Total Harmonic Distortion
The total harmonic distortion is a measurement of the harmonic distortion within a signal. It gives an impression of how much a signal is composed of frequencies that are a multiple of its base frequency.
We created a 1kHz sine wave with SigmaStudio and then measured the output amplitude of the emerging harmonic distortions (multiples of 1kHz). We reached an THD of 0.0903% when setting the level of the sine to 4.55 VRMS and 1.29% at 7.08 VRMS. This is expected as higher power levels lead to a higher distortion ratio.
One of the main purpose of the ALLinONE is signal processing. The quality of the signal is mainly influenced by two disturbance values. First, there is the noise power. Just by limiting the digital resolution, a digitalization noise is produced. Additionally, a thermal noise of every component is expected. Further, capacitive behaviour, cross talk, PN junctions and component tolerance may influence the result. To gather up the sum of these, the noise power PNoise is introduced. As the amplification of a component reaches the limits of linear range, amplifiers begin to produce distortions. These commonly show up as harmonics at the integer multiples of the base frequency of the signal. The sum of these harmonics hereafter is called the distortion power PDistortion. To classify the quality of the signal, the THD and the SNR are usually used. The SNR is the relation from signal power to noise power.
The THD relates the signal power to the distortion power. As there are not only harmonic distortions at the output of a Class-D amplifier, but also mixing products of the harmonic distortion, here the THD+N is measured, too. These relate the signal power to the sum of the power of the harmonic distortions and the noise power.
All of these measured values are directly related to each other and can be calculated from a single measurement. As the noise power as well as the power of the harmonic distortions usually is very low, a precise measurement is necessary.
Every element in the measurement chain adds noise and distortions. Therefore, these added values have to be differed from the noise and distortion caused by the DUT. To not distort the measurement unnecessarily, the measurement chain is kept as simple and short as possible. The measurement is representative if the distortion power created by the DUT is far bigger than the distortion created by the measurement chain. To classify the measurement chain, a calibration measurement is made.
The measurement setup contains the generation of a sine-tone at 1kHz. This signal is digitally transmitted from the soundcard [Fireface UCX] to the DUT (ALLinONE ). A 4Ω resistor was used as a load. The output amplifier of the ALLinONE has a floating ground. Therefore, the average has to be to be removed from the output signal before being measured by the soundcard. For this purpose an audio transmitter (Monacor LTR-110 [Monacor]) is used. After that, the signal is recorded by the soundcard again. It is important to take care, the signal peak value does not override the maximal applicable input voltage value of the soundcard. Finally, the signal processing functions of MATLAB are applied, to calculate the THD, the SINAD and the SNR from the recorded signal.
The results are shown in the following table:
| Supply Voltage/V | Voltage at Resistor/V | THD+N/dB | THD/dB | SNR/dB |
|---|---|---|---|---|
| 12 | 3.1 | -73.3 | -78.7 | 74.8 |
| 12 | 3.9 | -74.1 | -81.3 | 75.0 |
| 12 | 4.7 | -73.9 | -81.8 | 74.7 |
| 12 | 6.1 | -36.8 | -73.8 | 36.8 |
| 12 | 5.1 | -35.6 | -63.3 | 35.6 |
| 20 | 5.1 | -73.5 | -80.8 | 74.4 |
| 20 | 6.74 | 28.0 | -49.9 | 28.1 |
The table points out that noise and distortion increase with rising output power. A very interesting point is visible in row 5 and 6, where we switched from 12V to 20V. The output power is the same, but all values improved by a great margin at 20V. Again, we recommend using a higher supply voltage. Getting closer to full scale, a high distortion of the signal is visible. Just a fun fact: The SINAD equals the THD+N, if the signal power is much larger than the noise and distortion power.
