Richard wrote:
Maybe I used the wrong term. I think lots of HiFi tuners have very wide
filters much greater than 75 Khz. For DXing it seems then you need no more
than say 75Khz. A tuner with that bandwidth would, in a sense, (Ithink)
compared to a regular HiFi tuneer be a narrow bandwidth tuner.

Yes, you used the wrong termgrin.

"NBFM" has a specific technical meaning, IIRC a system where the
modulation index (ratio of peak deviation to maximum modulating
frequency) is less than 1.

For FM broadcast, the peak deviation is 75KHz and the maximum modulating
frequency 15KHz. (OK, I'm ignoring stereo...) So the modulation index
is 5.

For police radio, the peak deviation is roughly 3KHz and so is the
maximum modulating frequency. So the modulation index is 1.

BTW, what would be the result if you used say a 20Khz filter on a FM signal
with 75 Khz deviation? Would you get distortion or a perfectly copyable
signal. I mean is it the analagous to using a 2Khz filter for an AM signal
transmitted at 6Khz wide?

Lots of distortion.

In AM, distance from the center of the channel correlates to modulating
frequency. Restricting the filter bandwidth in the receiver restricts
the frequency response - the high audio frequencies ("treble") are
rolled off. But it has no effect on the range of *amplitudes* that can
be received; a loud sound within the bandpass of the filter will still
be reproduced accurately.

In FM, distance from the center of the channel correlates to modulating
*amplitude*. A loud sound will push the transmitted signal to the outer
edges of the channel. If the receiver's filter is too narrow to pass
that, the signal peaks will be chopped off, resulting in severe
interference.

Remember that the 75KHz peak deviation for FM broadcast is 75KHz *either
side* of center. To get the actual bandwidth required you have to add
the peak modulating frequency to that. 165KHz, not counting stereo.

You can chop some of that off at the expense of some distortion. I've
found in practice, 110KHz filters work fine for DXing though I wouldn't
want to listen to a symphony through them. 75KHz would probably be
reasonably intelligible.
--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com