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SIGNAL INTEGRITY : Equalisation magic: handle with care

In the last article I looked at pre-emphasis and, more importantly,the care needed in its application when you are implementing thenew high-speed serial buses. An alternative—or addition to—preemphasisis equalisation, which is a cornerstone of LVDS (low-voltagedifferential-signal) high-speed bus systems. You can argue thatequalisation is far better than pre-emphasis—although generallymore complex and expensive to implement. Equalisation is complex;as an aid to understanding it, the following is a somewhat novel descriptionof its operation. Equalisation is also presenting a new set of real-time measurement challengesthat you can address with some newand exciting ways of looking into achip in real time to measure—and, ifnecessary, set the required degree of—equalisation.

Equalisation forms the foundation of successful high-speed bus implementation. Modern PTSN (publictelephone switched-network) broadband modems, which use twisted-pair telephone wiring, owe their existence to equalisation, as do many other highspeed data-communication systems. As an academic, it might be appropriate for me to give a theoretical account of equalisation and then discuss the mathematical design of its implementation. However, some time ago, a friend and colleague, Dr Peter Harding, illustrated equalisation using merely sinusoidal waveforms, which captured the essence of the topic and offered an insight into the concept of equalisation. Peter noted that in a nonlinear system, the sinusoidal components or different harmonics of a digital signal travel at different speeds in a copper conductor and arrive out of phase. He explained the common misconception that equalisation is put in place only to compensate for the attenuation of high-frequency components: phase effects also matter.

Peter demonstrated the simple addition of the harmonic components of a digital signal. He then showed the addition of the harmonic components with incorrect amplitudes, and finally he demonstrated the effect of adding out-of-phase signal components. Every textbook has an example of the sinusoidal harmonic components that add, with suitable amplitudes, to give a square waveform, and there is one in the Web version of this article at www.edn-europe.com/article. asp?articleid=3224. However, Figure 1 shows the same signal components with unchanged amplitudes but with a range of phase shifts. The summed (purple) waveform shows significant distortion due to these phase shifts alone. Equalisation can—indeed, it must—compensate for both transmission- line high-frequency attenuation and phase errors.

Today many LVDS serial-bus receiver chips are supplied with built-in adaptive or programmable equalisation filters that compensate for the highfrequency losses and correct out-ofphase signal components, effectively reshaping the received digital waveform. The dilemma facing the engineer is to assess the effect that the equalisation filter is having on the received signal; if the equalisation circuit in the chip is not having the desired effect, it might be necessary to reset the degree of equalisation or redesign the transmission path. Fortunately, one option on modern high-performance oscilloscopes is an equalisation-emulator function allowing the engineer not only to probe an LVDS bus in real time, but to estimate the effect of the equalisation circuit on the waveform, further along the signal path where the engineer cannot directly probe it. The equalisationemulator function is in general a true DSP (digital signal processing)- programmable filter that mimics the built-in adaptive or programmable equalisation filters in the LVDS serialbus receiver chips. Figure 2 is an example of a serial-data link with an emulated eye diagram; one of the most important attributes of an oscilloscope-based serialdata- link-analysis equalisation emulator is the instrument’s ability to visualise and give a comparison of eye diagrams before and after equalisation.