Agilent Technologies 4294A Specifications download pdf (Page 15)

SECTION 2

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 corresponding Agilent typical instruments, their applicable fre-

quency range, and the typical applications for each method. Considering only measurement accura-

cy 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 measure-

ment 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

1000 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 combina-

tions of L, C, and R components as the bridge elements, are used for vari-

ous applications.

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Agilent Technologies 4294A Specifications Page 15