ABorel,
I’m not sure what you’re doing on the receive side, but something like the BB60D, using the digital modulation analysis tools, would be able to lock on and demodulate the (amplified then attenuated) QPSK waveform, and give you an EVM (error vector magnitude) related to the linearity of the amplifier. It can be fairly tricky to check your I and Q symbols without some sort of digital modulation analysis, as there is a delay and phase rotation between the transmitter and receiver, even on the same time base.
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