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Examining closed eyes: CRJ and CDJ

26 Jan 2016  | Ransom Stephens

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Inter-symbol interference (ISI) closes eye diagrams at high data rates. The skin effect and dispersion—the frequency dependence of the dielectric "constant"—combine to cause the messy low-pass nature of channels, or equivalently, in the time domain, to smear the pulse response across many bits. As data rates increase and higher frequency components are introduced, ISI gets worse.

Figure 1: Closed Eye (photo by Ransom)

The combination of one, two, or three types of equalisation open the eyes enough for the decision circuit to identify symbols—feed forward equalisation (FFE) at the transmitter, continuous time equalisation (CTLE) at the receive, and/or decision feedback equalisation (DFE) also at the receiver. For non-return to zero (NRZ) PAM2 (2-level pulse amplitude modulation) signals, there's just the one eye with the base band symbol for 1s high and 0s low. For PAM4 (4-level pulse amplitude modulation) there are three eyes, one separating each of the four symbol levels, since PAM4 encodes two bits in each symbol. In both cases, at high rates, the signal that enters the receiver has closed eyes.

Because ISI closes the eyes, the trick to analysing them is a combination of "embedding" the receiver equalisation scheme(s) and using carefully chosen test patterns. The test patterns are chosen to control ISI. "Embedding" receiver equalisation amounts to emulating it within your test equipment or applying the equalisation to a captured waveform offline. With the eye opened, you can see how much ISI is left uncorrected and the impact of other signal impairments—just like you do for open eyes.

Emerging 56 Gbit/s specifications offer new approaches to gauging signal impairments independent of ISI.

Start with a square wave clock-like signal, 1010... for NRZ-PAM2 or (11)(00)(11)(00) for PAM4, and measure CRJrms (rms clock random jitter), and CDJpp (peak-to-peak clock deterministic jitter). Since there's no signal on the data, no symbols to interfere, CRJrms and CDJpp measure RJ (random jitter) and DJ (deterministic jitter) independent of ISI, but retain the random noise and other uncorrelated impairments like crosstalk, PJ (periodic jitter), and any other EMI (electromagnetic interference). Apply the transmitter equalisation scheme, if there is one, and embed the receiver CTLE and/or DFE as appropriate so that the square waveform also includes the equalised version of all the signal impairments that are left over.

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