Swept Measurements

The QA401 software supports the ability to make swept measurements. You can even write your own measurements in DotNet (C++, C# or Visual Basic) and automate mundane measurements required as part of production, if needed.

In the 1.511 release, we've made some changes to the swept measurements. Some of these are visible, but others are purely related to internal performance. As part of testing, a lot of data was generated and some of it is being shared here. 

QA401 THD Measurements

The THD spec on the QA401 is -108.5 dB, measured in loopback mode, single ended, L- shorted to ground, attenuator off, 32K FFT, Hann windowing, 48 KSPS and -10 dBV. 

Because the measurement is made in loopback mode, it's hard to distinguish where the DAC performance ends and the ADC performance begins. In a previous blog post, we took a look distortion limits of the QA401 when using a notch filter. 

In this post, we'll take a look at some sweeps to see where the sweet spot is for the QA401 in loopback mode. 

In this first plot, you can see a THD sweep from -22 dBV to -8 dBV. Best case THD appears around -14 dBV and 2 KHz

Focusing on that operating point, we see the graph below. Using markers, we can see the first harmonic at 4 KHz is -120 dB below the fundamental, which is exceptional. The measured THD at this point is -115.7 dB--considerably better than the spec'd value for the QA401 of -108.5 dB. Of course, some QA401 may be slightly better and some may be slightly worse than this -115.7 dB figure. 

At 1 KHz, the optimal point for 1 KHz operation we can read off the swept plot is at -16 dBV. Below is a plot of that measurement:


While not as impressive as the 2 KHz point, the above shows that the second and third harmonics are both about 120 dB below the fundamental.


The improvements to the Frequency Response and THD plugs-in should help make faster swept measurements. Make sure you are taking advantage of these tools in your characterization. They are covered in the section labeled "Test Plugsin" in the manual. 

Have an idea for a plug-in you'd like to write in DotNet? Let us know, we're here to help. 


  • Are phase measurements possible? Really need to produce BODE plots of open-loop amplifiers.

  • I note that on swept THD measurements I can only get plots out to C. 8kHz. Are there any plans to extend this to 20 kHz? Would be useful for power amp development. One other comment I have is that on the LF response, it seems only to go to about 20 Hz. Extending this down to 1 or 2 Hz would be great for measuring turntable noise and for rumble filter development.

    Let me take this opportunity to congratulate you on a first class product. There has long been a need for an alternative to expensive instruments like the AP and HP and this is it.

  • Dear development team,

    first of all: thanks for this great product!

    I found a small bug in your latest software release, which was documented here (post #2371):

    Further, I would love to see a python3 example connecting to the QA401. I was not successful yet in adopting the work of mark melvin for the QA401.


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