Training eye doctor ii equalizers – Teledyne LeCroy Eye Doctor II User Manual
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Eye Doctor II Software
bits, which depends on its ability to accurately decode bits - a case of circular logic. Depending on the
strength of DFE applied, a single bit error may lead to a long run of errors.
Erasure DFE is used for this problem and improves the situation. Erasure DFE effectively sets a band
around the threshold value. When the signal falls inside the band at the time of slicing, it indicates an
uncertainty in the bit decoding. The receiver, although obliged to decode the bit, can then decide not to
apply this bit to the decision feedback, since the decoding was not certain enough. In operation, if era-
sure DFE is utilized and the signal is within the voltage delta about the threshold specified, a voltage
value that is the average of the ideal +1 and -1 values is applied to the DFE delay taps, thus causing the bit
to have no effect on decision feedback.
The Erasure DFE checkbox enables or disables erasure DFE. This feature excludes certain bits within the
specified Erasure Delta from the decision feedback.
The Erasure Delta control allows you to set the indecision band around the DFE threshold.
Training Eye Doctor II Equalizers
Eye Doctor II provides receiver equalization with its Equalizer components. Equalizer capability is made
up of software implementations of
Continuous Time Linear Equalization (CTLE)
, and
Decision Feedback Equalization (DFE)
. Each can be enabled or disabled. The data signal
can pass only through the DFE (which may be off, in which case the signal is unchanged); or through the
CTLE and then the DFE; or through the CTLE and the FFE and the DFE.
DFE requires a clock, which Eye Doctor II recovers from the data after CTLE and/or FFE. As the following
cases illustrate, if DFE is enabled you may have to enable one or both of CTLE and FFE to make sure Clock
Recovery can accurately recover a clock.
Both the FFE and the DFE can automatically Train or find their own tap values. The Train FFE & DFE but-
ton on the main Equalizer dialog first trains the FFE, finds the bit rate (shown on the
dialog), and then trains the DFE including clock phase adjustment.
Three basic cases are covered in this section.
1.
- The first (and simplest) has the signal is slightly degraded, so it forms an open eye without
equalization.
2.
- The second case has the eye is closed, so some equalization is required before Clock Recov-
ery.
3.
- The third case has the eye severely degraded (beyond just being closed). In this severe case,
DFE automatic training may not work unless a bit of help from the CTLE is applied and the input from
the DFE is taken from CTLE output.
Generally, the following pictures show a zoom of the input to Eye Doctor II (EDrIn), in the upper left; a
zoom of the output of Eye Doctor II's equalizer (EDrEqu), in the lower left; and an eye diagram formed by
SDA II using EDrOut as the signal source, and EDrClock as the explicit clock source.
This setup has Eye Doctor II doing the clock recovery, it has the PLL setup; SDA II does not have a PLL ena-
bled.
Eye Doctor II also needs to know the signal's bit rate to a reasonably high accuracy. For this reason there
is a Find Frequency button on its Clock Recovery dialog; if the signal is appropriately equalized for Clock
Recovery, it does work; and SDA II needs to know the signal's bit rate (at least nominally) to form an
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