## Page 34

# EDNE MAY 2015

designideas Rotational (or linear) measurement using an optical mouse sensor
is shown in Figure 1. The output from the channel
is connected to the DSO (yellow trace). The
attenuation and dispersion caused by the channel
have significantly degraded the waveform.
In the math option of the scope, there is a linear
equaliser block which takes filter coefficients
as input. Insert the linear equaliser block in the
signal path and enter the filter coefficients. The
output of the block is the green trace. The FIR
filter removes the ISI effect of the channel and
restores the waveform quite well.
Figure 1. Sdd21 of the PCB trace
The FIR coefficients obtained from the MATLAB
code are:
0.766, -0.115, -0.097, -0.119, -0.090, -0.099,
-0.083, -0.033, 0.028, 0.015, 0.042, 0.013,
0.024, -0.008, 0.002, -0.019, 0.000, -0.025,
0.005, -0.014, 0.011, -0.007, 0.021, -0.002,
0.014, -0.009, 0.009, -0.018, 0.003, -0.022
Figure 2. BertScope PRBS7 output (magenta);
signal at PCB trace output (yellow); FIR filter
output (green)
The filter has significantly improved the signal
integrity by removing the attenuation caused by
the PCB trace. Processing can be done in an
oscilloscope, offline, implemented in hardware,
etc.
There are some differences between the
BertScope output and filter output, which could
be due to the following:
There is some peaking in the first UI. This
is due to the large values of filter coefficients
a2, a3, and a4. Most of the high frequency amplification
is done by these coefficients, which
causes the peaking. The algorithm could be
modified to correct this.
H1(s) should be band-limited by using a
LPF function. This will give a band limited filter
and the error should be minimised over the
entire transfer function. This is also expected to
reduce the peaking in first UI. In this article, the
filter is band-limited by defining fmin and fmax,
which introduces errors.
The phase information is not included in
determining the filter coefficients, which introduces
some errors.
Calculation of the filter coefficients could be
improved by making these corrections in the
design algorithm.
Download the Matlab files (zipped archive)
from here.
33 EDN Europe | MAY 2015 www.edn-europe.com

EDNE MAY 2015

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