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Measuring impedance using an oscilloscope

There is a range of methods of measuring impedance, and today some using very sophisticated techniques come within the budget of hams. For some reason, the availability of these instruments has stunted thinking in applying simple techniques to the impedance measurement problem, and those whose budget does not extend to the latest technology cry that they are disadvantaged.

Well, the new technology wasn't there is the past, yet we made do with simpler, even if less convenient and less accurate methods.

Method

An old technique for measuring impedance which uses a very inexpensive test jig was to insert a known resistor in series Rr with the unknown impedance to ground (or vice versa), excite the combination with a sinusoidal source at the desired frequency, and measure the source voltage and the voltage across the grounded element and the phase difference using an oscilloscope.

The resistor Rr must be a pure resistance at the frequency of interest, the calculator assumes it has negligible reactance.

The following measurements are made:

For best accuracy, |V2| should not be too far from |V1|, and preferably |V1|<|V2|. To achieve this, it may be possible to configure the reference resistor on the high side or low side, though some DUT may have a grounded terminal which dictates that the reference resistor is on the high side.

Provision is made for the loading effect of the oscilloscope probe.

Calculator

This calculator takes the measured voltages and phase and calculates R and X.

Inputs:
 
|V1| (V)
|V2| (V)
V2 phase (°)
Rr (Ω)
Rprobe (Ω)
Cprobe (pF)
Frequency (MHz)
Zo (Ω)
 
Results:
R (Ω)
X (Ω)
|Z| (Ω)
Phase (°)
VSWR

SVN:

The calculator does not do a lot of error checking, if you enter nonsense, it will produce nonsense. VSWR is calculated wrt Zo. If V1>=V2 it is a measurement error, no results can be calculated. NaN means Not a Number, check the input values.

Table 1: Input field descriptions
Input field Meaning
|V1| Magnitude of the source voltage
|V2| Magnitude of V2, the voltage across the grounded series element
V2 phase Phase of V2 wrt V1
Rr The reference resistor.
Zo  Impedance for calculation of VSWR

Links

Changes

Version Date Description
1.01 03/11/2017 Initial.
1.02    
1.03    
1.04    
1.05    

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