开云体育

Hello,
A friend of mine has just received a new NanoVNA-H bought from an ebay seller in Essex.

It's one of the cased units with black front and all accessories very nicely presented in a box and even a usb-c to usb-c which works with his Android phone app.

The problem is that after calibrating it and looking at a VSWR plot of the supplied 50 Ohm load there is a massive spike at around 300MHz.

He set the centre frequency to 297MHz with a span of 10MHz and re-calibrated, in order to better see the issue. Scale was set to 0.2/Div.

Attached is a screen shot.

Has anyone else seen this issue, or know the cause?

It can still be returned, but if it is a known issue with a simple fix then that may not be necessary.

Thanks,

Barry
G4MKT

it's hard to say something, because there is not enough data on the screen and screen quality is very bad, because this is not screenshot, this is photo.

You can get real screenshot with NanoVNA MOD v3:

Very-very old firmware may not support capture screenshot function. In such case I recommend to update firmware with more new.

For further assistance, please show screenshot (please not a photo, because it's hard to read) which includes at least S11 LOGMAG with 10 dB scale and Smith chart.

You can also try to update with NanoVNA-Q, it has hardware check and will show indication if there is si5351 issue.


Also, note old firmware versions have big spikes at 300 and 600 MHz.

Datasheet for the si5351 says it works up to 225 MHz.
The original Nano VNA firmware pushed it up to 300 MHz, which has proven to be a wee bit too far.
Above 300 MHz it used the third (and fifth) harmonics of the square wave out of the si5351 instead of the fundamental.
This is fixed by using different firmware, try downloading firmware that claims a maximum of 800 mhz instead of 900 mhz,
This firmware only asks the si5351 to give a max of 800/3 = 267 MHz, which all si5351 chips seem capable of.

Different si5351's crap out at slightly different maximum frequencies.
Just a natural variation due to slight differences between parts,
even if in the same manufacturing batch.

Jerry, KE7ER

This is fixed by using different firmware

In newer firmware, use shell command `threshold`,
followed by `saveconfig`
/g/nanovna-users/files/NanoVNA_Console_Commands_Nov-4-19.pdf