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Balanced Circuit Measurement
with an Impedance Analyzer/
LCR Meter/Network Analyzer
Application Note 346-2
INTRODUCTION circuit is typically used in two conductors are at every point
communications equipment equal in amplitude and opposite in
How a Balanced Circuit Differs because a balanced circuit has the phase. Figure 2 shows an
from an Unbalanced Circuit advantage of better spurious noise unbalanced cable which is an
suppression. example of an unbalanced circuit.
A balanced circuit has its Most measurement circuits in HP's
electrical midpoint grounded. An Figure 1 shows a balanced cable impedance analyzers and LCR
unbalanced circuit, however, has which is an example of a balanced meters are unbalanced.
one side grounded. A balanced circuit. The voltages of the cable's
+V
+V
-V
0V
Figure 1. Balanced cable Figure 2. Unbalanced cable
1
Measuring a Balanced
Circuit with an Unbalanced
Measurement Instrument
+V +V
High
A balanced circuit cannot be V terminal
directly measured with an unbal-
anced measurement instrument 0 -V
Low
A terminal
because of the difference in their Balanced cable
configuration.
When measuring balanced circuits, Measurement Instrument
the unbalanced measuring instru-
ment requires a balun (balanced to Figure 3. Balanced cable measurement configuration
unbalanced) transformer. A balun
is a type of impedance-matching
RF transformer. don't have to use the balun which To Calibrate/Compensate
has the same impedance with the for (1):
Figure 3 shows the configuration circuit under test. However, when
for measuring a balanced circuit you measure the transmission or 1. Perform open/short/load
with an unbalanced instrument. reflection of it, you have to use a calibration at the APC7