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1.4 System Perspective on Jitter, Noise, and BER

This section briefly discusses jitter, noise, and BER within a high-speed linksystem. It also covers the role that clock recovery plays in providing the timing reference and in tracking low-frequency jitter, as well as jitter transfer functions.

1.4.1 The Importance of Reference

The beginning of this chapter defined jitter as any deviation from ideal timing. This definition is from the point of view of a "static timing reference" (see Figure 1.13). In other words, the ideal timing reference is a fixed timing point. This definition is very useful from concept and mathematical views, but it needs to be enhanced to be useful for the system application. Although it's true in a wide sense that jitter is any deviation from the ideal, if the properties of the reference are considered, the resulting jitter can be quite different. For example, a data signal with a sinusoidal timing jitter referenced to an ideal clock with a perfect period (i.e., zero-jitter) has a larger peak value than when it is referenced to the same clock but modulated with the same kind of sinusoidal, because the reference clock moves "in phase" with the data signal in this case.

Figure 1.13

Figure 1.13 The same jitter source results in two different jitter estimations with two different jitter references. One is a static ideal timing, and the other is a synchronized timing giving rise to zero jitter estimation.

This is in analogy to Newton's law for motion. Whether or not an object moves critically depends on the reference. In parallel, we can fairly say that whether or not a signal has jitter depends on the reference signal used to determine the timing. For illustration purposes, we will focus on a timing reference signal in the context of serial data communication. However, the general concept applies to other systems.

1.4.2 Jitter Transfer Function in Serial Data Communication

Serial data communication embeds the clock signal in its transmitting data bit stream. At the receiver side, this clock needs to be recovered through a clock recovery (CR) device where phase-locked loop (PLL) circuits are commonly used. It is well known that a PLL typically has certain frequency response characteristics. Therefore, when a receiver uses the recovered clock to time or retime the received data, the jitter seen by the receiver follows certain frequency response functions. Figure 1.14 shows a typical serial link system with a transmitter (Tx), medium or channel, and receiver (Rx).

Figure 1.14

Figure 1.14 A schematic block diagram for a serial link composed of three key elements: transmitter (Tx), medium (or channel), and receiver (Rx). Clock for Tx data generation and clock recovery (CR)/PLL for receiver are also shown.

A PLL typically has a low-pass frequency response function HL(f), as shown in Figure 1.15.

Figure 1.15

Figure 1.15 A typical PLL magnitude frequency response.

Any good estimation methodology should emulate the actual device behavior. In the case of receiver jitter, noise, and BER estimation/measurement, the model/measurement setup should estimate/measure the jitter as what a receiver sees. A receiver "sees" jitter on the data from its recovered clock.15 Therefore, it is a difference function from clock to data, as shown in Figure 1.16.

Figure 1.16

Figure 1.16 A jitter estimation/measurement system emulates jitter as seen by a serial data receiver. Note that the data latch function of "D" flip-flop in Figure 1.14 is replaced by the difference function to emulate the receiver jitter behavior.

Because the clock recovery (or PLL) device has a low-pass transfer function HL(f), the jitter output has a high-pass transfer function of HH(f), as shown in Figure 1.17. HL(s) + HH(s) = 1, where s is a complex frequency.

Figure 1.17

Figure 1.17 Jitter frequency response as seen by a serial receiver or as measured by a difference function.

The high-pass jitter transfer function shown in Figure 1.17 suggests that a receiver can track more low-frequency jitter at frequencies of f < fc than at higher frequencies of f > fc. This implies that a receiver can tolerate more low-frequency jitter than high-frequency jitter, with a jitter tolerance function being the reciprocal of the jitter output function, as shown in Figure 1.17. Figure 1.18 shows the jitter tolerance mask corresponding to the jitter transfer function in Figure 1.17.

Figure 1.18

Figure 1.18 The receiver jitter tolerance mask corresponding to the jitter transfer function shown in Figure 1.17.

Notice the same magnitude but different polarity slopes in Figures 1.17 and 1.18 at frequencies at f < fc. For a receiver tolerance test, a receiver should be able to tolerate more jitter than those defined in the Figure 1.18 mask. So the mask is a minimum jitter magnitude as a function of frequency that a receiver must satisfy. When the mask has a second-order slope—namely, –40dB/decade—a receiver with a first-order jitter transfer function with a slope of 20 dB/decade does not meet the tolerance requirement. A second-order jitter transfer function may meet the tolerance requirement, and a third-order jitter transfer function with a 60 dB/decade slope will exceed the tolerance requirement.

The jitter transfer function is a very important element in estimating the relevant jitter in a serial link. Without this building block, it is not possible to estimate the relevant jitter for the system and related BER performance in a rational way. We will give detailed discussions by using the jitter transfer function when we discuss specific communication link technologies in the upcoming chapters.

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