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

[Bob kb8tq]

Hi

The AR-60A is a pretty good “telecom” grade Rb. Depending on which one
you have it will have a 1 second ADEV in the 5x10^-12 (rare) to 1.5x10^-11 
range. 

Once you get inside the 1 second range on any Rb, the ADEV is going to be
dominated by the OCXO in the device. The telecom grade units are rarely 
targeted at super duper low phase noise. 

In any setup that compares A to B and gives you a result, the numbers will
be dominated by the performance of the worse of the two devices. VCO’s
rarely have good ADEV …..

Bob

> On Nov 28, 2021, at 1:19 PM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> 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)  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.
> Erik.
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