[time-nuts] Modulation on Rb sources

[Jeff Mock]

I have a couple of Rb sources and went looking for spurs close to the 
carrier.  One was particularly surprising.  First is a GPS disciplined 
Rb oscillator from Berkeley Varitronics.  I'm not a big fan of this box, 
but that's another story.

Here's plot of 500Hz around 10MHz, it looks like the 10MHz signal is AM 
modulated with a 128Hz signal, the spurs are about -75dBc.  I did the 
plot twice, once during normal operation, and once with the GPS control 
loop disabled from the front panel.  The plots were identical.  So, my 
question is whether this modulation is part of the Efratom module or 
introduced by the Berkeley Varitronics box?

    http://www.mock.com/test/rhinoii/am-128hz.png

Here's a similar plot of an LPRO-101.  This is just the module with a 
semi-crappy power supply.  There's a little 60Hz (+harmonics) that I 
figure is power supply related leaking through the mixer.  The 10MHz 
signal is +8dBm, so the 60Hz spurs are at least 98db below the carrier, 
much better than the previous graph and more like what I expected.

    http://www.mock.com/test/lpro-101/am60hz.png

Does anyone know what's going on with the first box?

jeff