How to Tune Transmit Finite Impulse Response Taps

+ Functional Interconnect Test Solutions

Automate Lane Tuning Through Lossy Fixtures

Lossy electrical fixtures and interconnects can degrade a high-speed PAM4 transmit signal before it reaches the measurement point, reducing the eye opening and making accurate evaluation more difficult. Engineers can compensate for this loss by adjusting transmit Finite Impulse Response (FIR) tap settings, but manual tuning requires repeated changes to the tap values followed by repeated waveform measurements. As the number of lanes and supported signaling rates increases, that process can become time-consuming and difficult to repeat consistently. Engineers need a method that measures the signal after it passes through the actual lossy path, determines how the transmitted waveform should be adjusted, and applies the updated FIR settings back to the source. The tuning process also needs an appropriate timing reference so the digital communications analyzer can evaluate the incoming waveform and eye correctly. A defined Pseudo-Random Binary Sequence Quaternary (PRBSQ) pattern provides the stimulus used during the tuning process, while the selected signaling rate determines the tap configuration that must be optimized. The objective is to improve the transmitted eye through the specific fixture or interconnect before additional measurements are performed.

A practical approach requires a closed connection between waveform analysis and transmitter adjustment rather than relying on repeated manual changes. The analyzer measures the incoming PAM4 signal, calculates new transmit FIR tap values, and returns those values so they can be applied to the transmitted signal. Engineers then verify that the updated settings have opened the eye and review the resulting eye statistics. The tuning can be applied to any selected electrical lane and supported speed, allowing different paths to be optimized individually. The setup may use a second lane to provide a square-wave clock with a divider that matches the analyzer clock input rate, or the analyzer can use Clock Data Recovery (CDR) as an alternative timing source. Engineers can begin with PRBSQ9 and a defined signal target amplitude, then use longer patterns or additional analyzer functions after the initial tuning if needed. Precision time-base measurements and filtering can also be applied after the first tuning pass to review the resulting signal. The goal is to reduce the time required to determine suitable FIR tap settings, apply those settings consistently, and verify the resulting eye before moving into subsequent signal-integrity or error-performance measurements.

Automatic Transmit FIR Tuning Solution

Keysight's Functional Interconnect Test Solutions (FITS-8CH) addresses transmit FIR tap tuning by integrating pattern generation and automatic coefficient updates with a Keysight digital communications analyzer. Engineers select a transmit lane and supported speed, generate a PRBSQ pattern through the lossy fixture or interconnect, and use the analyzer to evaluate the resulting PAM4 eye. Through the FITS-8CH GUI, the Automate Eye Tuning function coordinates the measurement and tuning process. The analyzer calculates updated transmit FIR tap values and communicates them back to FITS-8CH, which applies the new settings to the transmitted signal. A second lane can provide a square-wave clock with a divider to match the analyzer input rate, or analyzer-based CDR can be used. Engineers can set the target signal amplitude and the number of taps for the selected signaling rate, then verify the optimized eye and review eye statistics after tuning. The resulting configuration and measurement data can be saved and reported, providing a repeatable way to tune individual lanes through lossy electrical paths before continuing with broader signal-integrity or error-performance validation.

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