Agilent Technologies 4294A Specifications download pdf (Page 111)

Measurement accuracy can be improved by taking advantage of the equivalent circuit analysis.

Figure 5-47 (a) shows an Ls-Q measurement example for an inductor. In this example, an imped-

ance analyzer measures the Q value at 10 MHz. Measured data read by MARKER is Ls=4.78 µH and

Q=49.6. The Q measurement accuracy for this impedance at 10 MHz is calculated from the instru-

ment’s specified D measurement accuracy of ± 0.011, and the true Q value will be between 32 and

109. The reason that the uncertainty of the Q value is so high is that the small resistive component

relative to reactance cannot be measured accurately. It is possible to measure the resistive compo-

nent accurately if the inductive component is canceled by the capacitance connected in series with

the inductor. When a loss-less capacitor of 1/(ω

L)=53 pF is connected, the inductor will resonate at

10 MHz. (In this example, a 46 pF capacitor is used for resonance.) Figure 5-47 (b) shows the |Z|-θ

measurement results when a 46 pF capacitor is connected. This result can be modeled using circuit

mode D, and the value of R is calculated to be 8.51 Ω. The value of L is obtained as 4.93 µH. Since

the equivalent circuit analysis function uses approximately 8.51 × √

—

2 Ω data to calculate the R

value, the specified measurement accuracy for a 12 Ω resistance measurement can be used and is ±

1.3%. Therefore, the Q value can be calculated from Q=ωLs/R=36.4 with an accuracy of ± 2.4% (sum

of the L accuracy and R accuracy.) In this measurement, capacitance value does not have to be

exactly the calculated value but the loss of the capacitor should be very small because it will affect

the calculated Q value.

Figure 5-47. Q measurement accuracy improvement

5-37

(a) (b)

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Agilent Technologies 4294A Specifications Page 111