Teledyne LeCroy AORM - Advanced Optical Recording Measurements - Manual (Page 122)

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ISSUED: June 2013 923133 Rev A
Roughly, the phase steering target for the VCO is that the data transitions should happen on the
falling edge of the VCO output. To be more precise, we steer such that the VCO phase will be
180 degrees at the sample where a zero crossing in the data is detected. Because the software
VCO works on a sample-by-sample basis, there is, on average, a half-sample delay from the
VCO falling edge's zero cross to the data zero cross. At 20 samples per T, this half-sample error
is 2.5% of T, not noticeable without zooming in. At 10 samples per T, it is 5% of T. The following
small figure shows part of a zoom on a rising data edge and a falling clock edge sampled at 500
MS/s (2 ns per sample). The horizontal scale is 2 ns per division. The samples are bold. Note that
the data crosses zero at 1 ns after the falling edge of the VCO output crosses zero. This is the
expected result.
The signal used is a 4.36 MHz square wave,
which has a transition every 3T when 1/T = 26.16
MHz. During the first 50 µs or so the phase
settles in from initial startup, after that all the
zero crossings are half a sample apart, as
shown in this picture.
JitterTrack of Frequency (requires JTA option) of the extracted clock. The startup frequency was
correct to within a few kHz, and the PLL did not slip. It is possible that the starting frequency was
precise but the starting phase was not; the effect would be the same. JitterTrack shows frequency
as a function of time. The vertical scale is 20 kHz per division; the cursor is positioned at 27.107
MHz. The horizontal scale is 0.1 ms per division.
How the Starting VCO Frequency and Phase are Determined
The PLL's VCO is started at a frequency of 1/T. Due to the accuracy required, T is determined in
two steps. The first step produces an estimate of T starting with very few assumptions. The
second step starts with the estimate of T and refines it. The information used in both steps is the
sample at which a transition (through zero) occurred in the sliced data, for up to the first 2000