NanoVNA-H output signal level

11.11.2020 19:13

When searching the web for the stimulus signal level that is present on port CH0 of NanoVNA, most results seem to refer to the RF output figure given in the User Guide: between -13 and -9 dBm. I've seen posts also specifically claim that NanoVNA is suitable for measuring active components because of the low signal level at which it performs measurements:

"Basic performance" section of the NanoVNA User Guide.

However there seem to be a lot of different NanoVNA variants floating around, differing in both hardware and software. It's always good to double check these things. This is how the output signal of a NanoVNA-H I have (firmware version 0.4.5-1-gfbbceca, PCB revision v3.4) looks on an oscilloscope. CH0 on NanoVNA was connected to a 50 Ω terminated input on the scope. NanoVNA was set to CW stimulus at 1 MHz:

Output waveform of NanoVNA-H CH0 at 1 MHz.

The measured RMS voltage is 326 mV, which is around 3.3 dBm at 50 Ω. Much higher than what is stated in the manual. It falls slightly with frequency. At 10 MHz I've measured 2.8 dBm. This is still uncomfortably high. For example, it will likely damage a HackRF if you tried to measure the return loss of its antenna input. The E4000 tuner in an rtl-sdr has a absolute maximum RF input level of 10 dBm, so strictly speaking it will probably survive. Still, any kind of S11 measurement will show much worse results than in real life for low-level signals since it will overdrive the RF front end of the receiver.

According to this post, some versions allow you to set the signal level from the user interface or by using serial commands. As far as I can see my version only supports the serial command method.

These are the output levels I measured at 1 MHz using different power settings. For some reason the power serial command doesn't work if the NanoVNA is in the CW stimulus mode - the command only had an effect if I sent it when full sweep was enabled on the device. However after the command was accepted, the output power in CW mode was also affected:

serial command RMS voltage [mV] power @ 50 Ω [dBm]
power 0 88.7 -8.0
power 1 173 -2.2
power 2 253 1.1
power 3 324 3.2

From this it seems that power 3 setting is the default at power on. As you can see, there's really not much difference between the settings in the logarithmic scale. Even the lowest setting is still higher than what is stated in the manual. I also wouldn't trust these results too much. From what I read the output power is adjusted relative to output frequency and I can only measure it at the very low end. Hence I don't plan on using NanoVNA for measuring any kind of sensitive components.

Posted by Tomaž | Categories: Analog


1mW level is of less concern for many selective measurements than square wave rich in harmonics for us old analnog generation!
I enjoy AA-30 antenna analyzer. Even 100 MHz scope for current SSPA repair. Keeps me focused away from corona :-(

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