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|2nd January 2018, 07:46 PM||#1|
Join Date: Mar 2003
Location: Haarlem, the Netherlands
Simple second-order sigma-delta for checking raw DSD interfaces
I recently needed a signal source for checking the functionality of a raw DSD64/DSD128 interface. I modified a second-order sigma-delta I had built before for this. The schematic is attached, maybe it will be of some use to someone else who wants to do something similar. I built it with through-hole components on a piece of perfboard - which is far from ideal for a sigma-delta, of course.
The signal-to-noise ratio is rather limited because it is only second order. As you can drive it all the way to 100 % ones / 0 % zeros or 100 % zeros / 0 % ones, you can also use it to check what happens when the DSD signal is louder than it is supposed to be. As far as I know DSD is not supposed to ever have more than 75 % or less than 25 % ones, but rumour has it that not all DSD files comply with this, see Tweaking DSD to PCM - Roon Software - Roon Labs Community
With input signals up to sqrt(2) V peak (1 V RMS sine wave), the 75 %/25 % rule is met. With 2 sqrt(2) V peak (2 V sine wave), you drive it to 100 %/0 %.
Some components are used a bit outside their recommended range. The PC74HCT390 can typically work up to 55 MHz, but it is not guaranteed to work at 28.224 MHz. The 74AHCU04 and 74AHCT74 that drive the 100 ohm-220 ohm voltage dividers have to supply 15 mA of output current, which is above the maximum recommended value but well below the maximum rating. The voltage dividers reduce the high level to about 3.3 V and drive thin 75 ohm coax cables (without output termination!) to the circuit under test.
The low-pass filter in the right bottom corner is meant for checking the sigma-delta modulator's output signal. With a switch you can subtract the output from the input, so you only hear the quantization artefacts. Adjust the trim potmeter and trimmer capacitor for maximum cancellation of the input signal.
The dashed 33 kohm resistor is a resistor I should have placed on the board, but forgot. With it placed, the low-pass filter would have been a nice third-order Butterworth at 82 kHz.
The 27 pF capacitor that is in parallel with a 270 pF capacitor is a left-over from an earlier experiment. The same holds for the 220 pF capacitor that is in parallel with a 2.2 nF capacitor. It will probably work just as well without these components.
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