at the links I provided.

If a full wave rectifier is used, the AM envelope is updated twice as often.
It can be viewed as flipping the bottom half of the waveform up and using a
single diode. (Look at the waveforms for a power supply with half wave and
full wave rectification and this may be clearer.)

To receive a single sideband, you would need to tune the radio so that the
carrier is at one edge of the passband of the filter. Then one sideband
will be reduced somewhat, depending upon the shape of the filter, and the
other sideband will be in the middle of the passband. This generally will
not give good rejection of the unwanted sideband.

The other approach is to use the phasing detector as would be found in some
(sideband selectable) synchronous detectors or some SSB detectors. These
detectors process both sidebands and use cancellation to remove the
unwanted sideband.