Is this how the Omni VI and Omni VI accomodate FM?
Some multimode radios with FM have a separate IF and detector just
for the FM, and may have a narrower roofing filter in the IF used for
the other modes.
Yes, and there is another reason that you may as well use a whole
separate IF for narrow band FM detection. With wideband FM, like 75 kHz
used in broadcast FM, a discriminator circuit operating at an IF
frequency of typically 10.7 MHz will have plenty of audio output from a
wideband FM signal. When the FM deviation (modulation) is only several
kHz wide, you can get more audio voltage out of the discriminator
circuit if it is operating at a much lower frequency (typically 455 kHz)
so that the FM deviation is a higher percentage of the IF frequency
where the discriminator is operating. Most narrow band FM capable
receivers (including the Omni VI) eventually convert the higher IF down
to 455 kHz to do the FM detection. Some of the JA rigs get the high IF
after the roofing filter, before sending it on to a FM IF which
downconverts to 455 kHz. Ten-Tec did "the right thing" and tapped it off
before the roofing filter, instead of compromising performance for SSB
and CW by using a wide roofing filter. Using a wider roofing filter also
allows the bandbass tuning or IF shift to work over a wider frequency
range. Which I think is kind of neat, but I really like the close in
strong signal rejection of the Omni VI. You can't have it both ways.
Ken N6KB
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