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Applying Equalization in High Frequency Network Higher-speed networks require new methods to maintain signal integrity, thus achieving high bit rates while maintaining low error rates. At lower data rates (say, below 2.5 Gbps), a clean transmitter and a good transmission path sufficed. Designers sometimes compensated for loss of high frequency content in the transmission medium by applying pre-emphasis to the transmitted signal. The idea was simply to boost the initial high frequency content. At somewhat higher rates (up to 5 Gbps), improvement of the channel to reduce losses and use of transmitter pre-emphasis become requirements. But at rates above 5 Gbps, more advanced techniques are needed. In particular, to keep the transmission medium’s cost reasonable over distances of more than a few inches, applying equalization at the receiver is essential to keep error rates low. But how does one model a backplane or an equalizer in such situations? Fortunately, new technology allows engineers to model the effects of several choices for transmission lines and to “virtually probe” the signal at the input to the receiver and after the equalization. The engineer can even remove the effects of attaching a probe to the circuit, of passing the signal through an oscilloscope front end, or of any other fixtures/connectors/cables in the test setup. For example, LeCroy’s Eye Doctor enables these key measurements through software applied to a signal captured at the transmitter’s output. Of course, one can model signal shape and system performance in any electrical circuit. However, the largest modeling errors usually come in predicting what the output signal will look like from the transmitter. The new technique captures the real signal at the transmitter. The user can then test different models for the transmission line (“virtual” swapping of backplanes). And for a particular backplane, the user can also predict the signal shape at the output of an equalizer in the receiver. He or she can even try modeling different equalizers by specifying the number of taps of DFE (Decision Feedback Equalizer) or FFE (Feed Forward Equalizer). The user can also specify the properties of the probe, oscilloscope front end, and fixtures used in the test setup and correct for any signal distortion. This becomes more important as edge rates rise and signal amplitudes decrease. For best accuracy, signals should be probed at the transmitter output where the transmission path has not degraded their high frequency content. Any high bandwidth scope or probe will add high frequency noise to the acquired signal. It is much easier to correct for this – before the equalizer boosts high frequency content – if the added noise is much smaller than the signal’s high frequency content (which the transmission medium may degrade as much as 60dB at 10 GHz). If you are designing or testing next generation networks, equalizers are an excellent technique. New test equipment allows you to check a variety of equalization approaches to see which produces the best results. Dr. Michael Lauterbach is Director of Product Management at LeCroy. You can reach him at michael.lauterbach@lecroy.com. |