Specifications

2.0 Impedance Measurement Instruments
2.1 Measurement methods
There are many measurement methods to choose from when measuring impedance, each of which
has advantages and disadvantages. You must consider your measurement requirements and condi-
tions, and then choose the most appropriate method, while considering such factors as frequency
coverage, measurement range, measurement accuracy, and ease of operation. Your choice will
require you to make tradeoffs as there is not a single measurement method that includes all mea-
surement capabilities. Figure 2-1 shows six commonly used impedance measurement methods, from
low frequencies up to the microwave region. Table 2-1 lists the advantages and disadvantages of
each measurement method, the Agilent instruments that are suited for making such measurements,
the instruments’ applicable frequency range, and the typical applications for each method.
Considering only measurement accuracy and ease of operation, the auto-balancing bridge method is
the best choice for measurements up to 110 MHz. For measurements from 100 MHz to 3 GHz, the RF
I-V method has the best measurement capability, and from 3 GHz and up the network analysis is the
recommended technique.
2-1
Figure 2-1. Impedance measurement method (1 of 3)
When a circuit is adjusted to resonance by adjusting a tuning capac-
itor (C), the unknown impedance Lx and Rx values are obtained from
the test frequency, C value, and Q value. Q is measured directly
using a voltmeter placed across the tuning capacitor. Because the
loss of the measurement circuit is very low, Q values as high as 300
can be measured. Other than the direct connection shown here,
series and parallel connections are available for a wide range of
impedance measurements.
Bridge method
Resonant method
When no current flows through the detector (D), the value of the
unknown impedance (Zx) can be obtained by the relationship of the other
bridge elements. Various types of bridge circuits, employing combinations
of L, C, and R components as the bridge elements, are used for various
applications.