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

[Erik Kaashoek]

Bob,
Thanks for the hint.
After adding overlapping ADEV calculation and extending the simulation to a
100 seconds measurement period I did some simulations using -80 dB to -120
dB leakage of another signal at 10, 1, 0.1, and 0.01Hz difference and
varying the noise level up to -80 dBc/Hz.
Worst case is a delta frequency of the two inputs of 1 Hz and noise and
leakage at -80 dB but even under these conditions the ADEV at tau of 1
second stays below 1e-12.
Given the above leakage and noise conditions the minimum reliable
observable frequency difference is 1e-5 Hz which is very promising.
The nanoVNA (or its better cousin the LiteVNA) do have at least 80dB
isolation between the inputs so I'm tempted to implement this on the actual
HW for validation.
Given the HW, without modifications, it can only work for (almost) equal
frequencies but this should be sufficient for many relevan use cases.
One area of concern are the close-in spurs of the SI5351 when used at small
offsets from 10MHz. Too difficult to simulate.
Erik.

Op ma 19 sep. 2022 om 22:02 schreef Bob kb8tq <kb8tq at n1k.org>:

> Hi
>
> The “typical” gotcha doing this is channel to channel isolation.
> Folks have tried it with various devices and that seems to be
> the first barrier they run into. There may be others further down
> the road …..
>
> Often tossed up isolation numbers from various sources get into
> the > 120 db range for signals that are very close to the same
> frequency. If they are not close, then you start talking about how
> close this or that harmonic is.
>
> Simple test is the same one you now are very familiar with. Step
> one input across the other and see what happens ….
>
> Bob
>
>
> > On Sep 19, 2022, at 10:47 AM, Erik Kaashoek via time-nuts <
> time-nuts at lists.febo.com> wrote:
> >
> > After reading about DMTD and how the VNWA is doing frequency
> measurements I
> > was curious if it would be possible to use a nanoVNA to create a DMTD by
> > only changing the SW.
> > The nanoVNA has two input channels (S11 and S21) and a reference channel.
> > By disabling the output of the reference LO in SW the S11 and S21
> channels
> > become two independent inputs. One via the reflection bridge (S11) into a
> > mixer and one directly into another mixer. Both mixers also have the
> > offset_LO as input which should be tuned so both mixers output close to
> the
> > IF frequency.
> > The output of the mixers is converted using a 16bit stereo ADC running up
> > to 96kHz. The 16 bit samples streams are converted to phase and amplitude
> > by doing a SW I/Q downmix to DC.
> > The number of samples to combine into one phase/amplitude measurement is
> > defined in the SW.
> > As I did not want to put a lot of effort into creating embedded SW I
> > created a one input channel simulation in Octave of the processing after
> AD
> > conversion.
> > The simulation uses a 1kHz input signal with added noise and a 48kHz
> sample
> > rate and combines 1k samples into one angle measurement. All sample data,
> > I/Q data, cosine and sine tables are rounded to 16bits as used in the
> > nanoVNA. The 48 angle measurements per second limit the frequency
> > difference between the input signals and the tuned frequency because if
> the
> > frequency difference is too high the unwrapping of the angle will fail.
> > After unwrapping the 48 angle measurements per second a linear regression
> > uses the angle measurements to calculate the angular speed per second,
> > dividing this speed by 2*pi gives the frequency deviation of the input
> > signal from the reference signal.
> > It would also be possible to output the 48 angle measurements per second
> > (or any subsampled number) as raw phase difference measurements and do
> the
> > rest of the processing in something like Timelab
> > Using 48kHz sample rate and 16 bit accuracy of the data and an added
> noise
> > level of 1e-5 (is this -100dBc/Hz (?)) the minimum observable delta
> > frequency in the simulation is about 1e-6Hz. Any lower delta frequency
> > falls below the 16 bit numerical resolution. A higher noise level, such
> as
> > 1e-4,  hides the 1e-6Hz difference.
> > To make a complete DMTD one would have to do this angular measurement for
> > both channels and subtract the measured angle.
> > It is assumed the internal reference cancels out in a dual channel setup
> > comparing the two inputs so the simulation assumes a perfect internal
> > reference.
> > Some questions.
> > 1: The measurement of the angle (phase) is actually a combination of 1k
> > samples over a 1/48 second period. Is this a valid way to measure the
> phase
> > of an input signal? A frequency offset will cause phase rotation over the
> > measurement period. Is this causing systematic errors?
> > 2: With a 10MHz input signal and a minimum observable frequency
> difference
> > of 1e-6Hz over a one second period the frequency resolution with a "gate
> > time" of one second seems to be in the order 1e-13. Could this be
> correct?
> > Is the noise level realistic? Would this translate into a phase
> resolution
> > of below 1 ps or am I making a big mistake?
> > Erik.
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>