[TenTec] Roofing filters

[Ken Brown]

> 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