4.6 S-Parameters, Impedance and Smith Charts in MWO
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4.6 S-Parameters, Impedance and Smith Charts in MWO
In this section we will introduce the reader to the basics of high-frequency circuit simulation
in Microwave Office.
Let us start with a circuit of the type shown in Figure 4.4-1 where the generator is connected to the
load through a transmission line of physical length and characteristic impedance equal to its
internal impedance. A signal is sent down this line all the way to the load. Depending on what the
signal finds as a load, some of it may be reflected back to the generator and we need to measure the
power of this reflected signal and divide it by the incident one (eq. (3.8-1)) to find the reflection
coefficient
( )
. From this value, we can work out the impedance seen by the generator by using
equation (4.4-1). But how do we actually measure incident and reflected power?
In a laboratory we would use devices called directional couplers, which are able to tap a tiny
sample of power off the transmission line and also allow us to work out which way the power is
flowing! In a simulation environment we have specific elements called ports. Ports are ubiquitous in
the simulation of high frequency circuits and you must understand thoroughly how they work!
Figure 4.6-1 S-parameter measurement setup
Figure 4.6-2 Equivalent S-parameter measurement setup in MWO
Let us first consider Figure 4.6-1 which is essentially the same as Figure 4.4-1 where we have
added directional power meters and have specified the length of the line by its electrical length
θ
instead of its physical length .
How can we implement this circuit in MWO?
ǡθ
(l)
θ
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