Agilent Technologies 4294A Specifications download pdf (Page 7)

SECTION 1

Impedance measurement basics

1-1. Impedance

Impedance is an important parameter used to characterize electronic circuits, components, and the

materials used to make components. Impedance (Z) is generally defined as the total opposition a

device or circuit offers to the flow of an alternating current (AC) at a given frequency, and is repre-

sented as a complex quantity which is graphically shown on a vector plane. An impedance vector

consists of a real part (resistance, R) and an imaginary part (reactance, X) as shown in Figure 1-1.

Impedance can be expressed using the rectangular-coordinate form R+jX or in the polar form as a

magnitude and phase angle: |Z|∠θ. Figure 1 also shows the mathematical relationship between R,

X, |Z| and θ. In some cases, using the reciprocal of impedance is mathematically expedient. In

which case 1/Z= 1/(R+jX)= Y= G+jB, where Y represents admittance, G conductance, and B suscep-

tance. The unit of impedance is the ohm (Ω), and admittance is the siemen (S). Impedance is a

commonly used parameter and is especially useful for representing a series connection of resistance

and reactance, because it can be expressed simply as a sum, R and X. For a parallel connection, it is

better to use admittance (see Figure 1-2).

Figure 1-1. Impedance (Z) consists of a real part (R) and an imaginary part (X)

Figure 1-2. Expression of series and parallel combination of real and imaginary components

1-1

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Agilent Technologies 4294A Specifications Page 7