Hi John,
The 9 MHz IF bandpass center frequency does NOT get shifted by the PBT
control. The PBT control shifts a VCXO on the Passband Tuning Board.
This 15.3 MHz VCXO converts the 9 MHz IF down to 6.3 MHz, and then back
up to 9 MHz. This effectively moves the 6.3 MHz IF bandpass center up or
down relative to the 9 MHz IF bandpass center and the BFO.
When you are using a filter in the 9 MHz IF which is significantly wider
than the filter you have selected in the 6.3 MHz IF, you will be able to
hear (or see) the range of audio frequencies moving up or down as you
move the PBT control. When you move the PBT control to extremes of it's
range the 6.3 MHz IF bandpass moves so that it is no longer coincident
with the 9 MHz IF bandpass, and you will hear (or see) the audio output
level drop off.
If you are using filters in both the 9 MHz and 6.3 MHz IFs, the range of
PBT control settings which keeps the 6.3 MHz IF bandpass and the 9 MHz
IF bandpass coincident becomes much smaller.
If you have selected a 250 Hz BW filter in both the 9 MHz and the 6.3
MHz IF, the setting of the PBT to get the two IF bandpasses coincident
is quite critical. Moving the PBT control around this center setting
(which may not be the center of the potentiometer rotation) will sound
(or appear) more like a gain control than a bandpass shift.
All of this behavior is normal in the Omni VI.
When using a narrow filter in the 9 MHz IF of the Omni VI, you are stuck
with a fixed audio note for signals in the center of the 9 MHz IF
bandpass (for each mode), because the BFO is not variable. For example
when in LSB or CW mode the BFO is at 9.000000 MHz. If you are using the
model 219 filter, which has a bandwidth of 250 Hz centered on 9.000750
MHz, you are going to hear audio tones confined to the range of 625 to
875 Hz (a 250 Hz range centered on 750 Hz). It doesn't matter what 6.3
MHz filter you have selected, nothing outside that range is going to get
through the 9 MHz IF filter. You cannot use the PBT control to shift
that range. You can only use the PBT to get the 6.3 MHz IF bandpass
coincident with that range.
When you select a narrower 6.3 MHz filter than the one selected in the 9
MHz IF, the PBT control will work as you would expect (from experience
with other rigs that have only a wide 1st IF filter, or use other
schemes which may include a variable BFO). When the 9 MHz IF filter
selected is about the same or narrower than the 6.3 MHz IF filter
selected, the PBT control will not "seem" to work as a PBT control.
None of these filters have infinite attenuation outside of their
specified bandwidths, so my simplified explanation is not exactly
correct. Of course you will be able to hear signals and audio tones
outside the range I have said, if they are strong enough.
Ken N6KB
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.
>
> 73 de K1ESE
> John
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>
>
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