Custom Firmware, Electronics Design, and PCB Layout

How to Measure S-Parameters

See our other Electronics Articles.

By Tim Daycounter

Most multimeter will measure all electrical properties of a component with the exception of inductance, so you are left to your own devices.  There are several methods which we will discuss in this tutorial article.  

Method 1 - Impedance Comparison to Resistor

The first is to place the inductor in series with a known resistor of good precision, for example a 100 ohm 1% resistor is a good choice.  Stimulate the circuit with a function generator, and view the junction between the resistor and inductor on the scope, as well as the input voltage.  Tune the function generator until the junction voltage is half the input voltage.  At this point the impedance of the two elements is equal, so you can use the following equation to determine the inductance:

L= R/2*PI*F

The advantage of this method is that it is accurate in that calibration resistors can be found with tolerances in the 0.1% range. 

Here's a simple calculator:

Method 2 - Resonant Point with a Capacitor. 

Another method is to place the coil in parallel with a capacitor of know value to form a tank circuit and then put this tank circuit in series with a resistor of any reasonable value.  Then a function generator is used to stimulate the circuit.  Place scope probes across the tank, and sweep the function generator, looking for the point of maximum response.  This peak value is the resonant point, and a simple calculation can be performed to find the inductance.   

L=1/(w^2*C)

The disadvantage with this method is that it's harder to find reference capacitors that have tolerances less than 10%.  

Method 3 - Voltage Current Slope 

The last method is most complex and requires a that a pulsed voltage be placed across the inductor, and then the current is monitored. The duty cycle of the pulse should be below 50%.  Also use a high frequency to avoid saturation.  This requires a current sense resistor, or current probe.  The current is displayed on the scope and the slope determines the inductance. The slope is the peak current Ipk divided by the on time Ton. 

 L= V*Ton/Ipk

Daycounter provides contract engineering services. Please contact us if you would like a quote to out-source your next engineering project. 

[Employment] [Downloads][Articles] [Contact Us]

Call us now at 801-523-8444  -  136 E. Steep Mountain Dr. Draper, UT 84020

© Copyright 2004 Daycounter, Inc.  All rights Reserved.