Is it possible to somehow verify that the U551 chip is the one causing problems?

It is - with the NanoVNA-F, version 2.0 or newer firmware. It saves the s1p files. ( Not a help for the -H models, though. (Not enough memory - already crammed full!)

Checked your picture, black and white are not made by us, but it seems that your black version uses the same circuit as ours, we only found a little difference on the PCB, as long as the test is norma

I am not a fan of the Si5351. It was intended to be used for digital clock generation for processors and peripherals, not for RF signal generation where reasonable signal quality is needed. Its only a

Minute, basically two 50ohm loads on a SMA tee

I repeater the nanoVNA measurements while running the nanoVNA from a clean 5Volt supply No differences in measuring the 25ohm resistor or the cables

Kurt wanted to see some pictures so here they are. The first picture is the dB logmag of S11 of a 30cm semi rigid cable (medium) and a 1 meter RG58 cable (long), both Open and Short at the end, both f

To analyze possible confounding factors, modify: main.c cmd_vbat() - chprintf(chp, "%d mV ", vbat); +/* needs adc_stop(ADC1); ?? */ +chprintf(chp, "%d %d ", adc_vbat_read(ADC1), adc_tjun_read(ADC1) );

Very helpful, Thanks Erik. Sanjay

The IP5303 inverter section is running at 350KHz when running off of the internal battery. That will create a pile of noise. For a good sense of supply line noise, the IP5303 needs to be removed/disab