I’ve had my QA403 for about a month now, and I am impressed with it’s capabilities. It is a very significant upgrade from my HP339A/HP3580A combination.
I recently retested one of my designs that was very challenging to test with the HP setup. The test tone is at 16KHz, with a +5dBV amplitude. Based on the image, the THD should be no better than 95dB but it is computed to be infinity, and THD+N is computed to be -108dB.
I wish the QA403 could work at 384KHz as that would increase the measurement capability of a 20KHz (and even a 10KHz) distortion measurement. If there is way to enable 384KHz with the internal ADC/DAC I would be very interested.
Hi. You can enable the 384Khz sampling frequency (the ADC will work correctly while the DAC will not) by specifying -384k from the command line (‘QA40x.exe -384k’).
As far as the THD anomaly is concerned, perhaps the measurement frequency range has not been specified correctly: with a 16KHz fundamental, the first useful harmonic for calculating THD (2nd Harmonic) is found at 32 Khz. Therefore in the THD context menu ‘Measurement Stop Frequency’ must be set larger than 32 Khz. By setting 50Khz for example, the second and third harmonics will be taken into account in the THD calculation.
You can enable the input (ADC) of the QA403 with a sampling rate of 384 Khz. It is then possible to perform FFT up to about 190Khz. Caution though! The output of the QA403 (DAC) does not work in this mode. In other words, you can use the QA403 to perform FFT up to about 190 Khz, but the signal generator of the QA403 does not work when you choose this mode and therefore you must use a generator external to the QA403 should you need it. Since the FR measurement uses the DAC of the QA403, and since with the enabling of the 384KHz mode the DAC does not work, the answer is that you cannot make FR measurements up to 192Khz automatically.
Hi Claudio,
Thank you, I did not set the Measurement Frequency Range, I could not find the setting. I’ll look for it and set it properly. Perhaps you can mention how to access the THD context menu.
Too bad about the DAC, I am using the QA403 for some R&D work and the more I can see, the better. But after using the QA403, I’m not going back to my HP339A/3580A test set.
Best regards,
Morty
Hi @MTarr. To activate the THD context menu, right-click on the “THD” button. The “THD Options” window will open, as shown in the figure above, and from there you can set “Measurement Start Frequency” and “Measurement Stop Frequency.”
