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Old 7th Dec 2014, 10:26 am   #21
Radio Wrangler
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Location: Fife, Scotland, UK.
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Default Re: Quasi-Synchronous Demodulation

Horses for courses as they say.

Plain diode:

These can be viewed in several ways. Think of it as a rectifier working on the IF waveform followed by a filter to dump the IF frequency components and to let the envelope waveform through. Or think of it as a switch being beaten up by the large carrier component and switching the smaller sidebands on and off at the carrier frequency. Or think of it as a square law device where the carrier and sideband components mix and intermodulate, Each sideband mixing with the carrier gives a baseband component, the other one of a pair adds extra and in the right phase. They also make components around twice the IF, which the lowpass filter hunts down and kills. All of these views are valid. SSB proves that to shift the information, you don't need both sidebands and you don't need a carrier. You can look on an AM transmitter as something which sends the carrier reinsertion signal needed by your product detector packaged neatly along with the signal itself.

Switching Demod:

THe diode above can be improved by making it into a switch. Extract the carrier, filter and amplify it then use the big squarewave to operate a switching mixer. This can give a very highly linear mixer and can operate with signals below what would be the threshold of a plain diode demod. Getting the phase right and keeping it right over a range of signal levels is the difficulty. The carrier path is a limiting amplifier and AM to PM conversion is a problem.

Flywheel:

In the carrier reinsertion switcher above, extraction of the carrier involves filtering. Make the filter narrow and it will ring to cover up any wobblies of the incoming signal's carrier. This can be good if the wobblies are in the amplitude dimension. It can also be bad if the sidebands have the same phase wobblies as the carrier because then a less filtered carrier would track them.

PLL:

This is the extreme case of the flywheel carrier reinsertion. Good for fading which drops the carrier and maybe one set of sidebands, poor for phase wobblies.

Pythagoras:

Most digital receivers downconvert the digitised IF (or sometimes RF) and make two channels centred on zero hertz. Usually called I (in-phase) and Q (Quadrature) to demod AM from these, any of the above schemes can be executed in digital form, but the simple rectifier approach needs to look at the amplitude of the vector sum of I and Q data, so Pythagoras' well known thing about hypotenuses is used. Square the numbers of both I and Q channels, Add the results, and take the square root of that. Unlike the diode, it doesn't have a problematic lower threshold and is quite linear if you don't take too many short cuts in the arithmetic.


So there are AM demods for all reasons. Do you want to quickly track phase variations? Do you want to steam-roller across amplitude variations? Do you want cheap? or aare you painting-by-numbers?

Which you choose depends on what you think the biggest problem is going to be.

David
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