[time-nuts] Methods for comparing oscillators

[Bruce Griffiths]

Tom

Without elaborate temperature control of the mixer and ZCD it isn't
possible to maintain picosecond stability (at the mixer input).
The greatest problems are crosstalk and isolation between the 3
oscillators.
Designing a ZCD to achieve 1E-12/tau or slightly better resolution is
relatively simple especially if one uses the Collin's technique for
which spreadsheets have been created to simplify the process.
Choosing a low dc offset and offset drift mixer is critical.
Something like an Minicircuits MPD-1 is a good start as the dc offset is
low and specified as is the isolation between RF1 and RF2  (> 70dB for
frequencies > 15MHz).
It also allows low frequency isolation of IF and RF grounds to reduce
low frequency crosstalk.
If one were to use simple isolation amplifiers with 40 dB or reverse
isolation together with a splitter with output to output isolation of
30dB to drive a pair of  offset frequency isolation amps that then:

1) DUT to offset source isolation of 110dB is achievable.

2) DUT to DUT isolation of ~ 140dB is achievable.

Which should be adequate for these purposes.

If one used a 1Hz offset frequency then a ZCD output jitter of 10us or
less would be required.
This is easily achieved using a 3 stage limiter with a slope gain of 10
or more.
With a few inexpensive modifications (optimised mixer matching and
optimal IF port termination ) considerably better performance is
possible (limited primarily by drift.


Bruce

Tom Van Baak wrote:
> 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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