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

[Andy Talbot]

I forgot the URL
http://g4jnt.com/10MHzDist.pdf

Andy
www.g4jnt.com



On Sun, 28 Nov 2021 at 18:39, Andy Talbot <andy.g4jnt at gmail.com> wrote:

> The way I look at short term stability is to multiply up to microwave
> frequencies then mix the two and look at the resulting beat note.   The
> popular ADF4351 Fract-N synthesizer is ideal for this.  Take two of them,
> and programme for two frequencies in the GHz region a 1kHz or so apart when
> driven by the two 10MHz freqs that are being compared    Apply the two
> multiplied signals to a mixer, and look at the resulting IF product with a
> PC soundcard input.   Using spectral analysis software with a waterfall
> display, such as Spectran or Spectrum Lab you can look at the multiplied
> frequency instability in real time.
>
> An example of the techniques used on a range of 10MHz reference sources
> can be found here (I used a different Fract-N synth with a smaller setting
> grid possible than the ADF4351 can give, but the same idea applies)
>
>
> Andy
> www.g4jnt.com
>
>
>
> On Sun, 28 Nov 2021 at 18:19, 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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>