Radio Frequency Integrated Circuit Design - Webs

The Use and Design of Passive Circuit Elements in IC Technologies
A number of tools are available that can do calculations of transmission
lines, and simulators can directly include transmission line models to show the
effect of these lines.
5.23.2 Transmission Line Examples
At RF frequencies, any track on a printed circuit board behaves as a transmission
line, such as a microstrip line (MLIN) (Figure 5.31), a coplanar waveguide
(CPWG) (Figure 5.32), or a coplanar waveguide with ground (CPWG). Differential
lines are often designed as coupled microstrip lines (MCLINs) (Figure 5.33)
or they can become coupled simply because they are close together, for example,
at the pins of an integrated circuit. For these lines, differential and common
mode impedance can be defined (in microwave terms, these are described as
odd-order and even-order impedance, respectively). On an integrated circuit,
all lines are transmission lines, even though it may be possible to ignore transmission
line effects for short lines. The quality of such transmission lines may suffer
due to lossy ground plane (the substrate) or because of poor connection between
coplanar ground and substrate ground.
For example, a 400-�m by3-�mline on-chip, with oxide thickness of
4 �m is simulated to have a characteristic impedance of about 72�. The
capacitance is estimated by
C = 3.9 � 8.85 × 10−12 F/m � 3 × 10 −6 � 400 × 10 −6
4 × 10 −6
= 10.35 fF
Because of fringing and edge effects, the capacitance is probably more like
20 fF, with the result that the inductance L is about 0.104 nH. Note that for
a wider line, such as 6 �m, the capacitance is estimated to be about 30 fF, the
characteristic impedance is closer to 50�, and inductance is about 0.087 nH.
In making use of a simulator to determine transmission line parameters,
one needs to specify the substrate thickness, line widths and gaps, dielectric
constant, loss tangent, metal conductor conductivity, and thickness. Most simu-
Figure 5.33 Coupled microstrip lines.
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