Advice re measuring DAC linearity/frequency response. Getting a lot of strange wobble <=100Hz

QA40x
1

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):

image
image1613×947 119 KB

And here are my settings in REW:

image
image537×698 60.3 KB

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:

image
image1118×897 33.2 KB

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.

2

Chirps tend to do that. You don’t have to use chirp frequency response, you can use the step-by-step one.

3

I’m more impressed you have got the software working on Linux! I tried and failed miserably. Would you be open to running through the steps (for dummys) in order to help me (and others) to get the QA40x SW working under Mint/XFCE etc? Cheers.

4

REW has an option for that? I can’t see anything like that in the generator settings…

5

It’s pretty unstable, but I can do a lot of what I need with specifically QA40x v1.219 [strikethrough](cant remember what happened when I tried more recent versions, but the end result was evidently that it didn’t work).[/strikethrough] (Actually 1.219 is basically the most recent. I was thinking of another piece of software that I use which needs to be an older version)

And I run it using mono QA40x.exe

6

So, install mono and it will just run and work?

7 
michael@michael-desktop:~$ mono --version
Mono JIT compiler version 6.12.0.200 (tarball Tue Jul 11 21:33:00 UTC 2023)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
	TLS:           __thread
	SIGSEGV:       altstack
	Notifications: epoll
	Architecture:  amd64
	Disabled:      none
	Misc:          softdebug 
	Interpreter:   yes
	LLVM:          yes(610)
	Suspend:       hybrid
	GC:            sgen (concurrent by default)

Yea, I’m sure I didn’t do much more than that. I note also that I’m not using the Output channels of the QA403, so I don’t know if they work on Linux.

1 Like
8

So the steps would be to install the QA40x software in a windows PC and then copy all the files/folders onto my linux (mint xfce) machine, then?

How does mono know where to look for the .exe? It keeps saying no such file or folder or words to that effect. (semi-noob on Linux here)

Also, my LLVM says supported, not enabled- is that necessary to have it somehow enabled like your config above?

Thanks for your help!