[TenTec] Omni VI Pass Band Tuning CW Narrow

[Dr. Gerald N. Johnson]

On Fri, 2007-04-20 at 18:22 -0400, Mark Erbaugh wrote:
> On Fri, 2007-04-20 at 17:03 -0400, John Huffman wrote:
> > I have a new-to-me Omni VI+.  I have a question concerning pass band tuning and the filters.
> > 
> > My Omni VI+ has these filters:
> > 9MHZ
> > (48058) 9429 8-pole -2.4 kHz
> > N1
> > (48058) 9918 8-pole -2.4 Hz
> > N2
> > 219 (48055) 6-pole -250 Hz
> > 
> > 6.3 MHZ
> > 282 (49054) 6-pole -250 Hz-
> > 285 (48053) 6-pole -500 Hz-
> > 288 (48075) 8-pole -1.8 kHz-
> > Standard - 2.4 kHz
> > 
> > I view my audio output using Spectrogram (wonderful for zero beating for those of us with 
> > a tin ear).  Using Spectrogram you can see the range of audio frequencies being sent to 
> > the headphones and speaker.
> > 
> > When I turn the PBT knob, I can see the passband shift up or down for each filter except 
> > the 219 (48055) 6-pole -250 Hz in position N2.  That filter doesn't move.
> > 
> > If the knob on the PBT is centered it shows passing a narrow band of frequencies (although 
> > not as cleanly as the other filters).  If I turn the knob fully CCW there is no change in 
> > the frequencies or width.  If I turn the knob CW, the audio fades and disappears as The 
> > knob is turned.
> > 
> > Is this normal?  The filter in the N1 position behaves as expected.
> 
> John,
> 
> I believe that the PBT control only affects the 6.3 MHz filters. The 9
> MHz filters don't move. Make sure that when you are testing the N1
> filter, you aren't seeing the effect of adjusting the 6.3 MHz filter.
> Both IFs will affect the audio.  This is why TenTec offers 9 MHz filters
> with the same bandwidth but different center frequencies.
> 
> 73,
> Mark
> 
The pass band tuning does NOT adjust the 6.3 MHz filters. It moves the 9
to 6.3 MHz offset oscillator which changes the effective frequency of
the fixed frequency 6.3 MHz filter. After the 6.3 MHz filter, the signal
is mixed with that same offset oscillator back to 9 MHz for detection
with the 9 MHz BFO signal.

When the second IF filter is wider than the first IF filter it won't
change the bandpass tuning until its tuned so far that the gain is
reduced. The slopes of a wide 6.3 MHz filter will not be as steep as the
first IF 250 Hz filter and so the side of the pass band won't be found
to shift.

If you switch in the 250 Hz in both IFs (and you appear to have that
option) you should detect a narrowing from one side or the other
depending on which way you tune the PBT tuning knob with no more than a
+/- 125 Hz shift in center frequency until you kill the signal
completely by having the two narrow filters on different frequencies. 

In my Corsair II (which works the same), I found that I could get narrow
selectivity with the PBT and two SSB filter to the point that I almost
didn't install a 500 Hz narrow filter. The Omni VI should do as well.

-- 
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer