Teledyne LeCroy AORM - Advanced Optical Recording Measurements - Manual (Page 118)

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ISSUED: June 2013 923133 Rev A
Since the acquired data may be a millisecond or less in duration, extracting the clock depends
critically on the scope’s ability to determine T (1/clock frequency) from the data and on starting
the PLL's VCO at that frequency and at about the right phase. When it can do that, the VCO
starts up locked and does not have to settle noticeably. If it cannot find the frequency, the warning
message: "ORDATA VCO start freq is 3.19*LP fc, didn't find it” will be displayed. As the message
says, if the scope cannot find the frequency, it starts the VCO at 3.19024 * LP fc. That ratio is
26.16/8.2 (to six significant digits). That is correct for DVD according to the specification;
however, it may not be within 0.1% for a real drive. Experience shows that drives read a couple of
percent fast.
To make the clock extraction successful, the scope must be successful in finding the starting
frequency from the data. Here are some things you should do to make this successful:
1. Capture as clean a signal as possible. Remember that a passive probe is 10 MΩ resistive
only at low frequencies and, therefore, may significantly load a high-speed signal. A
passive probe's response will roll off well below the scope's DC 50 Ω bandwidth.
Consider using a differential probe such as the AP033 or AP034, or an FET probe such
as the AP020. Remember to attach the ground lead.
2. Equalize properly. If the signal you are probing is already equalized but not very clean,
you can tell ODATA that it is RF anyway and set the boost to zero. That way the data will
be low-pass filtered, which greatly reduces noise. If you don't equalize when you need to,
or if you apply boost to an already equalized signal, the scope will probably not be able to
determine the starting VCO frequency from the data, you will see the warning described
above, and the extracted clock may not be good.
3. Sample at about 20 times the expected clock frequency. If you sample closer to 10 times
the clock or below that, the extraction algorithm may not be able to correctly separate the
peaks in the width distribution to determine the frequency at which to start the PLL. If you
sample much more than 20 times the clock, the widths (in samples) may be too spread
out from detectable peaks in the distribution. (See the following explanation "How the
Starting VCO Frequency is Determined" for more details.) Example: CD data has T =
231 ns, about 4.33 MHz. We can extract the clock from CD data at 100 MS/s (23x) and
200 MS/s (46x) or 250 MS/s (58x). At 50 MS/s (11.5x) and at 500 MS/s (115x), it
sometimes does not find the right starting frequency. Another example: DVD has T =
1/26.16 MHz, about 38.2 ns. We can extract the clock from DVD data at 500 MS/s (19x),
1 GS/s (38x), and 2 GS/s (76x). At 250 MS/s (9.5x) and at 4 GS/s (153x), it sometimes
does not find the right starting frequency.
Following are some interesting pictures to show what can be handled: