HP HP 8712ES, User Manual, Page: 125 / 507

ES User’s Guide 3-73
Making Measurements
Measuring Impedance using the Smith Chart
Measuring Impedance using the Smith
Chart
The amount of power reflected from a device is directly related to the
impedances of both the device and the measuring system; for example,
the value of the complex reflection coefficient
Γ
is equal to 0 only when
the device impedance and the system impedance are exactly the same.
Each value of
Γ
uniquely defines a device impedance.
The Smith chart is a tool used to map the complex reflection coefficient
Γ
to the DUT's impedance. In a Smith chart, the complex impedance plane
is reshaped to form a circular grid, from which the series resistance and
reactance can be read (R + jX). (See
Figure 3-33, “Interpreting the Smith
Chart,” for more information on the Smith chart.) Marker features on the
analyzer display the resistance in units of ohms, and the reactance as an
equivalent capacitance or inductance in units of farads or henrys.
This section uses an example measurement to describe how to measure
the input impedance of an insertable bandpass filter like the one that
was supplied with your network analyzer. An S
11
measurement is
demonstrated in this example, but the same general concepts apply for
an S
22
measurement.
TIP An optimum calibration is critical for achieving best measurement
accuracy. Refer to
Chapter 6, “Calibrating for Increased Measurement
Accuracy,” for detailed information about all of the various aspects of
calibration.