[Elecraft] Choice of signal source for KX3 extended temperature calibration?

[Frank O'Donnell]

Which of the following two signal sources would enable the best results 
from the KX3's extended temperature calibration?

--Elecraft's XG50 board, which puts out 49.380 MHz ± 5 ppm.

-- Siglent SDG 2042X waveform generator, with input from a Trimble 
Thunderbolt GPSDO. Maximum frequency on the Siglent is 40 MHz.

The calibration instructions call for using a signal source close to 50 
MHz. Is 40 MHz close enough? The XG50 is much closer to 50 MHz, whereas 
the Siglent/Trimble potentially has the capability of being considerably 
more accurate. I'm interested in squeezing as much frequency stability 
out of the KX3 as possible.

Thanks and 73,

Frank O'Donnell K6FOD