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Overcoming interference

Signal conditioning aids high speed interface design.

High speed signalling is tricky business. As we climb beyond Mbit/s to Gbit/s data rates, the signal path behaves as a transmission line. At these speeds, reactive parasitics, pcb materials and layout geometries play a major role. Even minor impedance discontinuities will cause reflections, degrading the transmitted signal.
Signal degradation can be split into two broad categories: transmission loss due to the characteristics of the signalling medium; and distortion due to impedance discontinuities.
As a simple example, let’s consider an ideal homogenous 30in FR4 trace whose termination matches its characteristic impedance. Here, we have a consistent impedance and can concern ourselves only with the effect of the transmission media. Signal attenuation increases with frequency, due to skin effect and dielectric loss, with the net result that the media responds as a low pass filter.
Although we may be transmitting at a set data rate, the signal is composed of a continuous (Fourier) frequency spectrum dependent on the edge rate and the data pattern. The higher frequency components of the signal will have greater attenuation, with this effect increasing per unit length of the media. High frequency signal loss causes intersymbol interference – known as ISI – which closes the ‘eye pattern’, increases deterministic jitter and can eventually cause bit error problems.
An effective method of combating ISI distortion is through the use of signal conditioning. Pre emphasis is a technique where the transmitter signal is modified to increase the high frequency content such that, after traveling down the lossy transmission line, the received signal closely resembles the original unmodified signal. Some transmitters have configurable pre emphasis, allowing the user to optimise the driver signal depending on the amount of loss.

Ken Filliter

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