Measurements are done with a variety of instruments namely :-
- Prism dScope III Audio Analyzer,
- Custom designed LPF for measurement of class-D amplifiers similar to Prism ds-LPF
- Agilent 34401A 6.5 digit Digital Multi-meter
- Tektronix Digital Oscilloscope models, TDS784A, TDS3012, TDS7054
- Tektronix CRO model 2246A
- Leader LDC-824 6.5 digit 500MHz Frequency Counter with Oven Controlled crystal oscillator
- E-MU 0404 external 24 bit 192KHz sound card
To measure class-D amps using an audio analyzer with digital processing it is prudent to filter out any ultrasonic carrier noise (300kHz and above) as this can fold back into the audio spectrum and interfere with the measurements by exaggerating the noise measurements even though the noise is not part of the audible spectrum and for all intents and purposes is essentially filtered out by the speakers and certainly cannot be heard by the ear !.
Both Prism Measurements and Audio Precision sell outboard filters designed for this very purpose however we decided to build our own based on Prism's own specs which is very similar to a Chebychev 3rd order low pass filter with a 0.05dB equi-ripple response in audio pass-band and at least 50 dB attenuation at 200kHz.
http://www.prismsound.com/test_measure/products_subs/filter/lpf_spec.php
(http://www.prismsound.com/test_measure/graphics/prod_images/LPfilter_response1.gif)
Our filter design yields a very similar response ;)
P1050039.JPG
The first incarnation resulted in a 1dB drop at 20KHz :(
P1050040.JPG
For some reason the calculated values for the components was yielding an attenuation of nearly 1dB at 20KHz but with some experimentation with the simulator a better match was obtained and now the filter is within 0.05dB from 20-20KHz
ds-LPF.png