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Hi group

I have measured a low pass filter, but experience some, for me expected, oddities around the maximum usable fundamental frequency of the Si5351A. Please see the attached picture that clearly shows an abrupt change in the attenuation between ~300 MHz and ~303 MHz. The NanoVNA should be able to run in 3 x 300 MHz segments, i.e. fundamental plus second and third harmonic. So far so good. However, my experience in working with the Si5351A, and the RFzero project in particular, is that the fundamental frequency from the Si5351A can be used up 280 MHz with 100% certainty, and up to somewhere below 300 MHz depending on the actual device. But once the frequency increases, then the PLL will suddenly not lock, the frequency is of course unstable and the S/N is VERY poor.

The PLL in the Si5351A I have on my RFzero right now can lock at 299,6 MHz, but not at 299,7 MHz. But I have seen Si5351As that cannot lock above ~287 MHz. I don't think it is possible to bet on the max freq. even vs batch.

So why can the NanoVNA say it has 900 MHz, 3 x 300 MHz, usable bandwidth and not e.g. 840 MHz, 3 x 280 MHz, or use the fourth/fifth harmonics? I am a bit troubled with measurements in the 280 MHz - 300 MHz, 560 MHz - 600 MHz and 840 MHz - 900 MHz segments. Should it be an option, in the S/W, to set the max fundamental frequency? I am not thinking of the 1,5 GHz possibilities as such.

Bo

This is why hugen came out with his 800MHz version of the firmware.

did you tried original edy555 firmware?

Firmware from hugen79 and from edy555 have different frequency modes. So they have different behavior at different frequency boundaries.

On Fri, Sep 27, 2019 at 10:04 PM, Bo, OZ2M wrote:

Should it be an option, in the S/W, to set the max fundamental frequency? I am
not thinking of the 1,5 GHz possibilities as such.

There is no such option. You can modify firmware but the logic for frequency boundaries is not easy, needs to deal with this.

By the way, edy555 firmware uses different frequency segments boundaries. You can try edy555 firmware, may it will help to solve your issue with no need to change firmware code.

Hi

Thanks! I think I will change the firmware since "sounds" numbers are more important to me than the last 100 MHz.

Bo

For those interested in the spectrum of the Si5351A here is a series of pictures, using my RFzero, at 267 MHz (i.e. 800 MHz "NanoVNA version"), 280 MHz, 298 MHz and 299 MHz. The pictures clearly show the behavior of the PLL and also the signal-to-noise performance.

More spectrum plots are here:

Bo

Bo

I am the owner of RFzero (THANK YOU!) and of three, soon to be four, Nanovnas. While it does not fundamentally alter the discussion here, The Nanovna only has two frequency ranges: 55kHz to 300 MHz is accomplished using the fundamental. 300.01 MHz to 900 MHz is accomplished using the third harmonic of the Si5351 with the fundamental level boosted about 9 dB for this range.

Nanovna uses a second 5351 oscillator as a "tracking" source that operates 5 kHz above the measurement frequency and is used as a heterodyne source to the mixer which generates the 5 kHz audio signal for measurement. I believe (I have measured it but cannot find the data) that Nanovna uses the third harmonic for this "tracking" source from 300 - 600 MHz and then switches to the 5th harmonic for 600-900 MHz.

So it goes like this:

55kHz - 300 MHz: Measurement = Fundamental; "Tracking" = Fundamental

300MHz - 600 MHz: Measurement = 3rd Harmonic; "Tracking" = 3rd Harmonic

600MHz - 900 MHz: Measurement = 3rd Harmonic; "Tracking" = 5th Harmonic

BTW..... I have a HUGE problem trying to decide between RFzero and Nanovna as which is my favorite instrument!

WA8TOD

Hi

I have now upgraded my NanoVAN with the NanoVNA-H__800_ch_20190924.dfu firmware. However, as the attached picture show there is not much real change except that the abnormality has moved down in frequency. Is it the right firmware I have loaded since I can set a stop frequency to 900 MHz. I didn't expect that.

Thanks Warren for clarifying the use of the harmonics in the NanoVNA. I don't know the NanoVNA anatomy as well as I know the RFzero. They are indeed nice devices.

Bo

I have now upgraded my NanoVAN with the NanoVNA-H__800_ch_20190924.dfu firmware.
However, as the attached picture show there is not much real change

My understanding is that flashing Nano**800_aa*.dfu
("aa" instead of "ch") is needed for changing max frequency.
Then load "ch" version for that functionality..

It's somewhat different, does not really matter
Till 300MHz both test signal and 5kHz offset LO are fundamentals.
From 300 to 900MHz test signal is 3rd harmonics so actually from 100 to 300MHz and the LO is using 5th harmonics so actually is from 60 to 180MHz.

Hi

I have tried the nanoVNA_800_aa_20190802.dfu F/W too. The trace is the same as the NanoVNA-H__800_ch_20190924.dfu.

I strongly believe that when approaching 300 MHz, with the fundamental frequency, the NanoVNA doesn't work properly which the below spectrum plots also show. Is there a F/W that goes to e.g. 266 2/3 MHz and then has a max of 800 MHz, or 250 MHz/750 MHz...? If so I would love to try it.

Unfortunately I can't use the 8753 these days - maybe next week. But, I have measured the LPF using my signal generator and spectrum analyzer, and there is no abrupt change in the attenuation.

Bo

Due to cheap chines manufacturing one may expect out of spec SI5351 to be used so having a firmware where one can set the max SI5351 fundamental frequency does make sense.
I will submit a request to edy555

Thanks!

If at all possible please make it so, that the max usable fundamental frequency (MUFF) can be set through the USB also, e.g. from NanoVNA-Saver.

Bo