I'm interested to know what baseband digitiser rate we need to provide an accurate measurements of EVM. I am evaluating VSA with a standard PSA analyser which seems to be able to produce data at 15Msps, but a QPSK signal at 3.84Mcps doesn't seem to produce very good EVM results.

Does the VSA software interpolate between measurement samples when it calculates the EVM? It seems to me that if it does not then I will have to run the baseband sample rate at least 16 samples per chip to get close enough to the symbol instants.

What is the recommended sampling rate to make measurements at to get an accurate EVM?

Does the VSA software interpolate between measurement samples when it calculates the EVM? It seems to me that if it does not then I will have to run the baseband sample rate at least 16 samples per chip to get close enough to the symbol instants.

What is the recommended sampling rate to make measurements at to get an accurate EVM?

The sample rate of set in the hardware is determined by the frequency span selected in the VSA software. The sample frequency is determined by the following formulas and the software will resample the data from the hardware if the the desired sample rate cannot be set directly in the hardware.

Fs = Span * 2.56 (baseband mode)

Fs = Span * 1.28 (zoom mode)

The VSA software will then resample the data again for the demodulation using the symbol rate and points per symbol setting defined in the demod properties dialog box so that the data is sampled at the symbol times.

What sort of results are you seeing?