[TenTec] Fixed BFO freq = Fixed Beat Note

[Ken Brown]

There has been a lot of discussion about Omni's 1st IF filters, and the 
CW audio note that we hear using them. I  want to state the situation in 
a way that might help understand it.

When we have a filter with sharp skirts and high rejection outside the 
filter band pass, only the signals inside that bandpass will be heard. 
This is used to make a receiver hear only one sideband and when the 
filter bandwidth is very small it also limits the signals we hear to 
that very narrow bandwidth. But what determines which sideband we hear 
and what audio note is produced by signals that make it through this 
filter? The BFO frequency relative to the filter bandpass frequency. If 
the BFO frequency were variable we could set it so that we listen to any 
audio note we like, or even listen to the opposite sideband, or both 
sidebands. I can do this with my Hammarlund Super-Pro and my Kenwood 
TS-440. Of course these radios are lacking in other kinds of performance 
compared to the Omni VI.

In Ten-Tec Omni VI the BFO frequency is fixed, for any given mode, using 
crystal oscillators. So for a narrow filter with a specific center 
frequency, only a specific narrow range of audio tones will be heard. 
Why not make the BFO variable so that we can adjust to any audio not we 
desire? There are a number of reasons. If the BFO were adustable, the 
frequency display would be incorrect (except when the BFO is just at the 
right frequency) or some kind of compensation would have to be made to 
the digital readout, or to the 1st local oscillator (VFO) frequency, 
without changing the displayed frequency. To accomplish this would 
require more control control loops of more oscillators, more complexity 
and likely more phase noise.  Another method (which is used in the Omni 
VI second IF for the PBT) is to mix a single oscillator in at two 
different places, one converting in one direction (down to 6.3 MHz in 
the Omni PBT) and the other converting back in the opposite direction 
(back up to 9 MHz in the Omni VI PBT scheme) This also adds complexity 
and more possibilities for images and birdies, as well as the noise in 
the additional oscillator.

If we were willing to give up some accuracy in the frequency readout of 
the Omni VI (say for instance when not operating close to band edges) we 
could just bend the BFO a little bit and get any audio note we like. 
Perhaps with some firmware modification, and some interconnection with 
the RIT control, we could make an upgrade that could give us control 
over the audio beat note, without giving up frequency readout accuracy. 
I'm not ready to take this challenge on right now. Maybe somebody else 
will, and Inrad can sell the kit.

I hope someone finds this enlightening.

DE N6KB