I agree that for active devices the NanoVNA would not be acceptable. I usually start measurements at -30 dBm with my scalar network analyzers when measuring amplifiers, to avoid overload. By carefully watching the DC current into the amplifier and slowly raising? the input RF level, you can tell when it is into saturation.
Trying to turn the NanoVNA, which as far as I can tell was born as an antenna analyzer for hams, into a multi-thousand dollar VNA is fraught with peril.
I am sure that there is room for improvement and growth for this very nice, inexpensive piece of test equipment, but don't expect too much.?
Stuart K6YAZLos Angeles, USA
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-----Original Message-----
From: alan victor <avictor73@...>
To:
[email protected] <
[email protected]>
Sent: Mon, Aug 12, 2019 10:55 am
Subject: Re: [nanovna-users] Measurement of nonlinear (active) devices
The fundamental issue in using the NanoVNA for measuring active devices is FIRST its output power level. It is specified at -9 dBm to -12 dBm. If you consider a bipolar device operating at a quiescent current of 1 mA, these power levels are too large! The small signal S parameters are just that, small signal. A bipolar transistor had a Vbe value of 26 mV/mA of emitter current. Hence, once you subject the DUT to such a Pin level, you will notice the quiescent bias level to increase! That is NOT a small signal measurement. As a rule of thumb, I would start at -30 or -40 dBm and get a S21 dB measure. So you will need a 20 or 30 dB pad.
You may also pad the output of the VNA, CH0,? to achieve an appropriate level. I would also pad the CH1 of the VNA to be sure that the VNA is not over driven as the gain of the device, S21,? at audio and HF may be quite large.
Also, be sure your DUT is stable! I have seen 100k$+ VNA's go up in smoke as a measurement of a power device was unstable. The device will oscillate and the VNA is destroyed.
You will have to construct bias tees. More on that latter.
Alas... its only a 50 buck VNA... go for it. But your warned. 73' Alan
