[time-nuts] Re: Using a nanoVNA as DMTD, simulation only

[Erik Kaashoek]

Bob,
I extended the simulation to fully include both channels.
Using some careful math it was possible to use two different IF frequencies
for the downconversion to DC. This implies that any leakage after the two
mixers can be reduced by using different IF frequencies because he phase
calculations for each measurement sample acts as a one bin FFT effectively
filtering out the leakage from the other mixer output
The errors due to the 16 bit quantization and mismatch between IF frequency
and downconverted input signals can mostly be removed by choosing IF
frequencies that are very close to the frequencies of the downconverted
input signals, which, due to the new calculation method, is now possible.
It's like creating a PLL where you tune the IF for best accuracy, e.g.
identical to the input frequency after downmixing
The only leakage that is still relevant is the leakage from one channel
into the other channel before the mixers and, probably, leakage between the
two LO frequencies (to be investigated)
I've also tested the impact of having harmonics in the mixer output and as
these are all blocked by the one bucket FFT there is no visible impact.
It's weird that a frequency measuring device becomes more like a dual
heterodyne receiver with the final IF at zero Hz acting like an
analog/digital PLL where you can measure the phase errors very accurately

ADEV with 0.1 Hz input frequency difference after the mizers, -80dB leakage
and with equal IF frequencies
[image: image.png]

Same with different IF frequencies
[image: image.png]
Whether this method will enable to measure ADEV in any useful way (e.g. are
the ADEV pictures above a good representation of the actual performance)
is still to be investigated.

Erik.



Op wo 21 sep. 2022 om 00:20 schreef Bob kb8tq <kb8tq at n1k.org>:

> Hi
>
> With signals in the < 0.1 ppb offset range, you should see effects at the
> -80 db isolation level. They should show up as ripples in what otherwise
> should be a straight line ( ADEV drops vs tau in a straight line ….. ).
>
> Bob
>
>
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