Tektronix 071-0590-00 user manual Bert Pattern Generation, Prbs Patterns

BERT Primer

BERT Pattern Generation

Almost all BERT generators include some type of pseudo-random patterns generation. These “PRBS” patterns are used to stimulate a “Device Under Test” (DUT) for the purpose of comparing actual received data with the “reference” data available to the receiver. These PRBS patterns are described below.

PRBS Patterns

A type of data pattern used by most BERTs is called “PRBS” - pseudo-random binary sequence. PRBS patterns are categorized by the bit-length of the binary word generating the pattern. For example, a seven-bit PRBS will be output as a serial stream consisting of 27 -1 (2 to the 7th power minus 1 which equals 127 bits). These 127 bits include all possible permutations of seven bits with the exception of “0000000”. Due to the nature of the Shift Register used to generate these codes, the code 0000000 never appears. Hence the PRBS pattern is always of the form “2n - 1”, e.g., 215 - 1, 223- 1.

ITU Specifications (CCITT O.150 and O.151) identify several types of PRBS patterns used for communications testing. These patterns are usually generated by hardware shift registers with appropriate feedback. If the shift register has n- stages, the maximum pattern length will be 2^n - 1.

If the digital signal is taken directly from the output of the shift register (non- inverted signal), the longest string of consecutive ZERO's will equal n-1. If the signal is inverted, n consecutive ZERO's will be produced.

Table 1 of ITU Specification O.150 lists several different types of PRBS test patterns. Some of the recommended test patterns use "non-inverted" signals (PRBS 9, 11, 23), some use "inverted" signals (PRBS 15), and some use both the "non-inverted" and the "inverted" signals (PRBS 20).

GB1400 User Manual B-3