Teledyne LeCroy AORM - Advanced Optical Recording Measurements - Manual (Page 97)

AORM Software Package
923133 Rev A ISSUED: June 2013
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In the case of threshold-insensitive parameters, it usually suffices to use a fixed threshold
somewhere in the middle of the optical recognition waveform. Even if the signal’s middle shifts,
the fixed threshold is usually adequate.
Additionally, if the signal is AC coupled, it will tend not to shift much, and the fixed threshold will
be perfectly adequate.
The problem arises when what is required is a DC-coupled signal with a threshold that changes
dynamically throughout the waveform. There remains a possible solution, but the scope setup is
slightly more complicated.
Consider the fact that AC coupling can be regarded as rejection of the DC component of a signal,
or subtracting it from the signal. In many systems, the threshold is determined in precisely this
manner by applying a low-pass filter to the signal, and then applying this value, with the signal
itself, to the input of a comparator. If a threshold value determined in this manner is available in
the circuit, the threshold signal, along with the optical recognition data signal itself, should be
acquired. Waveform math can then be used to subtract the threshold signal. This is done by
defining a trace as the Arithmetic Difference of the raw data signal and the threshold signal. The
new trace is then used as the optical recognition data signal in the parameter calculations.
Regardless of how the signal is coupled, there are other considerations involved in determining
the appropriate threshold. If waveform math is used, the threshold is always 0 V. Otherwise, the
optimum threshold is best determined using the pmidl parameter.
Some optical recognition standards define the middle level of the 3T signal as the “decision level.”
Pmidl
configured for the single 3T pits/spaces is an ideal candidate for the best threshold value.
Another candidate is the pmidl value calculated using the entire range of n indices possible. In
this way, pmidl calculates the best overall threshold level as a weighted average of middle levels
calculated for each n index.
In AORM, the ODATA function can be used to remove these effects. Its “leveled” output subtracts
the “threshold” (low frequency content of the signal) from the input data.