I need to measure the input impedance of a LNA designed to amplify RF signals around 80dBm. However, the source of my VNA can only be decreased down to -20dBm. .....Any idea of how I can do this measurement?
As you noticed, it is -80dBm and not 80dBm (typo error)...sorry for the inconvenience. Moreover, I confirm you that on the VNA, I use, the level of the power source can only be decreased down to -20dBm.
Yes you are right .
Pad in short means attenuation pad.
May I know why you need to test the LNA at -80dBm.
You can as well test at -30dBm or so.I mean if it works fine at higher powers(-30dBm even it can go slightly more than input P1dB) it means that he LNA will work at lower power too.
What are your LNA specifications?
Please share with us.
As long as your LNA is operating well below the 1 dB compression point (20 dB or so) then the input impedance isn't a function of the power. If the P1 dB of you LNA is -5 dBm or higher, just measure it at -20 dBm. If you use 60 dB of attenuation, all you are going to do is to be measuring the attenuator.
Just to give yourself confidence of your input impedance measurement, just
measure the LNA with Pin= +5dBm, then 0 dBm , -5, -10,-15,-20 dBm. So
you will be able to see the effects as a function of Pin. As it was
mentioned before, the impedance will only start to change as you operate
around the P1dB point.