Teledyne LeCroy Eye Doctor II - Manual (Page 33)

Operator's Manual
appropriate Eye diagram. SDA II has a Find Frequency button on the Clock Recovery tab of its dialog, if
the data signal is appropriately equalized, it does work.
PLEASE NOTE THE FOLLOWING:
Each of the training cases uses Eye Doctor II in simple ways.
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Auto Find Levels was left checked for the DFE.
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We only used tap values for DFE that were the result of our Training algorithm. We could have
pressed Edit/View DFE Setup, and entered our own tap values, if desired (and knew the values).
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We used the CTLE in Auto mode. In Auto mode, the only parameter shown on the CTLE dialog is
Boost.
The oscilloscope automatically places the zero and two poles (the second being the parasitic pole,
which we place at the bit rate, as previously explained).
If we pressed Edit/View CTLE Setup... while in Auto mode, the pole and zero frequencies are
shown with no change. In fact, all that could be changed is the DC gain, which is just a scale factor.
We could have set CTLE mode to Custom, and set the pole and zero frequencies manually. This is
a great way to experiment with the CTLE.
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Under Emphasis, we used Auto Add. Auto Add uses two taps. For 6dB, the two taps are approx-
imately -0.25 and 0.75; so that the first UI in a state is 1 and all subsequent bits in the same state
are 0.5.
We could have selected Custom, and entered up to eight tap values. PCI Express Gen 3 will use 3
tap TX equalization, which can be easily set up in Custom.
Case 1
S LIGHTLY D EGRADED S IGNAL (6G B / S T HROUGH 10” O F FR4)
With no equalization applied, this signal has an open eye; this shows that Eye Doctor II's clock recovery
worked on this signal without any equalization. We touched Find Frequency and otherwise accepted the
default state (PLL on, etc) on Eye Doctor II's clock recovery dialog.
The following screen-shot shows both EDrIn and EDrEqu traces with the same signal, because no equal-
ization is applied. The output of EDrEqu is reframed, meaning it is displayed on a different scale.
Since Eye Doctor II is not actually doing anything, we could have fed this signal directly into SDA II and
had SDA II recover clock from the data, to make the Eye show as in the following screen-shot.
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