What is the role of equalization in recovering a signal degraded by channel loss?
Channel Equalization
Equalization is the technology that has enabled the dramatic increase in serial link data rates from 1 Gbps to 112 Gbps over the last two decades, compensating for the channel impairments that would otherwise limit throughput.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
What happens if the channel loss exceeds the equalization budget?
The eye remains closed after equalization → the BER exceeds the target → the link does not function. Solutions: reduce the trace length (less loss), upgrade the PCB material (lower Df), add a retimer or redriver IC (regenerates the signal mid-channel), or reduce the data rate (lower Nyquist frequency → less loss).
How does the SerDes adapt the equalization settings?
Link training: during link initialization, the TX and RX negotiate the optimal equalization settings. The RX sends back-channel feedback to the TX indicating the optimal FFE tap values. The RX CTLE and DFE adapt continuously using the incoming data. The training converges in 10-100 ms, and the settings are maintained during operation. If the channel characteristics change (temperature drift): the adaptation tracks the change.
What is a redriver vs a retimer?
Redriver: an analog device that amplifies and equalizes the signal without recovering the data. Provides 10-20 dB of channel loss compensation. Does not remove jitter or bit errors (simply re-amplifies them). Low latency (< 1 ns). Retimer: a digital device that recovers the data (CDR), removes jitter and errors, and retransmits a clean signal. Provides a full channel reset (the downstream channel gets a fresh signal). Higher latency (5-50 ns). Required for PAM4 at 56+ Gbps (redriver cannot adequately equalize PAM4 over long channels).