Teledyne LeCroy AORM - Advanced Optical Recording Measurements - Manual (Page 123)

AORM Software Package
923133 Rev A ISSUED: June 2013
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edges. If the source waveform has less than 2000 edges, the accuracy of this procedure may be
reduced.
If the source waveform has less than 50 edges, the instrument does not even attempt to estimate
T. The PLL will start at 3.19024 * LP fc. Because of the low bandwidth of the PLL, it does not
make much sense to try to extract the clock from a very short waveform; the PLL will not have
time to react.
The first step calculates the width of the first (up to) 2000 pulses, sorts the widths, and finds the
first three peaks in the distribution of widths. The distribution is "smoothed" by a five-bin wide
boxcar filter to prevent small local events from misleading the peak detection. This is the primary
reason why the signal must be over-sampled by greater than 10x. The distribution of widths is
similar to a histogram of pwid (pit width) on "leveled" output of the ODATA function, using a
threshold of 0.0 mV and measuring All widths. The spacing between the peaks is approximately
T, close enough to determine the lowest nT. The instrument calculates the estimate of T from the
means of the first three peaks, which are assumed to be lowest n, lowest n + 1, and lowest n + 2
(i.e., 3T, 4T, and 5T). This estimate is generally good to better than 1%.
The second step uses the location of the first (up to 2000) transitions, in order. It uses the
estimate of T to calculate n between each pair of same-polarity edges. If the estimate is within
1%, we have at least 50% margin. A 50% margin occurs if a pair of same-polarity edges is 25T
apart. On a good waveform, the count is likely to be exact. On a noisy or distorted waveform, it
may be that some peaks are miscounted, but as long as some are long and some are short, the
final total will be nearly correct. Finally, T is computed as:
(time at the last transition - time at the first transition)/(total n between them)
If there are 2000 edges, an average of 4T apart, the separation between first and last edge is
8000T. If our count of n is off by 1, that is a 0.0125% error. We can tolerate up to 7 counts error
(0.0875%) before the PLL will not start locked. When the waveform is correctly equalized, this
does not happen.
A highly asymmetric waveform will not have clean peaks in the distribution of its pulse widths,
which also means that many of the pulses will be nearly (n + 0.5)T. On such a waveform, we may
not be able to determine T. The possible reasons for failing to determine T (and therefore the
VCO start frequency) are:
• Less than 50 edges in the waveform.
• Could not distinguish the first three peaks in the distribution of widths.
As mentioned above, you should sample at about 20x to 50x the clock frequency to make clock
extraction work reliably.
An attempt is made to start the VCO not only at the correct frequency but also at the correct
phase. The phase is pre-set such that the first edge in the waveform will occur on a falling edge
of the VCO output. The first edge is just as likely to be out of place as any other edge in the
waveform, of course. If the VCO starts significantly at the wrong phase it will either slow down or
speed up for a short while until it gets to the right phase. A JitterTrack shows this clearly. On a 4x
DVD waveform we captured, which just happens to have a significantly out-of-place first edge,