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
To see the actual publication please follow the link above