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No. It is not correct that interrupts are masked during transmission to get timing correct. A possible explanation for your more drifting CPU clock during more transmissions is that greater amounts of heat from the PA are reaching the 20MHz crystal and causing the frequency to change slightly.
Okay, I have the TCXO ready. I'll give it a try per Alan's suggestion. But I just realized that calibrating it might raise an interesting issue.
For a while, I was using the U3S to transmit alternately on 2 different bands. Then I resumed transmitting on just one band. I noticed that my timing drift was greater when making 2 transmissions in a 10-minute period than it was when I only made one. I guess this means that during a transmission, the CPU's internal clock doesn't increment the same amount as it does when not transmitting. I suppose the internal clock raises an interrupt from time to time (so to speak) and during a transmission the interrupts have to be masked so that the timing of the transmission is correct. Am I right? Has anyone else noticed this?
The consequence I anticipate is that once I install the TCXO to run the CPU and measure its frequency ( I think I can get it within 4 Hz), entering its exact frequency into the software might not work.