Re: Spurious RF at beginning of CW transmission in the uBitx

Arv Evans

On Mon, Jul 30, 2018 at 01:39 PM, <n1kw@...> wrote:
Bob N1KW

Every since the first BITX we have griped and complained about the 20 ms infamous "BITX Click"
that occurs in changing over from receive to transmit and from transmit to receive.  Earlier tests
indicated that it was somehow associated with hold-over voltage on capacitors between audio circuits
and the balanced modulator.  Is it possible that you have now found, and fixed, that long-standing

It appears that I have solved the problem! While analyzing the circuit further, I realized that the
large capacitor C52, which is charged during receive, would feed back through R52 and R18
keeping the receive path (Q10, 11, and 12) after the balanced modulator "hot" for a brief period.
When the transmit path is activated, the receive side of the circuit is going to remain on for some
period of time due to the time constants of C52 and its loads. It is understandable that if both
directions of the circuit are on, it could oscillate during that time!

To resolve the issue I simply added a diode in series with R52 (cathode toward C52) so that C52
can no longer back feed power to Q10, 11, and 12 upon initiation of transmit state. Now the transmitter
output looks perfectly clean on the spectrum analyzer at beginning if TX. Shorting the diode causes
the problem to show as before.

If this fixes that long-standing problem, then we all owe you a big THANK YOU


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