Joe Subich, W4TV wrote:
wider than necessary. With the FM filter, the 4.2 KHz high cut is
less than the 10 KHZ of IF bandwidth. Why is the IF bandwidth and
audio response excessively restricted with the FM filter? It should
be possible to achieve at least 12 KHz of IF bandwidth and 6 KHz or
more of audio bandwidth depending on the filter skirts.
FM bandwidth is a complicated issue, but I believe that, to a first
approximation, you can add the peak audio frequency to the peak
deviation and then double it for deviation the other way and the other
sideband. This covers the dominant sideband frequencies, but actual
sidebands are infinite.
The most common VHF deviation is +/-2.5kHz. For communications quality
audio, that gives a total bandwidth of about 10.6kHz.
I'm not sure how Elecraft define deviation. Maybe they define it as
peak to peak, i.e. +/1.5kHz, in which case they might get 3.5kHz maximum
safe audo frequency in 10kHz. They probably can't go much above 10kHz,
because they need to be compatible with 12.5kHz channeling on VHF, or
even 10kHz channeling on European CB and PMR 446.
Of course, for lower instantaneous peak deviations, one could get more
distortion free bandwidth.
I would guess that communications FM devices are designed for
communication type RF bandwidths, however it is possible that cheap, or
even most, devices don't explicitly limit the audio bandwidth, and
simply rely on their being little audio power at high enough frequencies
to suffer distortion as a result of IF bandwidth limits.
The important points are that the required bandwidth for FM is rather
more than twice the maximum audio frequency and that FM communications
equipment, whilst maybe not designed to police audio bandwidth, isn't
particularly designed for wide bandwidths.
--
David Woolley
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm>
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