[time-nuts] Modulation on Rb sources
dforbes at dakotacom.net
Thu Dec 6 09:35:33 EST 2007
At 9:56 PM -0800 12/5/07, Jeff Mock wrote:
>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?
>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.
>Does anyone know what's going on with the first box?
The modulation is how the device maintains lock. Read the manual for
more info. Yours could have too high a modulation level.
I don't know what the close-in spur level is supposed to be, but I
could drag my FRS-N into the lab and measure it if you're interested.
Just not this month since there's too much real work going on.
--David Forbes, Tucson, AZ
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