Measurement setup: Stanford Research Systems model SR-620 Squeezebox2 BNC connector grounded at s/pdif coax, wired to internal 11.2896 master clock. 3' RG59 cable
The absolute accuracy of this unit is specced at 25 picoseconds, but with enough samples, it's 1ps of resolution is enough to consistently measure the impact of phenomena as small as CPU activity (as speculated in the FLAC thread). Scope probes did not give consistent readings so I soldered a BNC directly to the board - at this resolution even slight changes in the position of the probe will throw the measurement off by 100ps or more, so a solid connection is definitely needed. These are standard deviation for 10,000 samples measured at DAC input. In addition to disabling the second oscillator, I also tested some different power supply configurations but those did not show any changes in sigma. However pk-pk measurements shown on the dscope analyzer (for the s/pdif out) did show differences - I haven't investigated that disparity yet. Anyway here's the data from the SRS-620: Unmodified SB2, playback paused: *65 ps* SB2 with 12.2880 crystal lifted: playback paused: *55 ps* playing AIFF: *57 ps* playing FLAC: *57 ps* playing mp3: *58 ps* Finally: Measured at the oscillator: *43 ps* A couple screen shots below (settings are identical for both measurements). Here's how to read these: the vertical line indicates an arbitrary cursor position (rel=48.797ns). The mean measurement is indicating the distance from this value. BTW the nominal value here would be 1/11.2896/2=44.289ns. The difference does not indicate any imprecision in frequency, but rather that the duty cycle is not exactly 50%. "sigma" is the number we're most interested in. At the bottom, note the scale is 100ps/div. You can see the base of the curve is about 3 divisions wide, indicating pk-pk jitter is about 300ps. A tighter distribution (less jitter) is indicated by a taller, narrower histogram. [image: http://www.seanadams.com/jitter_pics/1.jpg][image: http://www.seanadams.com/jitter_pics/2.jpg] -- seanadams _______________________________________________ audiophiles mailing list [email protected] http://lists.slimdevices.com/lists/listinfo/audiophiles
![http://www.seanadams.com/jitter_pics/1.jpg][image:](http://www.seanadams.com/jitter_pics/1.jpg][image:){kind=link}
