Specifications

2-17
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 the DUT
impedance. In accordance with this equation, measured reflection coefficient varies from –1 to 1
depending on the impedance (Zx.) The relationship of the reflection coefficient to impedance is
graphically shown in Figure 2-15. The reflection coefficient curve in the graph affirms that the DUT
is resistive. As Figure 2-15 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 lower and higher
impedance, causing deterioration of impedance measurement accuracy. In contrast, the principle of
the RF I-V method is based on the linear relationship of the voltage-current ratio to impedance, as
given by Ohm’s law. Thus, the theoretical impedance measurement sensitivity is constant, regardless
of measured impedance (Figure 2-16 (a).) The RF I-V method has measurement sensitivity that is
superior 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.
Figure 2-15. Relationship of reflection coefficient to impedance
Note: Measurement sensitivity is a change in measured signal levels (ΔV/I or ΔV/V) relative to a
change in DUT impedance (ΔZ/Z.) The measurement error approximates to the inverse of the
sensitivity.