Low Impedance Measurement
Low Impedance Measurement is used to measure impedance in a lower range. The reference resistor is placed in parallel to the sound card input impedance, and if its value is much lower than the sound card input impedance, which holds true in most of cases, the sound card input impedance can be ignored. Thus, it is not necessary to calibrate the sound card input impedance before measurement. However, if the default sound card input impedance file exists or a sound card input impedance file has been loaded, the sound card input impedance will be automatically taken into account for better accuracy.
The LCR Meter will be able to display the measurement range based on the reference resistor value and the test frequency used. The software uses 1% and 99% variation from the test tone reference level to suggest the measurement range, within which good measurement accuracy can be achieved. This is similar to the case of resistor measurement using an analog multimeter whereby the middle region of the swing of the needle has good measurement accuracy.
The procedure to make a LCR measurement is also similar to the procedure to measure a resistor using an analog multimeter, as described previously.
Connection for Low Impedance Measurement
The connection diagram for low impedance measurement is shown as follows.
where:
- Rr is the reference resistor.
- Zx is the impedance to be measured.
- Zsc is the sound card input impedance.
- Vo is the output RMS voltage.
- Vi is the input RMS voltage.
(Rr|Zsc) and Zx form a voltage divider and thus we have Vi/Vo=(Rr|Zsc)/((Rr|Zsc)+Zx), where (Rr|Zsc) is the resultant resistance when Rr and Zsc are connected in parallel. It should be noted that the sound card output impedance, which typically ranges from a few ohms to a few tens of ohms for Speaker/Headphone Out, is ignored in the above formula. This simplification has negligible effect on the measurement accuracy as long as the output impedance is negligibly small compared with the value of Zx+(Rr|Zsc), which holds true in most of cases.
Rr is an external reference resistor. You need to find the resistor and make the connection by yourself. In the Rr combo box, you should enter the actual value (in kW) you used. Different measurement ranges requires different reference resistor values. Some pre-configured resistance values are available for selection: 0.01 k, 0.1 k, 1.0 k.
If you are not using the above values, then you have to enter the resistance value manually. For advanced users, if you have your own set of reference resistors, you can modify the TXT file named “ResistorRef2.txt” under the software root directory so that your can have your own set of reference resistor values available in the Rr combo box.
Zx is the impedance to be measured. The Zx combo box will display “0” for Step 1 and “?” for Step 2. In other word, in Step 1, Zx should be bypassed and in Step 2, Zx should be connected.
Make a LCR Measurement
As described previously, the procedure to make a LCR measurement is similar to the procedure to measure a resistor using an analog multimeter. You need to take two steps to complete a measurement at first, and then the first step can be skipped if the measurement range has not been changed.
Step1--- Set Test Tone Reference Level
Choose a proper measurement range by connecting a proper reference resistor Rr and enter the corresponding value in the Rr combo box on the screen. Then, short the two test leads to bypass Zx, select Step 1, start the plan and adjust the output level via Windows Volume Control, and/or the input gain via Windows Recording Control, in order to set the test tone reference level such that the maximum allowable level is reached to ensure sufficient measurement accuracy. A peak level above 85% is recommended. Stop the plan after the test tone reference level is set.
In the following example, we used a 1kΩ resistor for Rr and the peak level at the test frequency (1000 Hz) was set to 98.33%.
Step2---Test with DUT
Keep the test tone reference level intact, connect the DUT (Zx) between the two test leads, select Step 2 and start the plan. The impedance value measured will then be displayed in the Result window and Zx2 column.
In the following example, we used 20 Ω resistor with a tolerance value of 1% for Zx, and the measured value was 20.064 kΩ, which is very accurate. Note that the sound card input and output impedance here were about 50 k Ω and 100 Ω respectively and the output impedance was ignored without any compensation.
| 
