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U. Glaeser

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FIGURE 31.1<br />

relationship in that segment defining an effective impedance (Zo = V/I). A signal source driving the<br />

segment only sees this impedance as a sink of the signal energy and has no immediate knowledge of<br />

other parts of the line.<br />

The two conductors can be either two signals wires driven differentially or a single signal wire over a<br />

reference plane where an image current 2 flows in the plane coupled to the signal. A coaxial cable has a<br />

center conductor for the signal and the outer shield as the reference (Fig. 31.2(a)). Similarly a PCB trace<br />

forms a microstrip line with a ground plane as the reference (Fig. 31.2(b)).<br />

An effective way to model a transmission line is to use capacitances and inductances to represent the<br />

electrical and magnetic energy storage and propagation. The entire line is modeled using multiple LC<br />

segments as illustrated in Fig. 31.2(c). 3 The impedance of the line and the propagation velocity can be<br />

represented as Z = L/Cand<br />

v =<br />

1/ ( LC)<br />

. An ideal transmission line propagates a signal with no<br />

added noise or attenuation. Imperfections in the construction of the line such as varying impedance or<br />

neglecting the image current path cause noise in the signal transmission.<br />

Reflections, Termination, and Crosstalk<br />

When a signal wave encounters a segment with a different impedance, a portion of the signal power<br />

reflects back to the transmitter and can interfere with future transmitted signals. The reflection occurs<br />

because the boundary condition at a junction of two impedances must be preserved such that (1) the<br />

voltage is the same on both sides of the junction and (2) the signal energy into and out of the junction<br />

is conserved. For instance, if a lower impedance is seen by a signal, a lower voltage must be propagated<br />

along the new segment so that the propagated power is less than the original power. The lower voltage<br />

at the junction implies that a negative voltage wave is propagated in the reverse direction. Similarly, at<br />

the end of a transmission line, the receiver appears as an open circuit (high impedance) and would cause<br />

© 2002 by CRC Press LLC<br />

Components of an I/O subsystem.<br />

FIGURE 31.2 Cross-sectional view of transmission line: (a) coaxial, (b) microstrip (PCB), and (c) LC model of a<br />

transmission line.<br />

2<br />

Image current, also called return current, is equal to the signal current.<br />

The L’s<br />

and C’s<br />

are per unit length.<br />

3<br />

clk ref<br />

clock<br />

generation<br />

t bit<br />

dataout D Q<br />

clktx predriver<br />

synchronize<br />

Transmitter<br />

Receiver<br />

driver<br />

amplifier<br />

synchronize<br />

datain Q D<br />

clkrx +<br />

-<br />

Vin+ Vref clk ref<br />

phase-locked<br />

loop<br />

Timing Recovery<br />

V sw<br />

L<br />

signal<br />

reference<br />

signal<br />

reference<br />

(a) (b) (c)<br />

Z o<br />

C<br />

Channel

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