Jitter, Noise, and Communication System Basics
The essence of communication is about transmitting and receiving a signal through a medium or channel. An early mathematical model for communication may be tracked back to Claude Shannon's famous 1948 paper.1 Depending on what kind of medium is used to transmit and receive a signal, communication systems are grouped into three basic categories: fiber, copper, and wireless (or free space) (see Figure 1). The bandwidths typically are a few THz for fiber and a few GHZ for copper media. Considering the constraints of bandwidth, attenuation, and cost, fiber-based communication is often used for long-distance (more than 1 km), high-data-rate (up to 100 Gb/s per channel) communication. Copper-based communication is used for medium-distance (less than 1 km) and medium-high data rates (1 Mb/s to a few Gb/s per channel). Wireless is used for medium distance (~ km) and medium data rates (up to ~100 Mb/s). The choice of a communication medium is largely determined by cost and application requirements. Clearly, fiber has the highest intrinsic bandwidth, so it can deliver the highest data rate possible for a single channel.
Figure 1. A simple communication system, including three basic building blocks: transmitter, medium, and receiver.
What Are Jitter, Noise, and Signal Integrity?
When a signal is transmitted and received, a physical process called noise is always associated with it. Noise is basically any undesired signals added to the ideal signal. In the context of digital communication, the information is encoded in logical bits of 1 and 0. An ideal signal may be represented by a trapezoid wave with a finite 0 to 1 rise time or 1 to 0 fall time. In the presence of noise, it is the sum of ideal signal, with the noise giving rise to the "net" or actual signal waveform. If no noise is added, the actual signal is identical to the ideal signal waveform. If the noise is added, the actual signal is deviated from the ideal signal, as shown in Figure 2.
Figure 2. An ideal signal versus a noisy signal for a digital waveform
The deviation of a noisy signal from its ideal can be viewed from two aspects: timing deviation and amplitude deviation. The amplitude of the digital signal for a copper-based system is the voltage, and for a fiber-based or radio frequency (RF) wireless system it is the power. The deviation of the signal amplitude:
is defined as the amplitude noise (or simply noise), and the deviation of time:
is defined as the timing jitter (or simply jitter). Those definitions will be used throughout this book. The impacts of timing jitter and amplitude noise are not symmetrical, though. Amplitude noise is a constant function and can affect system performance all the time. Timing jitter affects system performance only when an edge transition exists.