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Optimising equalisation for FFE, CTLE, DFE

Posted: 03 Nov 2015 ?? ?Print Version ?Bookmark and Share

Keywords:duty-cycle distortion? crosstalk? electromagnetic interference? electromagnetic interference? ISI?

Combining equalisation at both the transmitter and receiver in a high-speed serial-data channel allows designs to reach more than 28 Gbit/s. Equalisation will continue to play a key role as we switch from base band, two-level non-return to zero (NRZ) to four-level pulse-amplitude modulation (PAM4) at lane rates in excess of 50 Gbit/s.

The ideal equalisation scheme inverts a channel's frequency response. Such inversion, which can be implemented at the transmitter, receiver, or both, can remove intersymbol interference (ISI). That leaves just random noise, jitter, DCD (duty-cycle distortion), crosstalk, and electromagnetic interference behind.

Figure 1 shows the frequency response, SDD12, and pulse response of a pretty good channel. The number of UIs (unit intervals) over which the pulse response extends indicates the number of symbols that interfere with each other. The longer the channel and the greater the signal bandwidth, the smaller the UI, the larger the number of interfering symbols, and the greater ISI impairment.

Figure 1: The frequency and pulse response of a high speed serial channel drops as frequency increases.

An ideal equaliser would invert the channel response and convert the pulse response back into a square pulse.

Three types of equalisation are used in high speed serial designs.

Tx FFE (transmitter feed-forward equalisation) modifies the amplitudes of symbols surrounding transitions while keeping the transmitted power constant. In principle, Tx FFE should be able to invert ISI if the number of symbols modified, that is, the number of "taps," extends over the entire length of the pulse response. In reality, Tx FFE seems to peter out after about three taps. Three taps of Tx FFE amounts to a blunt instrument: increase the ratio of high frequency to low frequency signal components to counter the channel's low-pass nature. That blunt instrument doesn't help fix the ugly nuances of channel frequency response.

CTLE (continuous time linear equalisation) is a linear filter applied at the receiver that attenuates low-frequency signal components, amplifies components around the Nyquist frequency, and filters off higher frequencies, as shown in figure 2. CTLE gain can be adjusted to optimise the ratio of low frequency attenuation to high frequency amplification. Of course, it runs out of steam when the attenuation pushes the low frequency signal components down into the noise and, like 2-3 tap Tx FFE, CTLE only addresses the gross low-pass filtering effect of the channel.

Figure 2: CTLE filtering options.

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