Agilent Technologies 4294A Specifications download pdf (Page 32)

2-6. Difference between RF I-V and network analysis measurement methods

When testing components in the RF region, the RF I-V measurement method is often compared with

network analysis. The difference in principle is highlighted as the clarifying reason why the RF I-V

method has advantages over the reflection coefficient measurement method, commonly used with

network analysis.

The network analysis method measures the reflection coefficient value, Γx, of the unknown device.

Γx is correlated with impedance, by the following equation:

Γx = (Zx-Zo)/(Zx+Zo)

Where, Zo is the characteristic impedance of the measurement circuit (50 Ω) and Zx is DUT imped-

ance. In accordance with this equation, measured reflection coefficient varies from -1 to 1 depend-

ing on the impedance, Zx. The relationship of reflection coefficient to impedance is graphically

shown in Figure 2-14. The reflection coefficient curve in the graph affirms that the DUT is resistive.

As the graph indicates, the reflection coefficient sharply varies, with difference in impedance

(ratio), when Zx is near Zo (that is, when Γx is near zero). The highest accuracy is obtained at Zx

equal to Zo because the directional bridge for measuring reflection detects the “null” balance point.

The gradient of reflection coefficient curve becomes slower for both the lower and higher imped-

ance, causing deterioration of impedance measurement accuracy. In contrast, the principle of the

RF I-V method is based on the linear relationship of voltage-current ratio to impedance, as given by

ohm’s law. Thus, the theoretical impedance measurement sensitivity is constant, regardless of mea-

sured impedance (Figure 2-15 (a)). The RF I-V method has measurement sensitivity which is superi-

or to the reflection coefficient measurement except for a very narrow impedance range around the

null balance point (Γ=0 or Zx=Zo) of the directional bridge.

Note: Measurement sensitivity is a change in measured signal levels (∆V/I or ∆V/V) relative to a change in DUT impedance

(∆Z/Z). The measurement error approximates to the inverse of the sensitivity.

The reflection coefficient measurement never exhibits such high peak sensitivity for capacitive and

inductive DUTs, because the directional bridge does not have the null balance point for reactive

impedance. The measurement sensitivity of the RF I-V method also varies, depending on the DUT’s

impedance, because the measurement circuit involves residuals and the voltmeter and current

meter are not ideal (Figure 2-15 (b)). (Voltmeter and current meter arrangement influences the

measurement sensitivity.) Though the measurable impedance range of the RF I-V method is limited

by those error sources, it can cover a wider range than in the network analysis method. The RF I-V

measurement instrument provides a typical impedance range from 0.2 Ω to 20 kΩ at the calibrated

test port, while the network analysis is typically from 2 Ω to 1.5 kΩ (depending upon the required

accuracy and measurement frequency).

Note: Typical impedance range implies measurable range within 10% accuracy.

Moreover, because the vector ratio measurement is multiplexed to avoid phase tracking error and,

because calibration referenced to a low loss capacitor can be used, accurate and stable measure-

ment of low dissipation factor (high Q factor) is enabled. The Q factor accuracy of the network

analysis and the RF I-V methods are compared in Figure 2-16.

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Agilent Technologies 4294A Specifications Page 32