Thread: FM "deviation"
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Old 10th Apr 2010, 2:41 am   #37
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: FM "deviation"

Thanks for that, Dave.

Something else that since occurred to me is that the degree of upper sideband rejection required would not be all that great. The noise increases at 6 dB/8ve, so across the subcarrier and its sidebands, there would be maybe an 8 dB increase in noise level from 23 to 53 kHz. Accordingly, 10 or so db rejection might be enough. Also, the rejection would not need to be “flat” across the sideband, if the objective is simply to push upper sideband noise level down to a level comparable to that of the lower sideband. Overall, that would seem to ease the phase-shifting burden.

But then why not simply put the subcarrier and sidebands through a mild filter with 6 dB/8ve downslope from 23 to 53 kHz and synchronously demodulate. In fact with FDM decoders, I think it is normal to apply the de-emphasis to the subcarrier ahead of demodulation using a bell filter, so it would just be a case of adding the tilt to that filter, and ensuring that the tilt, if not exactly 6 dB/8ve linearly across the band, was symmetrical about 38 kHz.

Cheers,
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