On 8/14/21 9:14 AM, David Feldman via groups.io wrote:
I need to measure S11 going into an RF power amplifier (1-15 MHz range of interest.)
While the amplifier has proven stable (so far) when driven through a 10 dB attenuator (from RF source), I am concerned that the impedance of the Nanovna's S11 port (I am using a first-generation unit purchased mid 2019) which is likely not 50 ohms could create a risk of oscillation (which could put the nanovna's circuitry at risk.) The nanovna does not have separable testset (like "option 011" on HP 8753 series), so I don't think I can apply methods in HP app notes pertaining to measurements of RF power amplifier circuits.
You could calibrate, with the 10 dB pad in place.? The SNR of the measurement is worse, but it's still valid.
The other thing is that the output Z of the NanoVNA is probably pretty close to 50 ohms. It's a resistive bridge.
If I insert (example) 6 dB pad between nanovna S11 port and amplifier input port, and calibrate the nanovna (open-short-load) at the "far" end of the pad, what impact would this have on S11 measurements?
They'll be correct - just that you have 12dB worse SNR - which means that the "floor" is higher - that reduces the accuracy of measurements into a "good match" because you're looking for a tinier signal.
Consider this.. the S11 measurement is looking at the reflected power.? Say the VNA puts out 0dBm, your DUT reflects -20dB, and the VNA sees -20dBm coming back.? If you put a 6dB pad in there, the actual power arriving back at the VNA is now -32dB down.
At those kinds of match, you are fine.? But say you've got a super duper amplifier with a -40dB S11.? Now you're measuring -56dBm, and that's harder, against the noise floor of the analyzer.
Are there other methods that would reduce risk when measuring S11 going into a live amplifier?
Thanks for any suggestions/pointers.
