I might add that you need to remember, the Nanovna is an inexpensive hobby device and although it is quite accurate in many ways, it is not a lab grade device like R&S, Keysight, etc.??
There was only so much memory space to work with at the time and the dev that added the fft routines did a great job.??
You may want to contact edy555 or DiSlord via issues in their Nanovna githubs if you think there is something that needs attention in the firmware.?
On Wed, 23 Sep 2020 at 5:46 PM, John Gord via groups.io<johngord@...> wrote: Christian,
In the firmware (not necessarily the PC software), the "Low Pass Step" mode appears to give correct step reflection amplitudes when the selected format is "Real".? I tend to treat the impulse modes as "indication only".
--John Gord
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On Wed, Sep 23, 2020 at 12:52 PM, Christian Zietz wrote:
Hello,
please excuse if this has been discussed before ? perhaps I searched the
list archive using the wrong keywords.
Do you think that the time domain mode built into NanoVNA firmware (in my
case: hugen79's version 0.4.5) should correct the displayed amplitude for
losses in the IFFT due to windowing and zero-padding? Other VNAs do so: see
the attached files, where the same DUT ? an open-ended coax cable ? was
measured with a Rohde & Schwarz ZVA and with NanoVNA in time domain mode.
Neglecting the (very small) cable losses, the DUT presents a total reflection
and is shown as such (¡Ö 0 dB) by the R&S ZVA. In contrast, on the NanoVNA,
the peak depends on the selected mode (bandpass, lowpass impulse or lowpass
step) and on the selected window. This can be explained by looking at the
signal processing performed by the NanoVNA firmware. For example, in bandpass
mode with normal window, it applies a 101 point Kaiser window (shape factor 6)
[1] and zero-pads to do a 256 point IFFT. Therefore, the loss is
20*log10(256/sum(kaiser(101,6))) ¡Ö 14.2 dB. Actually, the peak in the
NanoVNA result (blue curve) reads as ca. -14.6 dB, which matches very well.
Imho, the firmware should correct these systematic losses that happen purely
by signal processing in order to give a result consistent to expensive VNAs.
(Keysight does the same as R&S here. [2]) But maybe this topic was already
discussed and a there was a good reason _not_ to correct the losses in
NanoVNA?
Regards
Christian
[1]
[2] Keysight's time domain app-note even states "There is also some scaling
and renormalization that takes place to ensure the value of the time domain
transform retains its physical meaning. For example, the frequency response of
the S11 of an ideal open circuit, with no delay, has a value of 1 for all
frequencies; its inverse transform is a delta function. However, when the data
is sampled and windowed, the time domain transform of the response of an open
circuit will be spread by the windowing function and does not return an
impulse of unity height. Therefore, it is necessary to renormalize to ensure
that the time domain response of the open circuit has a value of unity."
