Digital Signal Processing Reference
In-Depth Information
CHAPTER 17
Calculating Trace and Plane Electrical
Values
17.1 Introduction
With the formulas presented in this chapter, the signal integrity engineer can esti-
mate the impedance, time of flight, loss, and circuit values (
R
,
L
,
C
) for traces and
the circuit values for planes. Ways to calculate conductance (
G
) appear in Chapter
7 and are not repeated here.
These are very helpful in estimating the parasitic values of traces without re-
sorting to a field solver. In fact, the results from these equations can be used as a
first-order check of the results from field solvers when validating circuit models.
Some of the equations (especially for impedance and loss) are too complicated
to be performed reliably by hand on a calculator. However, they are easily solved
with numerical software such as MATLAB [1] or MathCAD [2]. Spreadsheets can
also be used, or the equations can be coded in a simple programming language
(such as C or even a scripting language such as PERL, particularly for the simpler
equations).
The dimensions for stripline and microstrip traces used in the formulas are
shown in Figure 17.1.
17.2
How to Convert from Decibel Loss to Signal Swing
Chapter 8 showed how to calculate loss in decibels (dB). That equation is repeated
here, in (17.1):
⎛
V
V
⎞
Gain in dB
=×
20
log
received
(17.1)
⎜
⎟
10
⎝
⎠
transmitted
For example, the gain is
−
6 dB when
V
received
is 0.5V and
V
transmitted
is 1V. Since
the sign is (
−
), this value represents a loss.
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