Understanding Bit Error Rate Testing (BERT) and Jitter Tolerance

A Bit Error Rate Tester (BERT) evaluates high-speed digital links by comparing a transmitted pattern against the pattern received. The test architecture consists of a Pulse Pattern Generator (PPG) and an Error Detector (ED). The generator transmits standardized test sequences, such as Pseudo-Random Binary Sequence (PRBS) lengths ranging from 2^7-1 up to 2^31-1, as supported by the Keysight Technologies (Agilent HP) N4903B.

At the receiver side, the error detector must synchronize with the incoming data. This process often relies on automated voltage and phase alignment or Clock Data Recovery (CDR) to reconstruct the clock from the incoming stream. Once synchronized, the detector performs a bit-by-bit comparison of the actual input versus the expected reference sequence. To test error-handling capabilities and system recovery, instruments typically support targeted error injection modes, allowing engineers to introduce single or fixed-rate errors manually.

In physical layer testing, a receiver must not only decode clean signals but also tolerate timing deviations, known as jitter, which occur in real-world transmission lines. Jitter tolerance testing evaluates receiver robustness by intentionally injecting timing distortion into the test signal and monitoring the bit error rate.

Advanced systems, such as the Keysight Technologies (Agilent HP) N4903B, act as jitter tolerance analyzers. These platforms feature built-in sources to inject various types of jitter, including Sinusoidal Jitter (SJ), Random Jitter (RJ), Periodic Jitter (PJ), Bounded Uncorrelated Jitter (BUJ), Duty Cycle Distortion (DCD), and Intersymbol Interference (ISI). For example, sinusoidal jitter can be swept across frequencies up to 1 GHz. The BERT determines the maximum jitter amplitude the receiver can tolerate at each frequency before the bit error rate exceeds a specified threshold.

When selecting a BERT, engineers must match the system capabilities to their interface data rates. For legacy or lower-rate applications, instruments like the Keysight Technologies (Agilent HP) 86130A cover 50 Mb/s to 3.6 Gb/s. For mainstream high-speed buses, the Keysight Technologies (Agilent HP) N4901B supports data rates up to 13.5 Gb/s, featuring a fast transition time of less than 25 ps and low output intrinsic jitter of less than 1 ps RMS.

For ultra-high-speed or multi-channel interfaces, modular architectures provide necessary scalability. The Anritsu MP1800A features a 6-slot chassis supporting up to 8 channels at 32 Gbit/s in a single mainframe, scalable up to 32 channels with multi-mainframe control. This platform covers data rates from 0.1 Gbit/s to 32.1 Gbit/s, extendable up to 64.2 Gbit/s with an external MUX/DEMUX, and delivers a typical rise/fall time of 12 ps.