Hi John,
On 08/03/2012 05:18 AM, John Miles wrote:
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. :)
Great! :)
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.
I forgot to mention that, as I knew it was the point about that move.
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.
Indeed.
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.
OK. Good to know.
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.
OK. Good to know.
It would help if the frequency between the reference oscillators could
be monitored one way or another.
They do need to have the same amplitude, within a dB or two, for the same
reason (to avoid triggering some error-detection code.)
Good to know. Need to look at that then.
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.
I have good cables, but can improve separation, as several of the OCXOs
hang of the same +24V source and is physically close to each other.
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.)
The FTS1200 is about 1.5 Hz below the reference, so maybe that reflects
around the reference? I will fiddle with that in a moment.
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.
Cross-talk on the power-supply is highly suspected in this case. I'll
toss in a spare power-supply for separation.
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.)
I'll have a look at how they are tuned and see if I can improve on the
situation. Meanwhile the oscillators remains heated. :)
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?)
It's interesting to note that these spurs is unique to the FTS1200
measurement. Maybe it need some service. Schematic anyone?
4) An RF crosstalk or leakage problem with the cables/adapters used
5) A power-supply regulation issue
Would not explain why only the FTS1200 experience this, while it shares
supply with other oscillator not having this property.
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.
Doable.
7) Some as-yet-unexplored effect related to beatnotes in dual-reference
measurements.
Will keep looking at it.
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.
True. The signal is so strong that it should be clearly visible anyway.
Cheers,
Magnus
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