[time-nuts] Low-Cost Rubidium Performance

Bob Camp lists at rtty.us
Thu Feb 9 18:03:28 UTC 2012


There are at least two possible sources of the spurs in the FE. One is the
DDS, the other is cross talk in the CPLD divider. In one sense it really
does not matter which it is from, they are indeed there on the output. In
another sense it does. If they are from the CPLD, you *could* hack into the
unit and buffer up the VCXO at 60 MHz to use it as the output. Not as cute
for a standard for your counter, but fine if you want to head off to


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of John Ackermann N8UR
Sent: Wednesday, February 08, 2012 9:36 PM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] Low-Cost Rubidium Performance

As threatened, I've measured stability (out to a trustworthy 10K 
seconds) and phase noise of the three popular telecom surplus Rb 
standards.  I looked at two units of the FE-5680, two units of the 
Efratom FRS, and one Datum LPRO.  (I have two more LPROs but don't have 
the mating connector on hand, and didn't want to solder on all three.)

The Cliff Notes version:

* The units had similar short term stability from 1 to 100 seconds.

* Above 100 seconds, the LPRO was the clear winner.  The FE and FRS 
split, with one of each being the worst two.

* The FE-5680 phase noise loses big-time, both in noise floor and in the 
quantity and amplitude of spurs.  That DDS comes with a price.

* The LPRO has the best phase noise floor and cleanest spectrum.

Pretty plots and the underlying data are at

Important note: there's known to be a lot of variability among these 
units, so my sample size of one or two each isn't a guarantee that your 
unit will perform similarly.

Next step is to do some longer-term stability measurements (out to 100K 
seconds).  You may see that data in a month or so.


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