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

[Magnus Danielson]

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