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

[Andy Talbot]

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 teh 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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