How to measure the EVM of HMC497? First time to measure EVM.

Thanks to the kind-hearted people in the forum, I was able to understand the basic I/Q modulator such as HMC497. Now I am testing the RF IQ output. I use the E8267D from Agilent as the I/Q baseband signal generator. In the beginning I test the QPSK signals, the symbol row was used as fixed four, such as "1001". Then I tuned the DC offset to suppress the LO emission. When the emission was suppressed by about 50dBc, I test the EVM using the E4440 spectrum, also from the Aglient, with the EVM about 0.3%. However, when I changed the fixed four symbol row, such as from "1001" to "1011", the EVM result would raise to a level of amore than 20%. The LO frequency and symbol rate remained unchanged. So how can I test the EVM? Welcome to any suggestions.