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Impedance Measurement Basics

This page covers impedance measurement basics and impedance measurement methods. The common methods used for impedance measurements are bridge method, resonant method, I-V method, RF I-V method, network analysis method and auto balancing bridge method.

As we know impedance is very important parameter. It is used to characterize electronic components and circuits. Impedance is defined as opposition to AC (alternating current) at given frequency when it is passed through electronic device or circuit.


impedance measurement

As mentioned in the figure-1 above impedance vector consists of real part (resistance, R) and imaginary part (reactance, X).

Z = R + j * X (In rectangular form)
Z = |Z| Angle (θ) (In polar form)
Admittance Y = 1/Z = G +j*B
Where, G is conductance and B is susceptance
Impedance is measured in Ohm while admittance is measured in Siemen.

When real (R) and imaginary (j*X) components are connected in series,
impedance (Z) = R +j*X

When real and imaginary components are connected in parallel,
impedance (Z) = j*R*X/ R+j*X OR Y = G +j*B , here admittance is used for simplicity.

Reactance are in two forms viz. inductive reactance (XL) and capacitive reactance (Xc).
XL = 2*pi*f*L and Xc = 1/(2*pi*f*C)
Where in f is frequency , L is inductance and C is capacitance
w = 2*pi*f is known as angular frequency.

Impedance Measurement Methods

In order to measure impedance, two values are required to be measured as it is a complex quantity. All the impedance measurement instruments usually measure both real part and imaginary part of impedance vector. Later on instrument converts these real and imaginary parts into needed parameters such as |Z|, θ, |Y|, R, X, G, B, C and L.

There are different impedance measurement methods based on various parameters such as frequency of operation, measurement accuracy, measurement range and ease in measurement. Let us understand impedance measurement methods viz. bridge method, resonant method, I-V method, RF I-V method, network analysis method and auto balancing bridge method. In addition to these, LCR Meter is used for impedance measurement. Following sections cover six of these impedance measurement methods with difference between them with respect to frequency, advantages and disadvantages. Based on following, one can select impedance measurement method of choice for low frequency and high measurement.

Bridge Method

This method is used in standard lab applications. It covers frequency range from DC to 300 MHz.

Advantages:
• High accuracy
• Wide frequency coverage based on different bridge types
• Lower cost

Disadvantages:
• Needs to be manually balanced
• Narrower frequency coverage using single instrument

Resonant Method

This method is used for high Q device measurement. It is ideal for measurement for 10KHz to 70 MHz frequency range.

Advantages:
• Good Q accuracy upto high Q

Disadvantages:
• Needs to be tuned to resonance
• Low impedance measurement accuracy

I-V Method

This method is suitable for grounded device measurement. It is ideal for frequency from 10 KHz to 100 MHz.

Advantages:
• Grounded device measurement
• suitable to probe type test needs

Disadvantages:
• operating frequency is limited based on transformer used in the probe.

RF I-V Method

This method is used for RF component measurements. It covers frequency range from 1 MHz to 3 GHz.

Advantages:
•  high accuracy
• wide impedance range at higher frequencies.

Disadvantages:
• operating frequency is limited based on transformer used in the test head.

Network analysis method

This method is also used for RF component measurements. It is suitable for any frequency above 300 KHz.

Advantages:
• High frequency range
•  Good accuracy when unknown impedance is very close to characteristic impedance

Disadvantages:
• re-calibration is needed when measurement frequency gets changed.
•  Narrow impedance measurement range

Auto balancing bridge method

This method is used for generic component as well as grounded device measurement. It is suitable for 20 Hz to 110 MHz frequency of measurements.

Advantages:
• Wide frequency coverage from LF to HF.
• High accuracy over wide impedance range of measurement.
• Grounded device measurement

Disadvantages:
• Higher frequency ranges are not available.

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