[time-nuts] FTS1200, OSA8600, OSA8601 phase noise and ADEV measures

[John Miles]

> The benefit is that I can de-correlate the reference oscillator noise,
> and measure near or even below it.
> 
> I have just started doing this, so this is really my first sloppy
> measurements for you to see where I am heading.
> 
> I expect John to chime in and comments on all my mistakes.

Nope, you've pretty much got it. :)  The idea is to fool TimeLab into
thinking that the signal at the TimePod Ch0 and Ch2 inputs is the
'reference,' and the signal at the hardwired Ch1 and Ch3 splitter is the
'input.'  That is the reasoning behind swapping the channels in the
subtraction expression.

You then connect two uncorrelated oscillators of similar frequency to the
Ch0 and Ch2 SMA input jacks.  The noise contribution from each oscillator is
removed by the same cross correlation process that gets rid of the ADC
noise. 

Technically it's not necessary for the two oscillators to be at exactly the
same frequency, and it's also OK if they phase-wrap during the measurement.
In theory they could be as much as a couple kHz apart, but certain
calculations that the TimePod driver makes to cancel internal clock drift
may be confused if they are more than a couple of Hz apart.  I also have not
looked carefully for instrument spurs and artifacts that might show up in
that case, so I usually tell people to tune the two references as closely
together as possible.  

They do need to have the same amplitude, within a dB or two, for the same
reason (to avoid triggering some error-detection code.)

What is critical is that there is no crosstalk between the two oscillators.
If so, it will show up as a beatnote.  All of the warnings in the manual
about using double-shielded cables at the input and reference ports apply to
the use of uncorrelated references as well.  

In the case of your FTS measurement, there's a very clear beatnote or other
artifact at about 3 Hz.  You can see this in the frequency difference view
by zooming in (see attached), in the ADEV plot at t=0.33s, and in the phase
noise plot as the dominant spur, reported at 2.9 Hz.  

There is also a spur at half that frequency.  The AM plot says that the 1.4
Hz spur and 2.9 Hz spurs are about equal in magnitude, while the phase noise
plot says that the 2.9 Hz spur is about 20 dB stronger.  (That's why it
dominates the ADEV and frequency-difference views.)  

So you should look for any possible differences in frequency between either
your two reference sources *or* between the reference source(s) and the FTS
1200.  In the presence of crosstalk between RF cables or power supplies,
either of these scenarios could happen.  

Given perfect isolation between your DUT and reference, you should not see
any PN spurs or ADEV artifacts at their difference frequency.  If you do,
they should be so faint as to be barely noticeable.  (I can say that fairly
confidently because I spent a lot of time chasing beatnote problems during
the R&D process.  However, I have *not* spent much time looking at what
happens when you use 2 reference oscillators that are not very close to the
same frequency, so I can't guarantee what might happen then, spur-wise.)

Bottom line, the large low-frequency spurs in the FTS plot at 1.4 and 2.9 Hz
will be caused by one of these conditions:

1) A problem with the reference source(s)

2) A problem with the FTS oscillator itself

3) A normal characteristic of the FTS oscillator (maybe its spur specs
weren't very good to begin with?)

4) An RF crosstalk or leakage problem with the cables/adapters used

5) A power-supply regulation issue

6) Coupling between inadequately-bypassed power leads.  This is a big
problem with some OCXOs where they apparently forgot to use bypass
capacitors inside the can.  I usually solder a 0.1 uF ceramic chip cap right
at the point of entry, if in doubt.

7) Some as-yet-unexplored effect related to beatnotes in dual-reference
measurements.

It may be possible to rule out cases (2) and (3), and definitely case (7),
by temporarily switching back to the normal single-reference configuration.

-- john, KE5FX
www.miles.io


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