[TenTec] Correction: Omni VI Pass Band Tuning CW Narrow

[Ken Brown]

Ken Brown wrote:
> 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 
NARROW
> 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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