|[AMRadio] Zero beating|
John E. Coleman (ARS WA5BXO)
wa5bxo2006 at pctechref.com
Mon Oct 16 13:34:43 EDT 2006
For what ever the reason is, "Zero Beating" has always been problem.
Certain people would complain about it back in the 60s. Or I should say the
lack of zero beating has been a problem. Sometimes guys would be so far off
frequency that they couldn't be heard.
I have had a couple of rigs in the far past where the VFO zero beat
in RCV mode was a KC or more removed from the XMT frequency. The trouble
with one of the VFO's keying circuit was that it was keyed via the
oscillator's cathode circuit. (Heath Kit I think) Anyway the spot switch on
the front of the VFO had less resistance than the circuit that keyed it from
the XMTR. The quick cure was to switch the VFO spot on before going to XMT
The next one that gave me trouble was a pulling VFO. That is RF was
getting back into the VFO and pulling the frequency. XMTR tuning would
change the VFO frequency while on the air. Amplitude Modulation of the final
would cause the VFO to FM. The cure for this was to make the VFO double in
frequency. That is to set the VFO to a 160 MTR frequency and double some
where in a buffer stage to have a XMTR freq on 80 MTR. This is a good idea
in any home brew XMTR project.
Last trouble that you see the most of is power supply regulation.
Both filament power and plate power to the VFO must be regulated stable.
Too many times with store bought equipment, these things are
overlooked and ignored. And I think there in lies 50% of the "ZERO BEAT"
trouble. The rest of the time, when guys are off frequency, it is just
plain carelessness and occasionally they are rock bound and not using a VFO.
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