I see that the SI5351a synth manual now specifically mentions FOX924B TCXO as having a problem with discrete frequency jumps. So I tested the frequency stability of that TCXO on my very fancy spectrum analyzer (an old $20 RTL-SDR dongle connected to free gqrx software), and it's really bad:
(each FFT bucket above is about 1 hertz)
Basically the crystal drifts quite a bit with temperature, not any better than the non-TCXO version supplied with the kit as far as I can tell, and then when some threshold is reached will jump by 2-3Hz instantly, causing a frequency shift of 9Hz at 100MHz! This likely confuses the wsjtx decoder if the jump occurs during a transmission, especially at higher frequencies.
My advice is not to use FOX924B at all. The non-TCXO crystal's slow and predictable temperature drift can be handled by software. There is also??but the link no longer works.
