Bit Error Rate Tester (BERT)
Any electronic equipment or components of equipment that communicate with other electronics do so by sending and receiving bits of data. A bit error rate tester is an electronic testing device that measures whether bits of data are being successfully sent and received. The measure of the success of bit transfer is accomplished through the bit error rate (BER). One of the most commonly used types of BERTs is the high-speed BERT, which is used in the testing and design of many of today’s high-speed electronic systems.
Bit Error Rate (BER)
In electronic and telecommunications equipment, the bit error rate (BER) refers to the ratio of unsuccessfully sent bits of data to successfully sent bits. Error rates are typically very low, so BER is expressed in exponential form. For example, a BER of 10-6 represents that one bit error exists in every 1 million bits transmitted or received. Bits with errors usually have to be resent, slowing the overall transmission of data.
Many electronics and digital circuits within electronic devices are synchronized or coordinated through the use of a clock signal, which is a metronomic oscillating signal. However, some electronics that use high-speed communications do not make use of a clock signal. Instead, they rely on frequency references to produce an approximate clock known as either clock recovery or clock and data recovery (CDR).
Data rate is the speed at which bits of digital information are transferred by electronic devices. Data rate is usually measured in terms of bits per second or megabits per second. However, some data rates may be measured in bytes per second or megabytes per second.
Eye Diagram Mask
Bit sequences are often analyzed visually as diagrams based on several bit transmission measurements. These diagrams are known as eye diagrams, and they allow testers to isolate and solve data transfer errors. One tool used to help in analyzing and testing through eye diagrams is an eye diagram mask. Eye diagram masks are templates of the acceptable parameters for an eye diagram. The eye diagram mask is placed over an eye diagram display to show if the measurements of the eye diagram are within the acceptable limits.
Eye Diagram Pattern
Also known as an eye diagram or an eye pattern, this visual image is created by measuring the data generated through bit transmission analysis. The data is then converted into a pattern that allows for the simplified analysis of signal quality. By analyzing an eye diagram pattern, testers can diagnose signal transmission or reception problems and isolate the source of any problems found.
Jitter is defined as the noise, or short-term phase variations, in electronic signals. Jitter can be generated from a single source, or it can be the accumulated noise from several sources. However, accumulated jitter is often referred to as "wander". It is necessary to measure and analyze the jitter in high-speed transmissions because it can adversely affect data rates and overall system performance of electronic equipment or computers. The presence of jitter is associated with high BER.
To test the response of equipment to high BER, jitter can be injected into an electronic device or component as a form of low level phase modulation. The modulation can then be followed through the device using a BERT. The final analysis provides stress testing parameters helpful in understanding BER tolerance limits.
MUX is an abbreviation for multiplexer, an electronic device used for sorting digital or analog signals and directing them through a single input line. A MUX allows electronic devices to accept multiple types of signals through a single communications line, and is sometimes referred to as a multiple-input, single-output switch. In contrast, a DEMUX, or demultiplexer, selects one of multiple output lines through which a single signal should be directed. A DEMUX is often referred to as a single-input, multiple-output switch.
Many BERTs are designed to split a test signal into multiple channels for a side-by-side comparative analysis. This is known as parallel BER, and is helpful when testers want to see the results of different tests being run simultaneously on a signal.
Pattern depth is the amount of data that can be stored and processed by a BERT to form an eye diagram pattern. Some BERTs allow for user-defined pattern depth to be input or a choice of pattern depth selections.
Pseudorandom Binary Sequence (PRBS)
Also called a PN sequence, a PRBS is a variable series of bits created by a PRBS generator. A PRBS is not considered a truly random sequence because after the sequence runs its course, it is repeated. A PRBS can be inserted into electronic signals to test for bit errors. A PRBS-injected signal is considered to represent a worst-case transmission scenario.
Pseudorandom Word Sequence (PRWS)
A PRWS is similar to a PRBS, but instead of a sequence of pseudorandom bits, a PRWS generator creates pseudorandom, 16-bit words.
Serial decoding is a means of analyzing low-speed electronic signals travelling through a serial link. Serial decoding is accomplished with a hardware device called an oscilloscope. Software serial decoding is also possible.
Stress patterns are a sequence of digital bits inserted into an electronic device by a BERT for testing signal transmission. Stress patterns are generated with a PRBS generator. Different stress patterns are used for different types of tests. For example, the QRSS stress pattern is commonly used by a BERT to measure jitter.