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G8HQP Dave · 12th May 2017, 11:53 am

200kHz IF bandwidth is not really enough for good stereo. You need more like 250-280kHz. FM uses the sidebands at odd harmonics of the baseband to carry the signal, and those at even harmonics to maintain a constant RF amplitude. Stereo MPX has signals up to 53kHz so even 318kHz IF bandwidth only just allows for the third harmonic at +/- 159kHz - and that ignores phase issues.

Looking at it another way, if there is much stereo information in the original signal then this means lots around 38kHz. You want this to get through undamaged so you need low distortion and accurate phase and amplitude. To get just the 3rd harmonic sideband through OK means an IF bandwidth significantly greater than 6x38kHz = 228kHz.

Another issue is that you do not necessarily need a flat IF passband, but good phase response. This means a rounded passband, quite different from AM; Bessel rather than Butterworth.

I suspect that some of the earlier FM receiver designers did not realise this and got poor stereo from their add-on decoders as a result.

12th May 2017, 11:53 am	#24
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Avantic BM611 Stereo Decoder 200kHz IF bandwidth is not really enough for good stereo. You need more like 250-280kHz. FM uses the sidebands at odd harmonics of the baseband to carry the signal, and those at even harmonics to maintain a constant RF amplitude. Stereo MPX has signals up to 53kHz so even 318kHz IF bandwidth only just allows for the third harmonic at +/- 159kHz - and that ignores phase issues. Looking at it another way, if there is much stereo information in the original signal then this means lots around 38kHz. You want this to get through undamaged so you need low distortion and accurate phase and amplitude. To get just the 3rd harmonic sideband through OK means an IF bandwidth significantly greater than 6x38kHz = 228kHz. Another issue is that you do not necessarily need a flat IF passband, but good phase response. This means a rounded passband, quite different from AM; Bessel rather than Butterworth. I suspect that some of the earlier FM receiver designers did not realise this and got poor stereo from their add-on decoders as a result.