How to test PXIe-4461 THD+N

Updated Sep 27, 2024

Environment

Hardware

  • PXI-4461

Other



 

THD+N (Total harmonic distortion plus noise) is a measure of unexpected harmonics and noise in the output signal of an audio device. It is measured by the ratio of the energy of harmonics and noise to the energy of the input signal. Therefore, it is important to be precise about the parameters used in THD+N measurements.

My customer bought a PXIe-4461 this year. But when he was calibrating the parameters of the instrument, he noticed that the THD+N test result is higher than the specification suggest. What’s more, after the repair, the test results did not change. He wants to know how to test THD+N and what caused the problem.
 

1. For test code, as it is testing AO and the test frequency is from 20Hz to 91.2kHz, the testing range is large. Therefore, it is important to note that the single frequency sine signal sent has enough complete cycles. If an incomplete cycle sine signal is sent, there will be breakpoints at the ends, resulting in a discontinuous sine signal, Resulting in larger test results.
2. For the DUT, we also need to ensure the accuracy of the audio analyzer. The customer previously used AG15C to test AO. The equipment used here had insufficient accuracy, resulting in a deviation in the results obtained. Through escalation inquiry, it was found that the specific equipment model used for NI factory calibration is AP2700. The customer has a more advanced APx555, and after use, it was found that the AO results are close to the spec, however there are still some problems with the AO indicators.
3. Ensure that the power supply wires of the instruments used are well grounded, and it is best to connect all instrument casings, as most THD+N parameter tests are caused by noise issues. We have encountered related issues before at Shanghai RMA, and the power supply used at the Shanghai testing station is isolated.
4. The customer is using PXIe-1088, which has high electrical noise. It is recommended to use PXIe-1092 or PXIe-1095 chassis (the results obtained from testing at a Shanghai testing station showed that PXIe-1088 did indeed result in worse results than PXIe-1092, but still met NI's PXIe-4461 spec)
5. Through inquiry with colleagues in Shanghai, we learned that they did not use the Embedded controller, but the remote controller. The board and controller were in different test rooms. By observing the received signal spectrum of APx555, it is found that there is a clutter in its spectrum at around 350 kHz, 450 kHz, and 550 kHz, which will be filtered out under the conditions of measuring BW at 204.8 k/s, 20 Hz to 80 Hz, and 80 kHz.
 

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NI 4461/4462 Specifications - NI

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