[TenTec] Correction

[Keith Hamilton]

Actually I am trying to reconcile the differences between the center frequency of the 500 Hz filter and the 250 Hz filter, both in the 6.3 mHz PB IF. The center frequencies should be the same -( 6.299250 I think) regardless of the PBT control setting (11  or 1 o'clock) or the 9 mHz filter in use (I was using the 2.4 hz filter in the 9mhz IF at the time.) Since the center frequencies are not the same, (I must make a frequency adjustment to obtain maximum signal strength) I wanted to know if I can adjust the center frequency of my 250 hz filter or is the filter just too old and drifted. I would also like to know if others have experienced this discrepancy between two filters installed in their radio. Is it common for the older Ten Tec filters to go off frequency?
Thanks!  Keith W8GX

On Jul 5, 2011, at 11:50 PM, Ken Brown wrote:

> Hi Keith,
> 
> There are three things which determine the audio frequency of the CW 
> note you hear when listening to a CW signal with an Omni VI. Those three 
> things are:
> 
> 1) The RF frequency of the signal you are listening to,
> 
> 2) The LO frequency that the Omni VI is generating,
> 
> 3) The BFO frequency that the Omni VI is generating.
> 
> The amplitude of the audio you hear is affected by many things. For this 
> discussion the most relevant thing is where the bandpass of the 9 MHz 
> filter is relative to where the LO has converted the incoming RF, AND 
> the setting of the PBT which moves the signals passing through the 9 MHz 
> IF around relative to the center of the 6.3 MHz filter in use.
> 
> When you have the PBT set for maximum "band noise" amplitude passing 
> through both the 9 MHz IF filter and the 6.3 MHz IF filter, the the two 
> IF bandpasses are as coincident as you can get them. The "center 
> frequency" of this noise should be equal to the sidetone frequency that 
> the Omni VI generates when you key it, and if you zero beat an incoming 
> CW signal to your sidetone when you transmit your signal should be 
> pretty near (in your case, being a piano tuner, damned near exactly) 
> zero beat on the signal you have tuned in.
> 
> Assuming that the Omni VI LO system is working right there are two 
> things you need to do to get the frequencies to work out right.
> 
> First you need to use the menu function that sets the sidetone 
> frequency. This controls the offset of the LO system when you switch 
> from receive to transmit, as well as controlling the frequency of the 
> sidetone generated.
> 
> Second you need to have you BFO crystal oscillators operating at the 
> correct frequencies. The are adjustable with trimmer capacitors.
> 
> You can always get the 6.3 MHz and 9 MHz IFs coincident by adjusting the 
> PBT control, but when using a narrow 9 MHz filter you are stuck with the 
> tone that passes through it. The best bet is to set your sidetone to 
> match that.
> 
> You could misalign your BFO to get the tone you prefer, and then set the 
> sidetone to match it, but if you do your frequency readout would be 
> incorrect by the amount you have misaligned the BFO. The BFO changes 
> frequency also between transmit and receive, so you would probably have 
> to misalign both frequencies equally to keep the zero beat with the 
> sidetone exact, and this would result in incorrect frequency display for 
> transmit as well as receive. If you stay far enough from the band edges 
> it would not be a problem.
> 
> I hope this helps,
> 
> Ken N6KB
> 
> 
> Keith Hamilton wrote:
>> I made a typo in my question. I meant to say I must tune the radio from 7.040.00 to 7.040.20 to center the 250 hz filter.
>> Sorry
>> Keith W8GX
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