Tektronix 071-0590-00 Received Data Pattern Reference, Data Input Amplifiers, Error Comparator

BERT Primer

Received Data Pattern Reference

If the transmitter is using some other known standard pattern, such as a particular Mark-Space Density pattern, a copy of that pattern must also be generated by the receiver. The BER reference data may be a User Defined pattern. The same pattern used by the transmitter must be loaded into the analyzer's memory. Finally, an application specific data stream, supplied at the customer site, may be fed directly to the BERT Receiver as an external Reference Data source. Not all BERTs have this feature, but our GB700 and GB1400 both do.

Data Input Amplifier(s)

The BERT input amplifier stage must accommodate a range of data signal amplitudes, DC offsets and input termination impedances. Most BERTs will handle either single ended or differential signals and provide a 50-Ohm termination. Some BERTs, including our GB700 and GB1400, offer either 50 or 75 Ohm input impedance. BERTs typically do not have a great deal of input gain to accommodate small signals, as oscilloscopes or spectrum analyzers do. Most of our BERTs require 500 mV signal amplitude for minimum data input levels.

Error Comparator

The BERT receiver compares the incoming data stream bit-by-bit with its internal pattern using an Exclusive OR gate (XOR) and responds to non- matching inputs by sending a “1” to a built-in error counter. The number of errors compared with the number of bits received determines the BER, or Bit- Error Rate. This is usually expressed as a power of 10 using a negative exponent, since the fraction of errored bits to total bits must be 1 or less.

BER Computation

Ideally, the BER will be smaller than 10-9(one errored bit in one billion). A rate of 10-6is considered marginal. The size of the counter registers used to count total bits and errored bits determines the range of BER which the BERT can measure. The limits will be in terms of how large a BER can be measured (i.e., how “bad” the error rate is), as well as how small (how “good”).

Other BER Measurements

BER measurements may be expressed in various ways: Simple errored bit rate, the ratio of errored to total bits, is usually stated in scientific notation using a negative power of ten exponent. Other BER statistics include: Total Errored Bits and Total Bits, Errored Seconds (the number of seconds which contained at least one bit error), Error-Free Seconds, and other definitions. Many of these are specified and defined in BellCore and ITU Test Procedures, and other standards.

B-8 GB1400 User Manual