Tektronix 071-0590-00 user manual

BERT Technical Articles

To stress the noise margin continuously, the tester alternates between 10001000 (pattern A) and 11101110 (pattern B) at a frequency below the AC coupling circuit’s cutoff frequency. Each pattern continues long enough for the circuit’s transient to die out. A duration of 3.14 time constants, or πRC, is sufficient. Therefore, the complete fixed pattern stored in memory has a period of at least 2πRC. The frequency of the pattern is at most 1/2πRC, which is the cutoff frequency, fL, of the coupling circuit.

Then if the bit rate is fc, the number of bits in the fixed pattern is:

N ≥ fc / fL

For example, if fl=32kHz and fc=1 Gbit/s, the fixed data pattern must be at least N = 1 Gbit/s/ 32kHz = 31,200 bits long.

Users can create different stress levels by combining the above patterns with patterns that have an equal number of 1s and 0s, thus causing no stress in a system with AC coupling. For example, the pattern 11001100 won’t change the DC content or noise margin. By combining various amounts of this balanced pattern with the previous unbalanced patterns, users can stress the system to varying degrees, up to 50% (see table below).

Table 1. Data patterns for noise margin stressing

Pattern A	Pattern B	Margin reduction
1100110011001100	1100110011001100	0 %
1000110011001100	1110110011001100	12.5 %
1000110010001100	1110110011101100	25.0 %
1000100010001000	1110111011101110	50.0%

Other patterns such as 1000000010000000, stress the noise margin by more than 50%, but they also stress the clock-recovery circuit, which may be undesirable.

Users who want to stress the clock-recovery circuit can do so by varying the transition density. This is because the system’s receiver gets its information from the received data signal. For nonreturn-to-zero data, the clock information is in the data transitions. Random data contains an equal number of 0-to-1 and 1-to-0 transitions, and a clock-recovery circuit is usually designed to expect this 50% transitions density. The circuit may have trouble with either a greater or lesser density.

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