measuring Error Vector Magnitude

Hello everybody,

Does anybody know how to measure the error vector magnitude (EVM) in the dqpsk modulation system?

I have my I&Q signal(discrete), then pass raised-cosine filter, then D/A and final, upconverter to some RF frequency. And now, I want to test the distortion of my circuit so that I need to downconvert and demodulate the I, Q signal to discrete form. does anybody know how to do that?

Thanks!