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


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.

The same issue could apply to C64 when transitioning from TX to RX but at least there will not be spurious emissions going out over the air. I will add a diode in series with R66 in the same manner just for fun!

My apologies for suspecting that it could possibly a firmware issue!!!

Next I will address the CW keying shape which seems to have about 0.5 ms rise and fall times, far too short!

Bob N1KW

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