This section discusses typical FFE characteristics and displays. Let us know if you would like the FFE Analysis Tool enhanced with additional capability.

A typical FFE for use with serial data at a bit rate (BitRate) of 6.25 Gbps has one pre-cursor and two postcursor taps. These taps are defined in units of dB emphasis with the post-cursor emphasis set to -1 dB, and the two post-cursors set to -4 and -2 dB. The overall FEE response is defined to have normalized gain of unity at its Nyquist frequency (BitRate/2 = 3.125 GHz). Additionally, the FFE is defined to have a low pass filtering (LPF) defined with two poles at 20 GHz and 40 GHz.

This FFE plus LPF design is simulated with 32 samples per bit (SamplesPerBit) to show its waveform and spectrum responses. The waveform response is defined for use with a bit pattern using a repeating set of 4 consecutive bit = 1 values followed by a set of 4 consecutive bit = 0 values (4_1_4_0). For the 6.25 Gbps data rate, one unit bit time interval (UI) is 160 psec.

The FFE + LPF waveform pattern is shown here.

Under typical system usage, the bit pattern is more random using a pseudo-random bit stream (PRBS) possibly with encoding. Use of the 4_1_4_0 bit pattern for the current analysis make the emphasis characteristic of the FFE more visible. Observe that the pre-cursor and post-cursor levels are as specified. The pre-cursor transitions from level -0.269 to -0.302 resulting in -1.0 dB emphasis (20*log10(-0.269/-0.302). The first post-cursor transitions from 0.537 to 0.339 (-4 dB). The second postcursor transitions from 0.339 to 0.269 (-2 dB).

The spectrum responses for the FFE and for the FFE+LPF are shown here.

Observe that as specified, unity gain ( 0 dB) occurs at the Nyquist frequency (3.125 GHz). Also observe, the typical FFE spectral amplitude vs frequency oscillating from -5.4 dB to 0 dB every 6.25 GHz. The additional filter poles apply a roll-off to the FFE spectral amplitude response.

The FFE Analysis Tool provides an easy exploration of the response characteristics for a defined FFE plus filter.