Teledyne LeCroy AORM - Advanced Optical Recording Measurements - Manual (Page 115)

AORM Software Package
923133 Rev A ISSUED: June 2013
113
• "Acquisition too small to apply EQ filters": The valid region of the waveform is reduced by
"EQ spacing" (see following explanation) on each side. This error message means that
the result would then have no valid points.
• "LP fc low & sample rate too high, can't EQ filter": This message is shown if current EQ
spacing is greater than 8191 samples, an implementation restriction. The EQ spacing is
set to correspond to 2T, assuming that the cutoff frequency is correctly set; it is
calculated from the cutoff frequency as follows:
EQ spacing in samples = 2.0/(fc * 26.16/8.2) * sample interval
Operational Notes
1. Even if the input data is already equalized, it is often helpful to tell the ODATA function
that it is not, but set the boost to zero. This greatly reduces noise. White noise has power
per Hz of bandwidth, and reducing the scope's bandwidth to around 8.2 MHz gets rid of
99% of white noise.
2. Applying high-frequency boost makes short pulses larger and has less effect on longer
pulses. The correct boost should not greatly increase the signal's overall amplitude.
3. The output of the equalization is not delayed, as it would be by an analog filter. We
compensate for the known delay through the digital filter and replace each input point
with the corresponding equalized point.
4. The FIR LP filter plus 3.2 dB boost from the three-tap EQ filter produces the transfer
function shown in the next figure when the FIR fc is set to 8.2 MHz. The highest peak is
20 log (dB) magnitude. The bowed trace below it is the real component of the TF. The flat
line at zero is the imaginary component of the TF. It is zero indicating that there is no
delay at all from input to output.
5. The computation time for the low-pass filter is generally longer than the time required for
the sum of the rest of the computations done by the ODATA math function. This is
because the low-pass filter is a finite impulse response filter (emulating the shape of a 6
th
order Bessel filter). It can require hundreds of multiplies-and-adds per sample in the
waveform. The higher the sample rate relative to the bit time, T, the longer the FIR is. It is
adequate to sample at 10 to 20 times the channel bit time, T. For 1x DVD, T is
26.16 MHz. Twenty times that is 523 MHz, so 500 MS/s is a good sample rate.
6. The three-tap EQ filter uses as input the point to be replaced and the points 2T away on
each side. Since 2T may not correspond to an integer number of scope samples, linear
interpolation between scope samples is used to get the values at exactly 2T away on
each side.