Agilent Technologies 4294A Specifications download pdf (Page 79)

5-2. Inductor measurement

An inductor consists of wire wound around a core and is characterized by the core material used.

Air is the simplest core material for making inductors, but for volumetric efficiency of the inductor,

magnetic materials such as iron, permalloy, and ferrites are commonly used. A typical equivalent

circuit for an inductor is shown in Figure 5-8 (a). In this figure, Rp represents the iron loss of the

core, and Rs represents copper loss of the wire. C is the distributed capacitance between the turns

of wire. For small inductors the equivalent circuit shown in Figure 5-8 (b) should be used. This is

because the value of L is small and the stray capacitance between the lead wires becomes a signifi-

cant factor.

Figure 5-8. Inductor equivalent circuit

Inductance measurement sometimes gives different results when measured using different instru-

ments. This is due to several reasons which are:

Test signal current. Cored inductors are test signal current dependent as shown in Figure 5-9 (a).

Many impedance measurement instruments output a voltage driven test signal. Even when two dif-

ferent instruments are set to output the same voltage, their output currents are different if their

source resistance is different. Refer to Figure 5-9 (b).

Test fixture used. When a metal object is located physically close to an inductor, leakage flux from the

inductor will cause eddy currents to flow in the metal. The magnitudes of the resulting eddy cur-

rents are different because of the different dimensions and physical geometry of different test fix-

tures, as shown in Figure 5-10 (a), resulting in different measurement results. This is especially

important for measuring open-flux-path inductors. Figure 5-10 (b) shows experimental measure-

ment data for an Ls-Q measurement to see this effect. When a 1.0 mm thick brass plate, 40 mm × 40

mm, is placed close to a small 100 µH RF inductor, measured Ls-Q values are changed.

Q measurement accuracy. Generally, the Q measurement accuracy in impedance measurement is not

high enough, especially, when measuring high Q values. Figure 5-11 shows relationship of Q accura-

cy and measured Q values. Because Q value is reciprocal of D (Q=1/D), the Q accuracy is related to

the specified D measurement accuracy as shown in the figure. The Q measurement error increases

with DUT’s Q value and, therefore, the practically measurable Q range is limited by allowable Q mea-

surement error. (For example, if the allowable Q error is 10% and if the instrument’s D accuracy is

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Agilent Technologies 4294A Specifications Page 79