[time-nuts] Re: Testing GPSDOs

[Andy Talbot]

Yes, 1Hz observed is 1Hz on the multiplied up signal, which at 5GHz is
2E-10 on the reference causing it

The FFT process does indeed smear out faster transitions, so you need to
flip between different  FFT sizes to see fully what is happening.

If you use a prog like SPECTRAN to view the audio beat note, it's quite
feasible to have several instances of that software open at the same time,
all fed from the same soundcard source.   Set each instance to a different
FFT resolution, one wide, one narrow and others as you like, and watch what
happens


Andy
www.g4jnt.com



On Mon, 21 Feb 2022 at 08:38, Erik Kaashoek <erik at kaashoek.com> wrote:

> Andy,
> Thanks for these references
> Using two 5GHz PLL's, one hard referenced to the in-house standard and
> the other to the TCXO under test and the outputs of the PLL's send into
> a mixer producing a 10kHz output send into a PC for FFT analysis made
> the performance of the TCXO very visible.
> Can I assume 1 Hz variation of the audio signal is 2e-10 variation on
> the 10MHz signal?
> My concern with the high resolution FFT is the length of the FFT, does
> this not hide fast frequency variations? Or will these variation become
> visible through blurring of the peak and as long as the peak is nice an
> sharp one can assume the signal is stable over the FFT length?
> Erik.
> On 20-2-2022 18:38, Andy Talbot wrote:
> > Dunno if this will do what you want ...
> >
> > Get hold of some PLL Fract-N synthesizers, like the ubiquitous ADF4351,
> so
> > you can multiply up from two sources. Mix the outputs and look at the
> > frequency deviation over time.
> > My tests using this technique can be seen at
> > http://g4jnt.com/10MHz_Reference_Source_Stability.pdf    and
> > http://g4jnt.com/ShortTermStabilityLeoBodnarGPSDO.pdf
> >
> > I used synths I had to hand, but anything that will multiply up your test
> > and reference signals to a pair of frequencies separated by a few kHz, so
> > you can look at the beat note using soundcard software.
> >
> >
> > Andy
> > www.g4jnt.com
> >
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