Tektronix 071-0590-00 user manual Μ F Avergage = Average =

BERT Technical Articles

	0.1 μ F
	D’
	C	R	50
			(a)
D’	Avergage = -0.5		Average = 0.5
1
-1				Time
	Pattern A		Pattern B	Pattern A
		1	(b)
F
2
1
-1				Time
	0.5		RC	0.5
			(c)
Figure 3. (a) Most transmission systems use AC coupling.

(b)Consequently, a bit-error-rate tester can measure the system's noise margin by generating signal patterns with an unbalanced number of 1s and 0s.

(c)The result is baseline wander, which reduces the margin between the received signal, F, and the threshold.

As long as the number of 1s and 0s in the data pattern is equal, the pattern’s DC content is fixed, and the coupling circuit doesn’t block anything important. If the balance changes at a low frequency, the coupling circuit may block that frequency, causing F to wander up and down. This baseline wander reduces the noise margin.

Properly designed transmission systems ensure that data patterns have no frequency components below the coupling circuit’s cutoff, essentially eliminating baseline wander. But a BERT can purposely introduce some baseline wander to measure the noise margin.

An example is an input signal D’ that starts with a 10001000 pattern and switches to a 11101110 pattern. During the first imbalanced pattern, the average voltage, -0.5 V is blocked by the coupling circuit, and the output F has an average of 0 V. When the pattern changes to 11101110, D′ averages 0.5 . After a transient with a time constant of RC=5μs, the average of F returns to zero, and the signal has wandered by half of its amplitude.

This baseline wander reduces the margin between F and the threshold by 50%, to 0.5 V from 1.0 V. If this reduction caused the error rate to become measurable--say 10-6--the conclusion is that the original margin was about 50%.

GB1400 User Manual B-25