2-14 Chapter2
Quick Start: Learning How to Make Measurements
Learning to Make Reflection Measurements
Learning to Make Reflection Measurements
This example procedure shows you how to measure the reflection response of a 125 MHz
bandpass filter. The measurement parameter values listed are unique to this particular
test device.
For further measurement examples, refer to the "Making Measurements" chapter in the
User's Guide.
NOTE Reflection measurements monitor only one port of a test device. When a test
device has more than one port, you must ensure that the unused port(s) are
terminated in their characteristic impedance (for example, 50Ω or 75Ω). If
you do not terminate unused ports, reflections from these ports will cause
measurement errors. Figure 2-8 on page 2-15 illustrates two ways to
terminate an unused device port with the proper characteristic impedance.
The signal reflected from the device under test is measured as a ratio of the reflected
energy versus the incident energy. It can be expressed as reflection coefficient, return loss,
or standing-wave-ratio (SWR). These measurements are mathematically defined as
follows:
reflection coefficient (Γ) = reflected voltage / incident voltage
= S
11
or S
22
(magnitude and phase)
magnitude of reflection
coefficient (ρ)
=|Γ|
return loss (dB) = −20 log (ρ), where (ρ) = |Γ|
standing-wave-ratio (SWR)
= V
maximum
/ V
minimum
= (1 + ρ) / (1 −ρ)
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