Thread: FM "deviation"
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Old 6th Apr 2010, 5:31 pm   #36
G8HQP Dave
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Join Date: Sep 2008
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Default Re: FM "deviation"

Quote:
The Hershberger design included relatively elaborate three-stage phase-shifters that were said to be accurate to plus/minus 3 degrees over the band 50 to 12 000 Hz, and provided at least 31 dB of unwanted sideband rejection. The need for appropriate component selection and matching – including the avoidance of carbon composition resistors – was heavily stressed.
That is pretty impressive performance from only three stages! I can understand why they wanted to avoid composition resistors.

Quote:
So moving to quadrature demodulation would not have been such a big step.
Yes, getting a 38kHz quadrature carrier is the easy bit - you just add a few extra transistors to your PLL chip.

Quote:
Here’s an interesting question – clearly random and unequal phase-shifting errors in the respective I and Q demodulated signal paths would affect the relative amounts of the lower and upper sidebands that found their way into the nominally lower sideband (L-R) signal after matrixing, but would the net signal then have major phase errors that in turn would compromise the final matrixing with the baseband (L+R) signal? Or would it be simply the case that the LSB/USB mix in (L-R) varied somewhat over the audio band, but basic (L-R) integrity was retained.
That is an interesting point - it may be why they could do it. Errors in the image-cancelling demodulator would, if not too bad, mainly just reduce the sideband rejection which would merely re-introduce some of the upper sideband noise which you are trying to reject. The wanted lower sideband signal would still be OK - so we mainly need accurate F and 38-F phasing, and not quite so accurate 38+F phasing. It is still hard, but perhaps not quite as hard as I first thought. A really clever designer might even be able to balance off errors in the two sidebands, as the main aim is to get an accurate L-R signal and noise reduction is secondary.
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