All good questions and point. Thanks.
I shall post my measurement plots. I did both data capture as well as photo shot of the VNA as it looks a bit better on the VNA screen as opposed to the data capture.
I did a calibration span from 453-457 kHz. So with 101 points that should be ~ 40 Hz step and should resolve the filter BW fairly well. And in fact with 2 markers on, marker 2 will read DELTA Frequency in relation to marker 1. Nice feature. And you can discern a filter BW of ~ 400 Hz.
However, there is most definite what I will call phase modulation as there are distinct sidebands along the lower and upper skirts of the filter. Is the LO phase noise sufficiently low that with (assuming) 40 Hz steps this phase modulation of the skirts should not be present? Need to go look at their LO spec. Or is the interpolation challenged? Any form of averaging would certainly help.
Alan
________________________________
From:
[email protected] <
[email protected]> on behalf of amirb <amir.borji@...>
Sent: Wednesday, July 10, 2019 7:28 PM
To:
[email protected]Subject: Re: [nanovna-users] Narrow band filters and LO phase noise/jitter?
I may be wrong, but:
* is nanoVNA not derived from the EU1KY antenna analyzer?
No, EU1KY is quite a bit different
* in which case, mixers basically mix down to DC (no IF)
- bandwidth limited by RC networks into ADCs with 5.5 MHz gain bandwidth (at
100 kHz)
in any case, you have a limited resolution bandwidth even if it is not apparent as a physical filter circuit.
although I think there must be a lowpass filter after the mixer (zero IF)
The question is how wide/narrow is it?
On Wed, Jul 10, 2019 at 2:57 PM amirb <amir.borji@...> wrote:
certainly they are not implementing a narrow FFT bandwidth filter in the DSP
"DSP math libraries provided by ARM, with FFT code highly optimized for speed"
I didn't get where this quote is coming from but it does not mean the NanoVNA designer has
implemented a narrow band low pass filter (resolution BW) in DSP for small spans. Maybe he can explain it...
