Feedforward Equalizer Filter Topology for Low-Power Extended Taps
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Solution Overview
Problem
Feedforward equalizers (FFE) face a challenge in improving performance without significantly increasing power consumption due to the high power usage of FFE taps.
Innovation Solution
Incorporating a passive filter between the (N−1)th and Nth FFE taps, which mimics the behavior of more than N taps without additional power consumption, using a first-order high or low pass filter, and employing unique sample-and-hold circuits with preamplifiers and switched emitter followers to generate time-delayed signals and transconductance outputs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the number of FFE taps is increased to improve performance, then signal integrity and ISI compensation improve, but power consumption increases significantly
Solution Approach 1:
A passive filter is introduced as an intermediary component between FFE taps to generate extended tap coefficients. The filter processes the output of the Nth tap to create additional post-cursor coefficients (N+1, N+2, etc.) without requiring additional active S/H circuits and transconductance stages, thereby achieving enhanced equalization performance with minimal power overhead
Solution Approach 2:
The patent creates virtual copies of tap coefficients by filtering the Nth tap output. Instead of physically implementing additional independent taps, the system generates coefficient copies with modified values through the passive filter response, effectively simulating the behavior of additional taps at fraction of the power cost
2Reliability
If additional FFE taps are added to compensate for more severe ISI, then equalization performance improves, but device complexity increases
Solution Approach 1:
The passive filter serves as a complexity-reducing intermediary that mathematically generates the effect of multiple additional taps from a single physical tap structure. This eliminates the need to physically instantiate and configure numerous additional S/H circuits and transconductance stages, thereby maintaining device simplicity while achieving the equalization performance of a much longer FFE
Solution Approach 2:
The patent changes the parameter representation from discrete tap coefficients to a filtered continuous response. By applying the passive filter with specific pole-zero configurations, the system transforms the discrete tap coefficient set into an extended coefficient set that mimics the impulse response of a longer FFE, achieving equivalent performance with fewer physical parameters to manage
Data Source
AI summary
An N-tap feedforward equalizer (FFE) comprises a set of N FFE taps coupled together in parallel, a filter coupled between the (N−1)th FFE tap and the Nth FFE tap, and a summer coupled to an output of the set of N FFE taps. Each FFE tap includes a unique sample-an-hold (S/H) circuit that generates a unique time-delayed signal and a unique transconductance stage that generates a unique transconductance output based on the unique time-delayed signal. The filter causes the N-tap FFE to have the behavior of greater than N taps. In some examples, the filter is a first order high pass filter that causes coefficients greater than N to have an opposite polarity of the Nth coefficient. In some examples, the filter is a first order low pass filter that causes coefficients greater than N to have the same polarity as the Nth coefficient.