Measuring error ratio. Introduction

In case of digital signal transmission the elements of the transmited and received signal may differ from each other. If an elementary signal (bit) of the transmited and received signal has different value we are talking about bit error The cause of the bit errors has a wide variety. Remember, that the transmission of the signals through the link happens on analogous way, so linear distortions can results inter-symbol interference, or noise will be added to the transmitted signal which causes that receiver make a wrong decision about the value of the elementary signal.

By definition Bit Error Ratio (BER) is

                          number of wrong elementary signals
                BER = ---------------------------------------------
                       total number of received elementary signals
When measuring bit error ratio transmitter of the measuring equipment sends a known measuring signal to the link. In the receiver the received signal will be compared to the measuring signal. We count the differing elementary signals and the received elementary signals.

Similar to the bit error ratio one can define error ratio for composite of the elementary signals (character, block, message, etc.), too.

Bit error measurers use Pseudo Random Binary Seguence (PRBS) as a measuring signal sequence. The transmitter and receiver of the bit error measurers use PRBS generators of the same construction. At the beginnig of the measurement a synchonizer circuit drives the receiver generator to the same position as the transmitter, then the comparision can be started. Synchonizer circuit interrupts the loopback of the PRBS generator of the receiver and refill its shift register with the bits of the received PRBS signal, which was sent by the transmitter. Then closes the loopback again and checks the existence of the synchronized state.

In addition to the displaying of the BER bit error measuring equipments visualize the number of errored bites received during the measurement in tabular form numerically and in histogram form graphically.

Beyond the above mentioned visualization measuring equipments can interpret the measuring results even more form. E.g.: