BB60C for Pi/4 DQPSK RMS EVM %

Dear
We have a BB60c Spectrum Analyzer and we need to measure a TETRA Base Station parameters. We actually use an Aeroflex IFR3901 to measure this base stations, but we want to replace this equipment with the new BB60c.
We test both analyzer to compare in this kind of measurements:

-Frequency error.
-RF power.
-RMS EVM (%)

We set the modulation to Pi/4 DQPSK, and we detect similar results in Frequency error and RF power, but very different in the RMS EVM (%) measurement (the IFR 2% and BB60c 67%). When we change the modulation to QAM1024, the measurement of RMS EVM % gave a similar result, but it is not the native modulation of the TETRA base station. Another observation is that the I/Q constellation diagram is not points in the quadrants every PI/4 (this drawing is clear in the IFR3901) but they are all points distributed by all the quadrants.
We need your support to know why this result is so different between the analyzer, and what would be the correct configuration (filter, IF BW, symbol rate, etc.) to obtain these results.
BR
Gastón