Tektronix 071-0590-00 user manual Ideal pulse

BERT Technical Articles

ISI errors are caused when particular patterns (symbols) of data interfere with each other, leading to blurring and smearing of signals. An example of ISI can be illustrated by following a square wave (1010....). When the signal leaves the transmitter, the corners of the square wave are

sharply defined. As the signal travels a couple of miles over a transmission medium, attenuation (a function of distance) sets in. The digital waveform experiences phase delay, dependent upon the frequency of the signal components. As a consequence, the square corners become more rounded and blend into each other, making it difficult to tell where a pulse starts and stops. The tails of data pulses interfere with following data and reduce the eye opening (example shown in Figure 2).

	Superimposed
Ideal pulse	pulses with jitter
	modulation
position
- Peak	+ Peak
	Jitter modulation

Figure 2. Example of a "Eye Diagram" as viewed on an oscilloscope. Pulse rounding is due to phase dispersion, causing ISI.

By purposely adding noise, jitter or stress patterns, you can stress your design. An external attenuator can crank a known dB of loss into the transmission path. BERTs can add jitter by using a dithered clock so as to impair timing. Baseline wander can be tested by pattern stressing (what happens after a long string of 0s?), causing dc drift and reducing the amplitude of the decision threshold.

B-16 GB1400 User Manual