I’m building a DAC and I’m trying to measure it’s frequency response up to (say) 20kHz.
The trouble I’m having is that I don’t have an analogue input, so I can’t do a fully closed loop set up with the QA403 managing both the output exponential sweep and recording the frequency response on the input.
I’ve had a lot of issues getting the exported exponential sweeps to play back properly with the trigger function (I’m on linux, and I get frequent crashes when trying to use the trigger and so on). Therefore, I’ve been trying to do it manually with REW.
First, here are my settings in QA40x (which also shows the frequency response I get at the end):
And here are my settings in REW:
With the above combination of settings, I can click “Run” in QA40x, then click “Play” in REW, then click “Stop” in in QA40x once it’s taken a single measurement.
Time domain looks like this:
Referring back to the frequency response in the first screenshot, I’m unsure how to interpret the oscillation for the first 100Hz or so. I’m not sure if it’s a problem with the DAC, or whether it’s a problem with how I’m performing the measurement. I note that in the time domain, my sweep looks like it is shifted up by about 1/5th of a volt for the first part, before settling down to being symmetrical around 0V. I think the QA403 inputs are AC coupled, so I’m also not sure what to make of that.
I would be grateful of any thoughts on this being a valid way to measure the frequency response of a DAC or not.