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