SSA3000X measures a tone, 10MHz external reference clock

[Performa01]

You should use a span of at least 250Hz (better 500Hz) for all marrowband measurements.

In your very first picture in the opening post, we can see that the 100Hz sidebands are dominant (probably from fullwave rectified mains), whereas the 50Hz spurs are almost negligible. Observing the 2nd and 3rd harmonics of the main frequency would be much more revealing.

[dpenev]

Hi Performa01,

Indeed you are right.

Note that my main goal was to see if using external reference will reduce the SSA phase noise.
I have got the Anritsu generator recently and it is with OCXO so it is suppose to have best clock quality in our lab.
After some measurements I came to a conclusion that external reference reduces the phase noise (at least for the case generator provides both reference and the measured signal)
in the аrеa 50hz around the tone. That's why I am using so short span.

For 50/100/etc Hz modulation your are perfectly right that we better refer to the stronger 100Hz spur.

[Performa01]

Yes, for lowest close-in phase noise an OCXO reference signal is mandatory - and of course this can only be verified by looking at a low phase noise input signal, so the only sensible setup is both the reference and the signal coming from a high quality synthesizer like the MG3700A.

I just thought this was already confirmed and now the only remaining question was where the mains harmonic modulation comes from - and how to minimize it.

One of the suggestions has been to observe the reference signal with the spectrum analyzer in order to see if the 100Hz sidebands can be seen - I think this was good avice and certainly the first thing I'd try myself in such a situation. But to do this, quite obviously a wider span than 100Hz is required to get a clear picture of the situation.

If it turns out that that interference actually exists at the MG3700A reference output, I'd check the PSU inside the MG3700A, replace old electrolytics and maybe add some additional supply rail filtering for the OCXO and the associated reference output buffer amplifier.

Best of luck and please keep us updated!

[edpalmer42]

Since the 10 MHz output from the MG3700A is labelled on the unit as being TTL, I'm skeptical of it's suitability for a low phase noise source.  When the TTL is at it's logical high level, it will leak all the trash on the +5V rail into the output.  Dpenev has stated that the signal looks mostly like a sine wave so they've obviously filtered the high frequencies, but it looks like there's some remaining low frequency trash.

Ed

[dpenev]

Hi performa01,

Observations so far:
-no 50hz and multiple in the Mg3700a 10MHz reference signal visible on my ssa3000x
-50hz and multiple with strong 100hz component visible if 1.1ghz(for example) sine wave from Mg3700a measured with ssa3000x using ext ref from Mg3700a. The 50hz and multiple not visible if internal Clock used with ssa3000x.
-funny enough 50hz and multiple are not visible on ssa3000x if Mg3700a generates sine with frequency below 300MHz and also provides Clock to ssa3000x.
-I don't know why the Mg3700a don't use pure sine wave for its 10MHz reference but I think that more square a like would have stepper edges and would be easier to lock to with less jitter. On a scope it looks as sine wave. Later i will provide measurement in bigger span so harmonics wisible.


Sent from my MI NOTE Pro using Tapatalk

[dpenev]

This is how the MG3700A 10MHz reference clock looks like

 

[rf-loop]

What ever happen using MG3700A but Siglent SSA3000X is extremely critical about external reference signal. Of course it need be stable and accurate enough but also it need be clean, as clean as SSA need reference clock.  It also is very sensitive for ExtRef amplitude modulation. Just: All need be clean.
Some other machine may work so that it only follow ext ref carrier. 

It looks somehow like that Siglent perhaps totally swap whole reference, internal or just external. And if this is case, dirty reference makes all results dirty. Just like garbage in - garbage out.

Here is three images one is not necessary but for better visibility.




Image 1.
100MHz CW from 30 years old HP Rf generator. In this image Siglent run with its own internal reference. (same as in image 3 trace D but other shot)




Image 2.
Signal used as External Reference for SSA in Image 3.
10MHz carrier AM modulated with 35Hz and modualtion depth 1% (35Hz sidebands -46dBc).   
Also this same signal is used with 10% modulation depth (35Hz sidebands -26dBc)




Image 3.
Trace D: as also in image 1. (SSA using internal reference)
Traces A and B:  SSA use ExtRef as image 2.

Also after done other many tests and my conclusion (until other real evidences)  is that do not use SSA ExtRef until you have  high quality very clean reference signal for this purpose.

[dpenev]

Hi rf-loop,

I don't understand your point.
You synchronize your SSA300X with 'dirty' 10 Mhz and obtain 'dirty' SSA3000X measurement.
In my opinion this is absolutely logical. Is it not the purpose of the external synchronization
to replace the whole internal clock system of the instrument based on the externally provided one?

Note that my original confusion is that my 10MHz reference appear clean but the SSA300X measurements
are dirty for signals above 300MHz.   

[TurboTom]

Did you try what I suggested here?

Since the REF output of your generator appears to be rather high level, maybe the load of the SSA's REF input (designed for a lower level) pulls it down so much that and internal power rail inside the generator gets unstable. Even more so since apparently at higher generator output frequencies, the intermodulation suppression of the power stage gets worse. It's a quick test to put a 50 Ohms terminator on the REF output and test again with the SSA running on it's internal reference.

It's just a thought that could very well be wrong but all your description of the problem maly also point into that direction...

[rf-loop]

Hi rf-loop,

I don't understand your point.
You synchronize your SSA300X with 'dirty' 10 Mhz and obtain 'dirty' SSA3000X measurement.
In my opinion this is absolutely logical. Is it not the purpose of the external synchronization
to replace the whole internal clock system of the instrument based on the externally provided one?   

This is perhaps bit sidepathway for your case. I do not know. I have not qualified your reference quality at all.

Do you think all instruments just swap whole internal reference with external reference if external is used directly without caring signal quality (example take only ref signal 10MHz carrier to adjust and lock internal freq to its freg).  I have "ton of" very accurate reference freq sources but not all are very pure if look AM sidetones or other impurities and I can use these with many equipments for frequency reference and no need care if there is some 50Hz or 100Hz mod or "ground loop hum".  But reference what run SA internals need be quite clean. Also, as can see my previous other test. I have used 10MHz quite clean reference oscillator and then injected just 50Hz via splitter (perhaps more high level than usually when just some ground loop). So it proof that it is also bit sensitive for  50Hz "hum" and not know where it produce 10M carrier modulation.


Example if look inside HP 859xE  SA.  External reference goes to External Reference PLL circuit. (if this HP SA have internal high accuracy reference or standard reference it is connected to external ref BNC using small coaxial bridge.)

This kind of system what HP have used is not so sensitive for external reference 1% or even 0.1% AM modulation, perhaps not even 10%. Or combined quite low level 50Hz "hum" + 10MHz strong carrier as reference. No it is not sensitive at all (this 50Hz is stopped by HPF. User can use dirty or clean reference as long as its frequency is stable (low phase noise etc).
But Siglent IS sensitive and this must be taken into account.
This is why I say: Use only very clean external reference for Siglent SSA. Period.

[SoundTech-LG]

this may/may not apply, but worth a read anyway...

http://literature.cdn.keysight.com/litweb/pdf/5990-9189EN.pdf

[dpenev]

Hi TurboTom,
Terminating the MG3700A ref output with 50 Ohm does not change the generated signal in terms of the 50Hz/100Hz modulating tones around the main tone.

Hi rf-loop,
I think I understand  your point now.
Amplitude modulation of the 10MHz reference signal should not affect the quality of the SSA3000X measurements.
As SSA3000X should contain PLL which should lock to the external reference, its phase(t) property but not to its amplitude(t) property.
Based on your measurements it is not the case and SSA3000X is sensitive to amplitude disturbance from the 10Mhz ref input. 
Good point!
It is interesting in my case that I measure the MG3700A 10MHz ref signal as a clean in the neighborhood or 10MHz