Re: Random jitter bandwidth and minimum width pulses
Dawson,
You are correct, the occurance of RJ giving an amplitude greater that
the 100ps should then be the BER, but to expect a minimum pulse width to
also occur is a little unexpected. You generated this gaussian in matlab
so I understood, by using a transfer function from the random number
generated; perhaps you would like me to take a look at it. IŽll dig out
my Gaussian function and send it to you in return.
I certainly do not think that we should be introducing a filter on to
this jitter as the assumption is that this RJ is white. In reality of
course it is a more a PLL filtered, 1/f3, 1/f2 and 1/f noise with a
superimposed white noise, but I donŽt think we should get into this yet.
Anthony.
"Kesling, Dawson W" wrote:
>
> 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>>
>
> ------------------------------------------------------------------------
> Name: Jit_20M.ZIP
> Jit_20M.ZIP Type: Zip Compressed Data (application/x-zip-compressed)
> Encoding: base64
>
> Name: Jit_raw.ZIP
> Jit_raw.ZIP Type: Zip Compressed Data (application/x-zip-compressed)
> Encoding: base64