What is the difference between BERT and eye diagram analysis?

BERT counts actual bit errors by comparing transmitted and received patterns, giving a direct BER number. Eye diagram analysis on a sampling oscilloscope overlays many bit transitions to visualize signal quality (jitter, noise, ISI) but does not directly count errors. Modern BERTs often include eye diagram capability, combining both error counting and signal integrity analysis.

Test & Measurement

Bit Error Rate Tester

Q: What is a PRBS pattern?

PRBS (Pseudo-Random Bit Sequence) is generated by a linear feedback shift register. PRBS-N produces a sequence of 2^N - 1 bits before repeating. PRBS-7 (127 bits) is used for short tests; PRBS-31 (2.1 billion bits) stresses the link with long run lengths and low-frequency content that exercises AC coupling and CDR tracking.

Q: How long must a BERT test run?

To measure BER = 1e-12 with 95% confidence, you need approximately 3e12 bits. At 10 Gbps, this takes 300 seconds (5 minutes). At 100 Gbps, 30 seconds. The general rule is: test duration = confidence_factor / (data_rate * target_BER). For 99% confidence, the factor increases to approximately 4.6.

A Bit Error Rate Tester (BERT) is a test instrument that generates known pseudo-random bit sequences (PRBS), transmits them through a device or link under test, and compares the received bits against the expected pattern to count errors and compute BER. BERTs are essential for verifying digital communication links, serial data interfaces (PCIe, USB, Ethernet), optical transceivers, and high-speed RF digital channels. Modern BERTs support data rates from Mbps to 100+ Gbps with integrated eye diagram analysis and jitter decomposition.

Category: Test & Measurement

Pattern: PRBS-7 to PRBS-31

Understanding BERT

A BERT consists of a pattern generator (PG) and an error detector (ED). The PG produces a known PRBS sequence at the desired data rate and feeds it through the device under test. The ED receives the output, synchronizes to the pattern, and counts every bit that differs from the expected value. The ratio of errored bits to total bits gives the measured BER.

PRBS patterns are chosen because they exercise all possible bit combinations within their period length. PRBS-7 (2⁷−1 = 127 bits) is used for quick tests. PRBS-31 (2³¹−1 = 2.1 billion bits) stresses the link's AC coupling and clock recovery with long run lengths and broad spectral content.

BERT Test Duration

Minimum bits for target BER:
N_bits = −ln(1−CL) / BER_target
CL = confidence level (0.95 for 95%)

Test time:
T = N_bits / Data Rate

Example: BER=10⁻¹², 95% CL, 10 Gbps:
N = −ln(0.05)/10⁻¹² = 3×10¹² bits
T = 3×10¹²/10¹⁰ = 300 s (5 min)

PRBS Pattern Comparison

Pattern	Length	Max Run	Use Case
PRBS-7	127 bits	7	Quick functional check
PRBS-15	32,767	15	General BER testing
PRBS-23	8.4M	23	Optical/SONET
PRBS-31	2.1G	31	Stress test, CDR/AC coupling

Common Questions

Frequently Asked Questions

What is a PRBS pattern?

Generated by an LFSR, PRBS-N produces 2^N−1 bits before repeating. PRBS-31 stresses AC coupling and CDR tracking with long run lengths.

How long must a BERT test run?

For BER=10⁻¹² at 95% confidence: ~3×10¹² bits. At 10 Gbps: 5 minutes. At 100 Gbps: 30 seconds.

BERT vs eye diagram?

BERT counts actual errors (direct BER number). Eye diagrams visualize signal quality (jitter, noise, ISI) but don't count errors. Modern BERTs combine both.

Related Terms

← Bit Error Rate Bit Interleaving →