Can I Use Network Analyzer for Impedance Measurement?

Frequently Asked Questions (FAQs)SummaryCan I use network analyzer to measure the impedance of a component? This FAQ talks about how to use network analyzer to measure the impedance and the limitation.QuestionCan I use network analyzer for impedance measurement?  AnswerThere are 3 methods for impedance measurement using network analyzer:        1. Reflection Method: The reflection 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 highest accuracy is obtained at Zx equal to Zo. The gradient of reflection coefficient curve becomes slower for lower and higher impedance, causing deterioration of impedance measurement accuracy. The impedance is calculated by following equation: Z = 50 x ((1 + S11) / (1 - S11)) The impedance measurement range of reflection method is typically from 2 Ω to 1.5 kΩ (depending upon the required accuracy and measurement frequency.)          2. Series Through Method: The series-through method measures impedance by connecting the DUT in a “transmission series” connection. Series-through is most effective when measuring high impedance values: the 10% accuracy range is about 5 Ω to 20 kΩ or roughly one decade higher than the reflection method. The impedance is calculated by following equation: Z = (50 x 2) x ((1 - S21) / S21)          3. Shunt Through Method: The shunt-through method measures impedance by connecting the DUT in the transmission-shunt configuration. This is good way to characterize very low impedance values, and it is commonly used to make measurements in the milliohm range (e.g., power integrity applications). The 10% accuracy range spans 1 mΩ to 10 Ω, which is lower than typical impedance analyzers can reach. The impedance is calculated by following equation: Z = (50 / 2) x (S21 / (1 - S21)) Below are the comparison among 3 models: