[time-nuts] Re: Seeking advice: Is this the right way to check very short term (below 1s) stability?

[Magnus Danielson]

Erik,

Den 2021-11-28 kl. 19:19, skrev Erik Kaashoek:
> As the collection of frequency sources and counters in my home lab is 
> growing I'd like to understand the performance of the frequency sources.
> Two different GPSDO do help to check long term stability.
> But the Rubidium frequency standard I have (Accubeat AR60A) is fairly 
> unknown and seemingly not of good reputation, more specifically its 
> (very) short term stability is doubted.
> So how best to check very short term  (below 1s) frequency stability. 
> The frequency counters available loose resolution quickly when the 
> gate time is reduced below 1 second and high performance phase noise 
> measurement equipment is not available so google helped with a search 
> for alternative measurement methods.
> What I found was a method using two frequency sources, one of the two 
> being  a VCO, a mixer and some filters and amplifiers.
> By weak locking (large time constant)Â  the VCO source using the mixer 
> as phase detector to the other source, the output of the mixer's IF 
> port should carry a voltage real time proportionally to the phase 
> difference and by filtering and amplifying it should be possible to 
> check for variations in the 1ms-1s range.
> Maybe even a scope can see the variations.
> When you know the amplification and the full range voltage you can 
> even do an absolute measurement.
> Would this method work?
> Any specific concerns to take note of when doing the measurement?
> Removing the DC component (or locking the VCO such that there is no DC 
> component) will be crucial I guess but given the slow speed of the 
> loop even an ADC->computer->DAC->VCO setup can work.
> Any suggestion is welcome.

So, in that region one typically transition into measuring phase noise, 
as for shorter taus your performance will be dominated by the wideband 
white noise, and counters isn't the best tool to analyze that.

The weak locking technique can be used to a limit, but to get good 
results, you need to calibrate it. I suggest you set up the loop in the 
analog domain and only digitize the residual noise. Then inject using a 
synthesizer side-tones to your carrier and know relationship in 
amplitude and frequency offsets, for which then the phase-noise should 
be known, and use that to build a calibration scale. This is described 
in the NIST T&F catalog. You can do that with a varity of sources, but 
eventually you will be limited by the noise of the other oscillator.

I use a cross-correlator setup in the form of TimePod most of the times, 
with quiet references.

Cheers,
Magnus