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A G R E AT E R M E A S U R E O F C O N F I D E N C E RF signals is one example of such a produc-
tion test.
Reconstructing the mechanism of mul-
tipath interference in a test system is a com-
plex procedure, because it requires that noise
and time-delayed signals reach the phone
under test with appropriate power levels and
phase relationships. Figure 1 shows a typical
signal conditioning/switching system appli-
cation for testing mobile phones.
For mobile phone receiver testing, the
output of the mobile station test set can be
switched through two types of paths: 1) a
Management of
path through instrumentation that simulates
multipath fading and noise interference, and
2) paths than can switch in gain to simulate
Microwave Signals
varying distances between the mobile phone
and the base station. For phone transmitter
testing, the output of the phone is directed to
in Automated the mobile station test set through either an
attenuated or unattenuated path. For moni-
Test Systems
toring purposes, the output of the phone or
the input to the phone can be transmitted to a
spectrum analyzer.
Figure 2 shows the control and signal
path cabling associated with this type of
system. Electromechanical switches are lo-
Robert Green, Keithley Instruments, Inc. cated in the foreground. Signal paths extend
to the rear panel through banks of isolators,
divider/combiners, and additional isolators.
M
Ribbon cables supply the signals needed to
INIMIZING reflections and loss, isolation, power handling capability, control the microwave relays.
signal losses in test systems and voltage standing wave ratio (VSWR).
associated with the design, Power losses are primarily a function The Importance of Understanding
characterization, and pro- of the resistive loss through the path and and Managing Power Loss
duction of microwave RF impedance mismatch losses through each Before attempting to perform any mean-
products can be a complex challenge. In path. Mismatch uncertainty, usually the ingful testing or calibration of products, it's
addition to issues such as cable and inter- largest contributor to power measurement critical to quantify the power losses through
connect quality, engineers must consider uncertainty, can be calculated from the mag- the RF test system to be used. Some first-
conductor length, physical layout, and other nitudes of the reflection coefficients of the time users of RF test systems may expect that
aspects of system design that have little ef- source and load: such systems will be completely "invisible,"
fect on DC circuits, yet are fundamental to implying perfect matching, zero insertion
Uncertainty = 20 log (1