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. 


  • Hi ACR, I made a mistake. In swept measurements you are correct that currently all THD measurements are limited to 20 KHz. I just changed the code so that beginning in the 1.6 release you can specify the stop frequency. However, it’s still incumbent on the user to correctly pick the sample frequency. That is, if you want to measure the harmonics of a 15 KHz tone, you need to be sampling at 192Ksps

  • Hi Richard, the phase information is shown in the upper right corner of the screen. And when you do a swept measurement, the phase information is saved in the file you export so that you can load it in the your app of choice.

  • Hi ACR, you can measure THD beyond 8K. Just remember that you need to move your “measurement stop” setting out further. This parameter is where you tell the THD routines to stop their calculations. This is set in the THD context menu. Also, you will probably want to set sampling rate at 192Ksps to ensure that you have enough runway after 20 KHz.

    For turntable noise rumble, the lower bound on the input is set by the input RC network, and it has a corner around 1 Hz or so. You can definitely measure down that low, you just need really, really long FFT sequences. If you take the output and route it to the input on the QA401, and switch to FR mode and set the source to white noise, and set the FFT length to 256K and averaging to 5, you should see a really nice plot emerge of the overall input response. You will see the overall response is down about 8 dB at 0.6 Hz.

  • 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.


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