It depends
If you want to measure the OIP3 you should use the internal attenuator as high as possible while still seeing the intermodulation products as long as the input level to the tinySA is below 0dBm and you probably will end up with about 15dB attenuation.
But starting with 20dB is safe.
Suggest to watch these two video's?
https://www.youtube.com/watch?v=HJKjrEZRAKg
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For more info on the tinySA go to https://tinysa.org/wiki/
