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

[Bob kb8tq]

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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