Peak envelope detector and ssb detection?

[neazoi]

Why a peak envelope detector cannot be used for detection of an SSB signal alone?
Why exactly?

 

[jiripolivka]

Why a peak envelope detector cannot be used for detection of an SSB signal alone?
Why exactly?

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.

 

[neazoi]

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.

 

[jiripolivka]

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.

To generate audio output you need the beat frequency between the carrier and sidebands. No carrier, no audio if SSB signal is detected. Read the basics, please.

 

[neazoi]

To generate audio output you need the beat frequency between the carrier and sidebands. No carrier, no audio if SSB signal is detected. Read the basics, please.

Unfortunatelly, your answer did not help

 

[Borber]

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.

 

[neazoi]

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.

Yes, this is normally done in IF in superhets. I would like to focus on your first and second sentences. As far as I know in AM the carrier is not modulated, only the sidebands, am I right? So what why we cannot detect the sideband amplitude variations with a simple envelope detector?
Maybe a good explanation of how the sideband is exactly modulated will answer the question.

This question relates to the operation of the envelope detector as well, ie. if it behaves as a mixer or as a simple rectifier, similar to PSU circuits.

 

[Borber]

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.

 

[neazoi]

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.

This seems a more detailed explanation!
However, if the sidebands are not modulated at all, how can we recover audio in an ssb superhet, where we inject an unmodulated carrier?
It does not make sense, the full modulation information MUST be contained in each of the sidebands, else we could not detect any of it at all.
What am I missing here?

 

[betwixt]

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.

Brian.

 

[neazoi]

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.

Brian.

That's just answers it and solves a long-time question for me, thanks Brian.
So the envelope detector when it demodulates an AM signal, works as a simple amplitude detector (to recover audio amplitude variations) but ALSO as a mixer (to recover audio frequency variations based on the reference AM carrier), am I right?

This also verifies my thoughts that the sideband in an SSB signal is both amplitude varied and frequency varied.
Am I correct in my above points?

 

[betwixt]

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

Brian.

 

[neazoi]

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

Brian.

I read somewhere that the carrier in AM, does not only provide the reference frequency for demodulation but also the reference phase and reference amplitude, which is useful for AGC receivers. Are these true? But why the reference phase is needed?

 

[betwixt]

Several points there:

1. AGC (Automatic Gain Control) adjusts the gain of signal amplifying stages to optimize the levels at the detector (if it's actually possible). It needs the complete AM signal to do that, if the sidebands alone were used, the AGC would track the modulation instead of the signal as a whole. Bear in mind that AGC should work even if the carrier isn't modulated. If you listen to SSB on a HF receiver, you will probably hear the background 'breathing' as the audio level goes up and down, that wouldn't be acceptable in regular AM which could be carrying voice or music.

2. Some receivers (I have one here) use synchronous AM detectors. It uses a PLL to lock to the received carrier frequency and phase as it leaves the IF stages then uses the PLL oscillator as a substitute for the transmitted carrier in a DSB demodulator. It has the advantage over a regular detector of making it immune to selective sideband fading but has the drawback that it needs enough carrier to lock to. It doesn't work at all on very weak signals but gives better quailty if the signal is strong enough.

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.

Brian.

 

[neazoi]

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.

Brian.

All right, so the phase AND the amplitude AND the frequency of the carrier is needed to correctly demodulate the sideband.

Thanks a lot!

 

[Audioguru]

I made a voice scrambling circuit with MC1496 balanced modulator ICs making a single-sideband suppressed carrier signal then adding the carrier and demodulating it. I used a switched capacitor lowpass filter IC to remove one sideband. An ordinary AM detector produced inverted audio frequencies so that speech was completely unintelligible. My demodulator circuit was almost identical to the modulator and its output produced perfect audio.

 

[betwixt]

Hence the avatar......:smile:

Brian.