Re: [TenTec] Tranceiving accurately

[Ken Brown <ken.d.brown@verizon.net>]

Hi Bob,

The reasons for transmit frequency to be different from receive 
frequency are as numerous as are transceiver designs. In many 
tranceivers the BFO is shifted in transmit mode and the balanced 
modulator is unbalanced so that the carrier (BFO) comes through instead 
of being nulled out. The BFO needs to be shifted in transmit so that you 
do not have to tune the other guy's signal to zero beat while receiving 
(zero Hz audio, a difficult note to copy, most people prefer something 
like 400 Hz to 1 kHz) in order to have your transmit signal zero beat 
with his. There are various methods of shifting the BFO. Sometimes two 
separate BFO crystal are used and sometimes a single crystal is used, 
and a padding capacitor is switched in for the offset. In all cases the 
relative frequencies of the two BFO frequency modes must be correct, in 
order for your transmit frequency to be the same as the frequency you 
are receiving on. Even if the radio is adjusted right, you may have a 
tendancy to listen with the audio note "wrong" for the way the radio is 
set up. If your CW sidetone is not the correct frequency (which should 
be the same as the difference between the two BFO frequencies) you don't 
have a good reference for tuning the audio note of the received signal.

If you are talking about SSB, the BFO should not shift between TX and 
RX. If some of the switching devices are "leaky" the BFO TX offset logic 
signal which should be switched off in SSB could be slightly shifting 
the BFO when it should not at all.

If it is a rig that uses PLLs for all of the BFOs, then it is a whole 
different ballgame. Both the VFO and BFO can be shifted to move the BFO 
carrier relative to the IF filter bandpass. If they don't both get 
shifted the same amount (between RX and TX) your RX and TX zero beat 
carrier frequencies will not be the same. The logic in the radio is 
supposed to get this right.

73 DE N6KB

Robert & Linda McGraw K4TAX wrote:

> Does anyone have a good method to confirm that a transceiver is indeed
> transceiving accurately?  I've come to a conclusion that one of mine is
> actually shifting frequency some 20 to 30 Hz between receive and transmit.
> As to why, I haven't found a clue.
>
> I'd like to hear your thoughts and suggestions on this topic.
>
> 73
> Bob, K4TAX
>
>
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>  



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