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Agilent AN 346-2
Balanced Circuit Measurement with an
Impedance Analyzer/LCR Meter/Network
Analyzer
Application Note
Introduction Measuring a balanced circuit with an Selecting a Balun
How a balanced circuit differs from an unbalanced measurement instrument There are several types and brands of
unbalanced circuit A balanced circuit cannot be directly balun transformers. When selecting a
A balanced circuit has its electrical measured with an unbalanced meas- balun, ensure that frequency is com-
midpoint grounded. An unbalanced urement instrument because of the patible with your measurement
circuit, however, has one side ground- difference in their configuration. requirements. When you measure the
ed. A balanced circuit is typically impedance parameters of a balanced
used in communications equipment When measuring balanced circuits, circuit, you don't have to use the
because a balanced circuit has the the unbalanced measuring instrument balun which has the same impedance
advantage of better spurious noise requires a balun (balanced to unbal- with the circuit under test. However,
suppression. anced) transformer. A balun is a type when you measure the transmission
of impedance-matching RF trans- or reflection of it, you have to use a
Figure 1 shows a balanced cable former. balun which has the same impedance
which is an example of a balanced with the circuit under test to keep
circuit. The voltages of the cable's two Figure 3 shows the configuration for impedance matching. Table 1 shows
conductors are at every point equal measuring a balanced circuit with an recommended balun transformers.
in amplitude and opposite in phase. unbalanced instrument.
Figure 2 shows an unbalanced cable
which is an example of an unbal- Note: In balanced cable measure-
anced circuit. Most measurement cir- ments, residual current in the balun
cuits in Agilent Technologies' imped- or the measuring instrument can
ance analyzers and LCR meters are cause measurement errors. To reduce
unbalanced. the degree of error, perform
open/short and load compensation at
the measurement terminals of the
+V balun.
-V
+V +V
High
V terminal
0 -V
Low
A terminal
Figure 1. Balanced cable Balanced cable
Measurement Instrument
+V Figure 3. Balanced cable measurement configuration
0V
Figure 2. Unbalanced cable
Measurement Configuration with a Short standard PN 1250-0929 Transmission measurement configuration
Balun and Compensation 50 load standard PN 11593A with a network analyzer
Impedance measurement configuration Figure 7 shows transmission meas-
with Agilent 4294A impedance analyzer 3. Remove the connector and connect urement configuration (3) with a net-
Figure 4 shows impedance measure- the DUT. Measure the DUT. work analyzer.
ment configuration (1)/(2) with the
4294A. Impedance measurement configuration To calibrate/compensate:
with the Agilent 4395A(#010) Short the terminals closest to the
To calibrate/compensate for (1): Figure 6 shows impedance measure- DUT to the signal out and to the test
1. Perform open, short, and load ment configuration (2) with the port, then perform response/thru
compensation at the balanced termi- 4395A. calibration.
nals of the 16314-60011. Use the
furnished compensation standards of To calibrate/compensate:
the 16314-60011. Refer to "To calibrate/compensate for
(2)" of impedance measurement con-
Standards 0 PN 04191-85300 figuration with 4294A impedance
0S PN 04191-85302 analyzer.
50 PN 04191-85301
Table 1. Recommended balun transformers
To calibrate/compensate for (2):
1. Assemble a female BNC connector Unb/Bal. (W) Bandwidth Type No. Suppliers
as shown in Figure 5.
50:50 100 Hz to 10 MHz 16315-60011 Agilent Technologies
50:100 100 Hz to 10 MHz 16316A Agilent Technologies
2. Perform open, short, and load cali-
50:600 100 Hz to 3 MHz 16317A Agilent Technologies
bration at the BNC connector using the 50:50 0.1