Random jitter bandwidth and minimum width pulses
Jeff and Anthony (and other interested parties),
I see several minimum width pulses without the jitter filter in place in my
sim. This is surprising to me. Here's a description and some plot. Any
ideas?
The "jit_raw" file that is attaches shows (from left to right and top to
bottom) the jitter applied at each edge in sequence (measured with respect
to the ideal edge position), the difference between jitter at each edge and
the jitter at the previous edge (cycle-to-cycle jitter), the amplitude
spectrum of the jitter, and the probability distribution (histogram) of the
jitter. The RMS jitter is 24 ps, chosen to get 1e-2 BER with 0.35UI pk-pk
(=112 ps p-p) Gaussian RJ. The run time is 1000 baud, so I'd expect about 10
traces to cross the eye. As you can see from the plot, there are also about
10 occurances where cycle-to-cycle jitter is about -100 ps. These occurances
cause minimum-width pulses due to successive edges being shifted toward each
other severely. I'm surprised the frequency of occurance of minimum width
pulses is as large as the BER! These will definitely result in receive eye
violations after going through the channel. If this is real, it suggests
that our compliance system is not self consistent. One alternative is that
real transmit jitter is not so large and/or is bandlimited below half the
baud rate. Anthony, you mentioned that you are using band-limited jitter so
you might not see this effect if you limit well below 1.56 GHz. What
bandwidth and rolloff are you using? Any idea why Mysticom doesn't see this
either?
The "jit_20M" file shows the same jitter sequence after being filtered
through a single pole discrete time low pass at 20 MHz. (This filter is
applied to the sequence of jitter values, not to the jittered waveform.) The
filtering is apparent in the amplitude spectrum. The RMS of the pre-filtered
jitter was increased according to a derived formula to get the same
post-filter RMS jitter as the unfiltered case. In spite of the fact that the
resulting RMS jitter is clearly larger than theorectically anticipated, he
maximum cycle-to-cycle jitter is still reduced drastically. It stays below
about 20 ps, so no minimum-width pulses are generated. This filtering may be
extreme, but demonstrates the method I'm using to avoid minimum width
pulses.
all thoughts are appreciated.
-Dawson
<<Jit_20M.ZIP>> <<Jit_raw.ZIP>>
Jit_20M.ZIP
Jit_raw.ZIP