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I just designed and built a DAC which utilizes the CS8416 DIR and the CS2300 jitter supression chip. The CS2300 is essentially a FIFO DPLL which features very good jitter filtering of the incoming clock. I was probing various nodes around the PCB with my Tektronix scope when I decided to look at the output of the CS2300. The regenerated clock there was well shaped and low in noise. Then I put a second scope probe on the input to the CS2300. With the scope triggering on the de-jittered output clock, the input clock appeared to be 'wandering' around at low frequency, maybe a few Hertz or so. It could also be described as a random 'flutter' along the time axis, and appears to be quite severe. I'd estimate the flutter or wander as being almost 50% of the clock period, peak-peak.

I probed the S/PDIF signal itself directly at the input to the DAC and it too exhibits the low frequency flutttering. Needless to say, I was alarmed, and at first thought the DVD player I was using as a CD transport was defective, but I swapped it for an old Marantz CD63SE and there was no apparent difference. The DAC locks to the signal without trouble and plays music with no obvious degradation. Does anyone know whether this low-frequency flutter/wander is normal for an S/PDIF signal?

If anyone is curious, I discovered the source of this problem. The DPLL chip which I was attempting to use (CS2300-01) has too low a loop-filter bandwidth of 1Hz. I switched to a version with a 128Hz bandwidth (CS2300-03) and the flutter/wander went away. Perhaps, in a future DAC design, I might experiment with cascading the 128Hz bandwidth version in to the 1Hz bandwidth version and see if the final de-jittered clock output is stable then.