[time-nuts] Methods for comparing oscillators

Tom Van Baak tvb at LeapSecond.com
Mon Aug 3 00:41:12 UTC 2009


Hi John,

The hp 5370A or SR620 or other sub-ns resolution time interval
counters allow you to easily compare frequency standards at
mid- to long-term. In fact, if measuring the daily drift rate of Rb
or OCXO against your GPSDO is all you need then almost any
nanosecond counter will do the job. Collect data for minutes or
perhaps days -- and you will end up with some useful phase or
frequency deviation plots as well as frequency drift calculations.
It's quite simple.

But for short-term measurements, the main limitation that 5370
and equivalent counters have is their single shot resolution of
around 20 ps. This sounds impressive, but it clearly limits your
phase measurements to 2e-11 at tau 1 second, or 2e-12 at 10
seconds, or 2e-13 at 100 seconds, etc. This means you cannot
adequately measure devices with short-term stabilities better
than that using a 5370.

Since good OCXO have stabilities down in low -12's at tau 0.1 s
to 10 to 100 seconds, this means your counter is the limiting
factor when you compare your DUT against your REF. And by
"limiting" here I sort of mean "useless".

Now it depends on what your needs really are. If you are mostly
interested in frequency accuracy or drift rate over time, then I
think a 5370 is all you need. On the other hand, if you are
interested in short-term stability then you do need something
better -- at least 10x to 50x better.

Dual-mixer methods are the solution for this. Although there are
some high-end products out there, it seems to me a fairly basic
setup is all you need. I mean, you don't need 1e-13 or 1e-14
level of performance -- none of the oscillators you mentioned
are that good to begin with. Can it be done in less than $50?

Perhaps someone on the list can suggest the minimum effort
required to obtain, say, 1e-12 or 5e-13 level of dual-mixer
stability measurement?

I'm not talking about an extreme engineering solution that gets
down in the -14's or -15's -- just a simple, cheap, home-brew
solution that improves on a 5370 short-term resolution by a
modest factor of 10 or 100. I think that's all John needs.

/tvb


----- Original Message ----- 
From: "John Green" <wpxs472 at gmail.com>
To: <time-nuts at febo.com>
Sent: Thursday, July 30, 2009 9:01 AM
Subject: [time-nuts] Methods for comparing oscillators


> I've been hanging around and reading long enough to understand that when
> measuring the differences between oscillators the preferred methods are the
> HP 5370A Time difference counter or the dual mixer method. I want to
> evaluate some ocxo's and Rb sources against either a Tbolt or Z3801 and I
> don't have either method available. What I have used in the past is an HP
> Infinium scope with the reference fed to one channel which also provides
> sync and the DUT to the other. I have tested 2 ocxos that were so close that
> the two waveforms did not move by a detectable amount in a 30 minute period.
> I realize that this method will require very long observation times when
> looking at more stable sources. I am not looking to get absolute data, just
> comparative. Given what I have to work with, is there a better way? I use an
> Agilent 89441A Vector Signal Analyzer for signal quality measurements. I can
> see 60 Hz sidebands at least 60 or 70 db down and while I can't measure
> phase noise, I can  tell a clean oscillator from a dirty one. For instance,
> there is a world of difference between the signal generated by an HP8920 and
> a E4430B.





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