Hi everyone,

I just asked myself why in textbooks they always set up the polarity of the diode in AM envelope detector so that it always pick up the positive half of the AM modulated signal? why not the negative half? what's wrong with it?

and after a lot of thinking, i came to the conclusion that the negative half is nothing but a 180ْ phase-shifted version of the positive half.
and if we thought of the envelope of any half as a current signal, then the changes in the envelope current amplitude of the negative half will be the same as the changes in the envelope current amplitude of the positive half, but only it happens now in the opposite direction. So, if i transmitted a speech signal, and reversed the polarity of the diode at the receiver so that it could pick up now the negative half of the received AM signal, then what was previously a compression when the positive half was chosen, the speaker will produce it now as a rarefaction and what was a rarefaction will be a compression, but it will maintain the same amplitude changes. right?

Since our ears works somewhat like Fourier analyzers, they only feel change in frequency, but not in phase. So i think if i set up an experiment with the envelope detector diode connected in such a way so that it could pick up the negative half of the received AM modulated signal, then i believe i would hear an identical speech signal to the one i have just sent and i wouldn't be able to tell the difference between the two.

By the way, i have read this article about AM diode detector and i quote:

If the detector is to be used only for detection it does not matter which half of the envelope is used, either will work equally well. Only when the detector is also used to supply the automatic gain control (AGC) circuitry will the polarity of the diode matter.

http://www.radio-electronics.com/in...n/am-reception/diode-detector-demodulator.php

What're your thoughts about this?

 

It's only for visual clarity. Actually, there are some full wave AM envelope detectors. These allow you to use a shorter time constant on the post detection filter, thus less "diagonal clipping." I used to build hi fi AM receivers using this method.

Eric

 

I have a little information about envelope detector that is An envelope detector is AN electronic circuit that takes a high-frequency signal as input ANd provides an output that is that the envelope of the initial signal. The condenser within the circuit stores up charge on the rising edge, and releases it slowly through the resistance once the signal falls. The diode nonparallel rectifies the incoming signal, permitting current flow only if the positive input terminal is at the next potential than the negative input terminal.

 

I have a little information about envelope detector that is An envelope detector is AN electronic circuit that takes a high-frequency signal as input ANd provides an output that is that the envelope of the initial signal. The condenser within the circuit stores up charge on the rising edge, and releases it slowly through the resistance once the signal falls. The diode nonparallel rectifies the incoming signal, permitting current flow only if the positive input terminal is at the next potential than the negative input terminal.

Might mention that the RC time constant of the post detection filter is important. If it's too long, a particularly obnoxious form of distortion called "diagonal clipping" occurs. This is one reason synchronous detectors have replaced diode detectors in a lot of higher quality communications.receivers.

ERic