This method of impedance measurement is used with an electronic AC voltmeter and a signal generator. It requires the addition of a non-inductive resistor and an SPDT switch. Typical values for the resistor are in the range of 1Ω - 10Ω. The value of the resistor needs to be very low. So as to have the unknown impedance determine the current through the circuit, not the resistor. A schematic of the test set up is below. As always the accuracy depends on many factors. Namely, the test setup, the accuracy of the meter and signal generator used, and your prodedure.
Test Procedure:
- Setup switch as shown in the figure.
- Put switch S in the lower position (measure at the output of Zx) and adjust signal generator for an upper scale deflection of the voltmeter.
- Record the reading and label it as Er.
- Without disturbing the setting of the signal generator, move the switch S to the upper position (measure the input of Zx.
- Record the reading and label as Eg.
- Calculate the unknown impedance from:
| Eg × R | |
| Zx = | ——— |
| Er |
Example:
A 10Ω non-inductive resistor is used in the setup above. The readings are Eg =
1.5V and Er = 1mV. The unknown impedance would be:
| 1.5 × 10 | |
| Zx = | ——— |
| 0.001 |
In this case the unknown impedance is 15 kΩ. This setup is good for audio frequencies, but breaks down around 5 kHz.