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

[Bob kb8tq]

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

> On Sep 20, 2022, at 11:12 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> 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 <mailto: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 <mailto: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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