Measure amplifier non-linearity using two-tone signal
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HOWTO
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 # Take a USRP B200
 # Connect its TX to the amplifier you want to measure
 # Connect the amp to an big enough attenuator
 # Connect to the B200 RX

The amplitude ramp script is going to generate a two-tone (1kHz separation)
signal at 222MHz, and record outgoing signal magnitude and phase difference
against the source signal. The measurements are written into a CSV file.

It's your job to ensure you don't overload the B200 input signal power (-15dBm
last time I checked).

First step is to gather some measurements:

    ./amplitude_ramp.py --ampl-start 0.15 --ampl-stop 0.5 --ampl-step 0.01 --num-meas 300 --txgain 77

This creates a measurement.csv file you can analyse with

    ./plot.py

That's it for now.