Estimated DUT THD

Got an idea for a possible feature in the QA401 for excluding QA401 inherent THD.

Would it be possible to add a feature that calculated pure DUT THD based on relatively simple math. I know from the cable industry that intermodulation products are calculated with 20log rule for even harmonics and app. 15dBlog for odd harmonics.
If I eg. measure a power amplifier at 100W at 8 ohm to have THD = 0.002% (-95dB) and measure the QA401 with app the same settings I get 0.001% (-100dB). A simple equation would be Total THD= DUT THD+ QA401 THD or in this case with 15log rule: 0.002% (-95dB)= X THD+ 0.001%(-100dB). The result in this case is DUT THD -99dB or 0.001122%.

Given it is a simple estimation there is some uncertainties, but it is probably close.


Claus Dreisig

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Hi @a14352. Could you be a little more detailed about the method you propose? I find your proposal very interesting. Thanks

Hi CLaudio

The idea is is to use a set of know inherent THD data from the QA401 (eg at 1KHz) and characterize the QA401 back to back and deduct this from the measurement. A practical setup could (perhaps) be to use one set of output and input on the QA401 as “back to back” and the other output and input connectors as DUT connections. (with attenuation as the the amplifier gain).

There are probably some limitations but perhaps it could work


Claus Dreisig

Just one more thing!

The closer the two measurements are (QA401 ad QA401 + DUT) the more accurate the estimation can bo done. A relatively big difference will give more uncertainties.
It could be fun to compare how close one could get to more expensive equipment by estimations.



Very interesting.
One would need to apply a calculation algorithm in concrete at this point to observe the results that can be obtained. It is not very clear to me the rule of 20log for even harmonics and 15log for odd harmonics. In practice, how could it be applied since with the THD value you cannot distinguish which harmonics it is produced by. However, in my opinion, the whole thing deserves to try to apply it.

To be honest, I am not sure what the best approach is.

15log and 20log rule is used for IM calculations in the cable world. This is not IM but just simple voltage subtraction so perhaps it is best to start with a simple approach on all harmonics.

If the harmonics are in phase (DUT and QA401), then a 20log rule is probably best. (simple voltage addition). I would suspect this is the case if the DUT frequency and phase response is flat.

If anybody has an AP analyzer the estimate result could be verified?


Claus Dreisig