How to measure SNR ?

Using the Spectrum Analyzer (and a signal generator) you can measure only the Signal to Noise Ratio at the input of the demodulator, which is given by the formula:

SNR [dB] = 174 + RX_sens [dBm] ? 10*LOG(BW_Hz) ? NF [dB]

For QPSK SNR have to demodulate the signal and do a BER or EVM test.