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Re: BITX40 PTT Noise

 

Re. the millisecond carrier burst...

Many who are new to BITX and other similar designs will discover the presence of a carrier?
burst when transitioning from receive to transmit, and an audible click when transitioning?
from transmit to receive.? This was subject to much scrutiny and analysis in the early?
days of BITX20 and its many variants. ?

It seems that the culprit is the coupling capacitor between microphone amplifier and the?
balanced modulator, and the coupling capacitor between receive audio amplifier and the?
balanced modulator.? In either transmit or receive mode these capacitors get charged?
to a particular value.? When a transition is made from transmit to receive or from receive?
to transmit this charge causes a few milliseconds of bias upset in the balanced modulator. ?

Several design changes have been proposed to "fix" this situation but most seem to add?
more problems than they fixed.? This situation is a trade-off for simplicity and low cost?
versus complexity and high cost. ?

Arv K7HKL
_._


On Fri, Nov 18, 2016 at 7:15 AM, simon.jack68@... [BITX20] <BITX20@...> wrote:
?

Looked into this a bit more.?Scope is telling me that there is a?carrier (when trimmed to a minimum) of?1V peak to peak. This is after it settles down from the initial transient spike?which appears to be 60V peak to peak.? All into a dummy load.??Audio from a nearby receiver suggests?that it is the carrier tone that is the spike signal.

Tried unplugging the speaker, the mic, the volume control, no difference.
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