[time-nuts] Datum 9390 Rb phase noise

John Ackermann N8UR jra at febo.com
Sat Nov 17 21:19:04 EST 2007

John Miles said the following on 11/17/2007 09:06 PM:
> Well, I bought the other Datum 9390 that was for sale at the time Mike
> posted this message (eBay 260177272556), and it's certainly an impressive
> box of stuff for $150.  There is an Efratom Rb module, a couple of power
> supplies, and a bunch of cards with a wirewrapped(!) back plane.
> I hooked up an antenna and it's reporting '1 USABLE SATELLITE' so far, so I
> guess there's not much wrong with it if anything.  The last-known GPS
> location corresponds to "Cavium Networks HQ" in Google Earth, in Mountain
> View, CA.  The rubidium oscillator works and appears to be in calibration.
> The only fly in the ointment is the phase noise at the 10 MHz output:
> http://www.thegleam.com/ke5fx/compare.gif
> Is -120 dBc/Hz par for the course for Efratom FRS-C rubidium modules, or is
> there likely to be a problem with this one?

Yours is actually better than spec...  my Efratom FRS-C manual says the
spec is -70 at 1 Hz offset, -110 at 100 Hz, -130 at 1 kHz.  The notation
is a bit odd though -- it says "dBc/sqrt(Hz)".

My understanding is that the compact telecom Rbs put FM on the reference
as part of the lock mechanism; that makes the phase noise fairly ugly.


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