Choosing the correct instrument: LCR meter vs. VNA

LCR meter impedance measurement

LCR meters apply an AC signal to the DUT and measure the complex ratio of the resulting voltage and current. Using this information, they calculate the impedance of the DUT. The impedance is represented in rectangular form as:

Z = R + jX

where R is the resistance, and X is the reactance, which can be inductive (positive) or capacitive (negative). The phase angle (j) represents the phase difference between the voltage and current within the circuit.

VNA impedance measurement

Unlike LCR meters, VNAs are not specifically for measuring impedance. However, they are commonly used to indirectly measure the impedance of a DUT at high frequencies, when impedance matching and accurate characterization are crucial for proper functionality.

The VNA measures two key scattering parameters, known as S-parameters: S11, which represents the reflection coefficient at the DUT's input, and S21, which represents the forward transmission coefficient from port 1 to port 2. Using these parameters, the VNA calculates the impedance Z between the ports using three methods, depending on the applicable impedance range:

Shunt Measurement Configuration: Z= (Z0(1+S11))/(1-S11)

Shunt Through Measurement Configuration: Z= (Z0(S21))/(2(1-S21))

Series Through Measurement Configuration: Z= (2Z0(1-S21))/S21

where Z0 is the reference impedance of the VNA (usually set at 50 ohm).

The complex impedance Z can also be directly read from the network analyzer using the Smith chart.

LCR meter vs. VNA - advantages and disadvantages

The main difference between an LCR meter and a VNA is that the VNA uses a fixed reference impedance (Z0). This makes VNAs more precise and allows them to be easily matched to the transmission lines. This impedance matching eliminates potential concerns such as parasitic inductance and capacitance.

In contrast, LCR meters can modify their reference impedance based on the DUT and other requirements. This means that they produce more accurate measurements, but it also means that the you need to compensate for the fixture and cables, which will eventually start behaving as parasitic elements.

VNAs measure S-parameters, which can then be converted to impedance parameters. The S-parameters are essential to understand the reflection, transmission loss and insulation of RF devices (e.g., a coupler) as well as to understand radiation properties (e.g., antenna). All of this means that the VNA is important when working in higher frequency ranges, from kHz to GHz. LCR meters, on the other hand, are restricted to impedance parameters and operate at a lower frequency range from DC to MHz.

Another point of comparison is the number of supported ports. VNAs can support multiple port devices such as couplers, but LCR meters can only support single port components such as resistors, capacitors and inductors.

Key differences between LCR meter and VNA