[TenTec] OMNI V Mixing Scheme

[James R. Binkley]

This is my first post since joining this group, so please bear with
me.

I have been reading my OMNI V manual in an effort to become more
familiar with its internal workings.  I don't understand how the
receiver mixer scheme works on CW, if my understanding of what the
manual is saying is correct.  I will use a numerical example to
simplify my discussion.

Assume that I wish to receive a signal which is at exactly 7.0 mHz.  

As I read the manual, it says that in this case, the PLL board will be
putting out exactly 5.5 MHz, and the LO Mixer board will have selected
the 21.5 MHz crystal.  The output of the LO mixer board will therefore
be 21.5-5.5, or exactly 16 MHz.  So far so good.  

This 16 MHz LO signal mixes with the incoming 7.0 MHz desired signal
in the first mixer, Q3-6 in the Front end Board.  This produces 16-7,
or exactly 9.0 MHz.  

Now for the problem, this 9.0 mHz signal now must pass through the 9
MHz 2.4 kHz filter on the 9 MHz filter board.  The problem is that the
lower cut-off frequency of this filter is given as 9.0003 Hz. 
Therefore the 9.0 MHz signal is 300 Hz out of this filters passband. 
I don't know how much attenuation the filter has at this point, but it
must be substantial.  In addition this would make the response of the
receiver on each side of the desired signal very unsymmetrical.  This
seems undesirable to me.

It is possible that my assumption that the PLL is putting out exactly
5.5 MHz is incorrect, the manual does not explicitly say so.  The
microprocessor could set the PLL a little low in frequency, which
would move the 9.0 MHz desired signal up by the same amount, in this
way the 9.0 MHz signal could be moved up into the pass band of the 2.4
kHz filter.  If the microprocessor did this when in the CW mode, and
displayed the uncorrected frequency, then I suppose everything would
be OK.  

In addition I note that this 9.0 or 9.0+  MHz desired signal is
finally mixed down in the product detector, or final mixer in the
IF/AF board  with the  BFO signal.  TEN TECH never says what the
frequency of this BFO signal is on receive, the manual says that it is
9.0006 MHz on CW transmit, but is silent on the subject of what
frequency it is set to on receive.  It could be 9.0 exactly as it is
for LSB and the amount by which the PLL is set low would then
determine the CW beat note frequency, I don't know, TEN TECH doesn't
give enough information.

I would appreciate any comments from those of you out there who have a
more detailed insight into the mixing scheme than that afforded by the
manual.

Thanks Bob w4aos 


p.s.  On CW transmit, if the BFO is 600 Hz high (9.0006) then it would
seem that the PLL signal would have to be offset as well, in order for
the final transmit signal frequency to not be 600 Hz off.  I assume
that if the PLL frequency is moved slightly off for receive, then it
could also be moved off on transmit to make everything come out
correctly.

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