Bit error ratio (BER) measurement is the fundamental measurement of the quality of the fiber optic communication system. It measures the system s probability that transmitted bits will be correctly received as logic ones and zeros.
Bit error ratio is the ratio of the number of bits received incorrectly compared to the number of bits transmitted in a specified time interval or quantity of bits. The typical acceptable BER levels range from 1e 9 to 1e 12.
Equipment for testing BER
The equipment used to test a fiber optic system s BER is called BERT (bit error ratio tester). BERT has two fundamental parts: a signal pattern generator and an error detector.
What is a signal pattern generator and how does it work?
The pattern generator is responsible for producing a known data sequence to the system under test. The test patterns are often designed to intentionally stress some aspect of the system under test such as a pattern sequence that is difficult for the clock recovery system to synchronize.
Signal patterns being generated
The patterns being generated also must be similar to the real traffic that will be used in the system. The most common pattern is the pseudo random binary sequence (PRBS).
PRBS patterns can produce all possible combinations of ones and zeros for a given pattern length. For example, a 2^7 1 PRBS pattern will produce all possible combinations of 7 bit binary numbers.
Pattern generators can generate very long pattern sequence such as a 2^31 1 sequence which has all combinations of 31 bit binary numbers.
What is a error detector and how does it work?
The error detector is on the receiving end of the system being tested. Its functionality is to determine whether the received data matches the transmitted pattern.
The system under test is being fed with the data pattern from the pattern generator. The system has a decision circuit to determine whether the received bit is a one or a zero. The system s decision circuit output is then fed to the error detector.
The error detector has a built in internal pattern generator which generates reference pattern identical to the pattern from the pattern generator.
The error detector s internal reference pattern signal is then synchronized to the output from the system under test. The the bits are compared bit by bit between these two signals. Then the bit error ratio is determined by dividing the number of incorrectly received bits to the total number of bits transmitted.
The relationship between signal power level and bit error ratio (BER)
Another very import system characteristic is called power penalty. What it measures is the relationship between the attenuation between the transmitter and receiver and the bit error ratio. Simply put, the higher the attenuation the worse the bit error ratio. This is often being illustrated on a chart which is a line with negative slope.
In a typical BER characterization of a high speed system, as the received power is decreased, the signal to noise ratio is reduced and the probability of a bit being received in error increases.
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