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Buying a BERT: Making an Informed Decision

2021-03-22  |  7 min read 

You know you need a bit error ratio tester (BERT), so now comes the somewhat daunting task of determining which one is best for your application. The last thing you want is to purchase an instrument that meets your needs today, but not in the future. This can be difficult to achieve in the constantly evolving world of computer bus and communications interfaces. As each new standard is introduced, data rates increase and so do the design challenges. These higher speeds drive additional need for equalization techniques in transmitters and receivers to compensate for channel loss. Add to this the signal degradation caused by printed circuit boards or cables between the transmitter and receiver, and compensation can be a real challenge. To help reduce the amount of time you spend selecting a BERT, consider these four things:

1. High-Speed Interface Standards

If you are using (or plan to use) any of the following high-speed digital interfaces, then you know a BERT is a must have. Look for a BERT that not only offers these interface options, but also includes the necessary hardware to fully characterize your system.

  • High-Speed Serial Receiver - Computer Buses & Backplanes
    • Application Examples: QPI, PCI Express, SATA, SAS, USB3, TBT, DP, SD, UHS II, MIPI D-PHY/M-PHY, HDMI
    • Typical Requirements: Data rates < 16 Gb/s, calibrated jitter, SSC, ISI and S.I., clock recovery, pattern sequencing, no loopback, 3-wire or multi-level
  • Backplanes, Cables, SERDES, AOC, Repeaters
    • Application Examples: ​10GBase-KR/-KR4/-CR4, CEI, IB, TBT, CAUI, CAUI 2/4, CEI-56G/112G, 400 GbE
    • Typical Requirements: Data rates between 10 Gb/s to 58 Gb/s, de-emphasis, crosstalk, PRBS, PAM4, NRZ
  • Optical Transceivers and Subcomponents
    • Application Examples: 10/40/100/400 GbE, 32G/64G FC, CFP2/4, PON, OTN, 8G/16G FC, QSFP, SFP+, QFP
    • Typical Requirements: Data rates between 3 Gb/s to 58 GB/s, PAM4, NRZ, PRBS, framed bursts

2. Link Training

High-speed interfaces such as PCIe, SATA and 10GBASE-KR require that the transmitter and receiver communicate with each other to optimize TX de-emphasis and RX equalization. A BERT acts like a link partner in your system to help interactively perform this link training; reacting to both equalization and de-emphasis requests. A BERT needs to understand these protocols and react to these command requests.

3. Channel Loss Emulation and Compensation

With increasing data rates, it is critical to understand channel loss between the transmitter and receiver in digital designs. A BERT can emulate real-world conditions to provide insight on losses in your design and enable you to compensate for those losses. Here are some examples of how a BERT can help:

  • Signal De-Emphasis and Channel Loss Compensation: Common when operating above 5 Gb/s, you need to characterize receiver ports under realistic and worst-case conditions. Your BERT must provide a pattern generator signal that will accurately emulate transmitter de-emphasis, have an adjustable number of tap levels and compensate for loss in the test setup.
  • Input Tolerance Testing: Crosstalk effects can be emulated using multiple BERT channels with one channel used as an aggressor lane. Look for a BERT with adjustable data rates, several pattern standards and jitter functions. Random interference (RI) and sinusoidal interference (SI) options with software control on source and aggressor lanes are not necessary, but very useful. Ideally, the BERT should have several automated tolerance test capabilities to ensure repeatability and reduced setup time.
  • Emulate Channel Loss with Integrated and Adjustable ISI: With pulse amplitude modulation (PAM), one or more symbols can potentially interfere with subsequent symbols causing noise and degraded performance. This results in intersymbol interference (ISI). Today’s high-speed digital receivers are specified to tolerate a certain amount of ISI, but you still need to understand the impact to the system. A BERT needs to provide integrated and adjustable ISI to emulate channel loss during receiver characterization. To maximize your investment, look for a BERT which feature adjustable and independent ISI levels on each channel.

4. Characterizing NRZ and PAM-4 Signals Operating Up to 64 GBd

Signal integrity issues with 25 Gb/s NRZ and PAM4 device testing pose a challenge when characterizing those devices. Real-world conditions should be tolerated by the receiver under test without exceeding the desired bit error ratio (BER) level. To help simulate these conditions, look for a BERT with an input receiver test that includes jitter tolerance, interference tolerance and level sensitivity margins that are applicable for NRZ and PAM4 devices. Also, PAM4 receivers require additional margin testing for level non-linearity, crosstalk effects from adjacent lanes and vertical eye closure. Make sure these are included when evaluating PAM4 options with a BERT.

Next Steps

As with any major purchase, you want to make an informed decision when investing in a BERT and this guide will help. To truly get the full return on your investment, it is important to identify not only your needs today but also in the future. Device characterization using integrated jitter sources, de-emphasis capabilities, simulated channel loss and emulated aggressor lanes is becoming commonplace. If you want to make your job easier (and who doesn’t), look for a BERT that integrates transmitter, receiver, and test set-up connections. It will lead to more robust test results, simpler calibration and a reduced number of re-adjustments.

Keysight knows there is more to selecting a BERT than specifications. We understand BERT applications and the need to successfully test your system under real-world conditions. To learn more about Keysight’s BERT options, click here. If you still have questions, give your Keysight salesperson a call.