All,

Here’s a plot of frequency drift in the 70 cm Receiver – first on internal reference, then on external reference:

The first part of the plot is with the receiver and the signal generator set to 435.500000 MHz on internal reference.  You can see the slow steady drift upwards of about 15 Hz over a period of about 5 minutes.  Then I reinitialized the receiver for the external reference.  It locked right up and the output was exactly 10.700000 MHz where it stayed.

The most important thing I noticed is not on this graph…  With internal reference selected, but while feeding 10 MHz into the external reference input, (as it might be configured in flight) the two reference signals are beating together.  It wasn’t until I started listening to the output of the receiver by patching it into a distribution amplifier so I can feed my counter, the SDR-IQ, the TS-2000, and the HP 8566B all at once that I noticed it.  The result is that the input signal is FM modulated by the beat between the two reference oscillators.  I also noticed some unexplained jumps in frequency that I have not yet been able to capture.  I’ll dig into this again when I have more time.

73, Juan