[TenTec] Fixed BFO freq = Fixed Beat Note

[Ken Brown]

Hi Chuck,

>But doesn't the "ST" (sidetone) adjustment on the OMNI VI also adjust the BFO frequency?
>
No. There are only five adjustments to the BFO and they are trimmer 
capacitors for "alignment" and not controllable by front panel controls 
or by the microprocessor or control systems. The only electronic 
controls to the BFO select one of five possible frequencies for  CW TX, 
  CW/LSB RX /LSB TX,   USB TX/RX,  FSK MARK,   FSK SPACE.  Two crystals, 
five trimmer capacitors and a bunch of switching diodes and transistors 
are used to select from the five possible frequencies.

Changing the ST (sidetone) on the Omni VI has to accomplish two things. 
Number one (the most obvious) is that it changes the frequency of the 
sidetone generated when you key the transmitter. This must be done by 
changing some "divide by" value somewhere in the control board or in the 
microcode running there. Thing number two that must happen is that the 
amount of shift in the  5.0-5.5 MHz PLL between TX and RX has to be set 
to the appropriate value, so that when you zero beat in incoming signal 
with your sidetone, your transmitted signal will be zero beat with the 
incoming signal.

The BFO frequency used for CW TX is 9.000400 MHz. The BFO frequency used 
for CW RX (same as LSB RX and TX) is 9.000000 MHz. The only case where 
there is no shift of the 1st LO PLL from TX to RX is when you have ST 
frequency set to 400 Hz, or in USB or LSB (I don't know about FSK I 
haven't done the arithmetic), all of this assumes RIT and XIT OFF or set 
to exactly zero.  At any other ST frequency the PLL has to shift between 
TX and RX. For instance with the ST set at 700 Hz the 1st LO PLL is 
going to have to shift 300 Hz between TX and RX.

When the Omni is aligned properly, in CW mode the frequency display on 
transmit indicates where your CW "carrier" is and in receive it 
indicates the frequency of a received signal which is zero beat with 
respect to the CW sidetone, NOT the received frequency which is zero 
beat with the BFO. And if you are not operating split, or RIT or XIT 
they are the same frequency.

The ST frequency can only be adjusted from 400 Hz to 990 Hz, a range of 
590 Hz. The factory default of ST = 700 Hz puts the zero beat (with 
respect to sidetone frequency) near the center of either the 217 500 Hz 
or 219 250 Hz filters. When using the 217 500 Hz BW 9 MHz IF filter, at 
the extreme ends of the ST range the received signal (that is zero beat 
with your sidetone) is right on the edge of the filter bandpass. You 
could not use this full range with a 250 Hz BW filter, and that is no 
doubt why Ten-Tec used to have the both 219 and the 221 filters, which 
are both 250 Hz BW but with different center frequencies.

By the way if you start doing the math to figure this out, you will 
notice that the BFO is on the low side for LSB and on the high side for 
USB. This is because the relative frequencies get inverted in the first 
mixer using high side local oscillator injection. USB on the "sky 
frequency" (to borrow from radioastronomy terminology) becomes LSB in 
the 9 MHz IF.

>Your posting reminds me of another question that has nagged at me. Before I acquired my OMNI VI, I used a Kenwood TS-850S (which also had an adjustable sidetone/offset).  However, I don't recall having to worry about the offset frequency of the IF filters-- even the narrowest of filters (270 Hz, 250 Hz, and 125 Hz).  I'm not sure what design characteristic of the OMNI series radios requires us to purchase a first IF filter with a fixed offset frequency.  Does this have to do with the OMNI being double-conversion versus triple-conversion?
>
It is really because the BFO is fixed, not variable. Since the ability 
to shift the 1st LO PLL is there, the only thing missing to give the 
Omni VI the capability to have adjustable ST AND keep the zero beat 
(with respect to the sidetone that is) received signal in the middle of 
the 9MHz filter bandpass, would be to have an adjustable BFO and some 
firmware to make to 1st LO and BFO track each other as the ST frequency 
is adjusted. It does not seem like too great a stretch of the 
imagination to make the BFO operate as a VCXO, the same way the 15.3 MHz 
oscillator does in the PBT part of the Omniv VI. The difficulty would be 
to get the offsets of the 1st LO and the BFO to track exactly, or at 
least very closely. A DAC driving the BFO VCXO and a conversion curve 
implemented in the firmware seems plausible. Collins used to do it with 
a clever mechanical linkage of the PTO and BFO.

When all the oscillators are digitally controlled this becomes easy. And 
then you can reduce the number of different kinds of filters you have to 
stock. Also the tolerances of the center frequencies of filters are not 
as critical, you can just program the rig to work with whatever filter 
you've got, within reason.

By the way, the number of conversions the receiver is really depends how 
you count them. If you count the down conversion from the 9 MHz IF to 
the 6.3 MHz IF and the conversion back from 6.3 MHz to 9 MHz as 
conversions, then it is at least a triple conversion receiver. And the 
down conversion from 9 MHz to audio in the product detector could also 
be counted to make it a quadruple conversion receiver. Historically from 
the days of AM detectors that last stage is not counted as a 
"conversion", even though using a product detector it really is a 
heterodyne conversion.

DE N6KB