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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