Digital Signal Processing Reference
In-Depth Information
CHAPTER 8
Signal Loss and the Effects of Circuit
Board Physical Factors
8.1 Introduction
In previous chapters we assumed that the copper traces were rectangular and had
smooth surfaces and that the stripline traces were embedded in a single block of
dielectric material (the dielectric was homogeneous).
These ideal assumptions are not valid in practice. Copper surfaces are rough,
often traces are trapezoidal in shape, and the glass-to-resin ratio (see Chapter 5)
determines the actual dielectric constant. All of these can alter the trace impedance,
delay, and loss characteristics, and it is important for the signal integrity engineer to
determine when it is necessary to account for them in simulation or when to discuss
them with circuit board fabrication shops.
8.2
What Are Transmission Line Loss and Attenuation?
Loss is the mechanism that reduces (attenuates) a signal and in circuit board traces
is caused by resistance in the conductor and leakage in the circuit board laminate.
Distortion is the change in pulse shape caused by the loss unequally affecting each
of the harmonics making up the signal.
8.2.1 What Effects Do Losses Have on Signals?
To understand loss, we must think about signals in the frequency domain. As de-
scribed in Chapter 1, a pulse is made up of many frequency components (harmon-
ics). Because the amount of loss depends on the frequency, each harmonic making
up a pulse will be attenuated differently as the pulse travels down the line. In fact,
each successively higher harmonic will be attenuated more than lower-frequency
ones. This means that when the harmonics recombine at the load their amplitudes
and phase are not in the same proportion as when they were launched. The pulse
has become distorted.
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