Why a peak envelope detector cannot be used for detection of an SSB signal alone?
To detect SSB you need the carrier to obtain an intelligible audio output. Peak envelope detector outputs a voltage proportional to RF input power, so only the full AM RF signal can generate a correct audio output.
Please go and read some basics, e.g. ARRL Radio Amateur¨s Handbook, any edition.
Hello, I know that you need the carrier as well, to recover the audio, my question is exactly why?
I tend to think the peak envelope detector as a rectivier of the peaks of the modulated RF, and the rectified voltage is then fed to a shunt capacitor to "keep the instantaneous charge" and recover the audio waveform. I think this is not the same as mixing two signals to recover the audio but like the tectification process in the PSU, althought this is not clear to me and is not specified in any of the documents I have read.
But in an AM signal, which is composed of the carrier and the lower and upper sidebands, these peaks occur only on the sidebands. In other words the AM carrier is not modulated, only the sidebands are modulated.
If this is the case, why these sidebands alone cannot be detected by the peak envelope detector?
I assume a single tone (say 1khz) for the current explanation, not a complex voice signal.
Envelope of SSB rectified with detector does not recover intelligible audio. AM has envelope which after rectification gives normal audio. One of method for demodulation of SSB is to first mix SSB with carrier to get AM and then rectify it.
At AM carrier is modulated with audio. Resulting spectrum contains carrier and two sidebands. Sidebands are audio signals shifted on each side of carrier. Neither of them has envelope similar to audio but all three together has equal to it. Sidebands are not modulated at all they are modulation signals shifted on upper and lower side of carrier.
In a nutshell - a 'normal' AM signal carries amplitude and frequency information. The amplitude can be recovered by rectifying a single sideband signal but the modulation frequency is found from the difference between the carrier and sideband. If the carrier is removed, the 'reference' by which the frequency can be determined is no longer there. The normal practice of inserting a locally generated carrier restores the ability to recover the modulation frequency. It also explains why misplacing the carrier (or the signal relative to it) results in the audio pitch being too high or low.
If you think about it, listening to 'Donald Duck' SSB in AM mode is exactly just rectifying the amplitude envelope in the way you suggested.
Yes, that's right. The amplitude is similar to the original audio and the sideband does move in frequency. The only thing stopping it being demodulated is a fixed point from which the frequency can be measured from. On it's own, all you see on a spectrum analyzer is the relative positions and amplitudes of the components in the modulation. With voice, that could be quite a complex assortment of frequencies and levels unless you can talk in sine waves
Several points there:
3. A regular detector picking up an AM signal can be mathematically modelled as a multiplier. The phase of the carrier comes into the equation, without it the two sideband signals may add or subtract, possibly distorting the recovered wave shape. It becomes like an IQ demodulator with two unlocked LO instead of one with a fixed phase shift.