I've seen this and its always the capacitor!
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At 15/06/2020, you wrote:
Hans and the group,
This is going to be fun. Today, I was fooling around with the Antuino and suddenly, it wouldn't read the return loss. I checked the driving signal. It was on, but off by about 200 khz. Naturally, the measurement receiver wouldnt pick it up.
I added serial.print into thr source code. Everything seemed to be perfect. Finally on a dare, I probed the crystal, it was at 27.2 mhz! I resoldered, etc. After a few minutes, it jumped back to its standard 27 mhz frequency.
I have never seen anything like this on the si5351. Spurious crystal modes are not uncommon. Anyone else has experienced this too?