High Impedance Measurement

High Impedance Measurement uses the sound card input impedance as one of the reference resistor. Thus, to enable this type of measurement, the sound card input impedance, which is sound card dependent, must be calibrated first. After calibration, 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 shown as follows.

Connection for High Impedance Measurement

The connection diagram for high 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, Zx and Zsc form a voltage divider and thus we have Vi/Vo=Zsc/(Rr+Zx+Zsc). This connection should be used for high impedance measurement where Zx is comparable to or higher than Zsc. 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 Rr+Zx+Zsc, which holds true in almost all cases. It can be taken into account by simply adding its value (if it is known) to the value of Rr.

Zsc is typically in the range of 600 Ω ~ 50 kΩ, depending on the sound card and channels (MIC In or Line In) used. It must be calibrated before you can start the real LCR measurements. The calibration data can be saved so that you do not have to calibrate it again as long as the same sound card is used. In the Zsc combo box, two options are available: “?” and “Zsc”. Choose “?” if you want to conduct Zsc calibration, and choose “Zsc” if you want to conduct a normal LCR measurement. Only “?” will be available if the software cannot find any calibration data in the current software directory. This is to force you to conduct Zsc calibration first.

Rr is an external reference resistor. You need to find the resistor and make the connection by yourself. An easier way is to use two VIRTINS Sound Card Oscilloscope Probes (P601), one connected to the input jack of the sound card and the other connected to the output jack of the sound card. Then you can connect the inductor/capacitor/resistor to be measured between them easily (see figure below, you do not have to connect the ground lead if they are connected within the sound card itself.). The impedance of the probes will act as the reference resistor. Note that for VIRTINS Sound Card Oscilloscope Probe (P601), the input impedance is 1 k, 201k and 10.001M for switch position 1, 2 and 3 respectively. In the Rr combo box, you should enter the actual value (in kW) you used. Different test steps or different measurement ranges desired may require different resistor values.

Some pre-configured resistance values are available for selection, most of them are relevant to the case where two VIRTINS Sound Card Oscilloscope Probes (P601) are used as both the test leads and reference resistor.

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 “ResistorRef1.txt” under the software root directory so that your can have your own set of reference resistor values available in the Rr combo box. In the following sections, to simplify the explanation of the measurement procedure, we will assume that two VIRTINS Sound Card Oscilloscope Probes are used. But bear in mind that you can always use your own test leads and reference resistors.

Zx is the impedance to be measured. When “?” is chosen in the Zsc combo box, the Zx combo box will always display “0”, which means during Zsc calibration, Zx should not be connected. When “Zsc” is chosen in the Zsc combo box, the Zx combo box will display “0” for Step 1 and “?” for Step 2. In other word, only the last step of the actual LCR measurement requires Zx to be connected.

Calibration of Sound Card Input Impedance

The software does not assume Zsc is a constant with regard to frequency. Thus the value of Zsc should be calibrated against a number of frequencies covering the entire range within which the test frequencies will be chosen. During the actual LCR measurement, the actual Zsc values at the actual test frequencies will be interpolated from the calibration result. To avoid the interpolation error, use the same frequencies during calibration and actual measurement.

The procedure to calibrate Zsc is similar to the procedure to make a LCR measurement. You need to take two steps to complete the calibration.

Step1---Set Test Tone Reference Level

To start the calibration, you need to choose “?” in the Zsc combo box. It is automatically chosen if this is the first time you conduct the calibration. Then, a locked dedicated test plan named Zscdefault.dtp will be automatically loaded (see figure below). By factory default, this plan will measure the Zsc values at 50 Hz, 100 Hz, 200 Hz, 500 Hz, 1000 Hz, 2000 Hz, 5000 Hz, 10000 Hz, 20000Hz.

“Step 1-Set test tone reference level” should be selected. The purpose of this step is to set the test tone reference level which is similar to the zeroing step when you want to measure a resistor using an analog multimeter. The reference level should be set as high as possible in order to get sufficient measurement accuracy provided no signal clipping occurs.

Before you start the test plan, make sure the test loop is connected correctly and the Rr value in the Rr combo box corresponds to the actual value used. For this step, a relatively low (compared with the one in the next step) Rr value should be used. The value can be zero. In the example, we used 2_[P1+P1] (=2 kΩ) for convenience purpose.

Zx is bypassed and its value is zero.

Once the test plan is started, it will generate each test frequency one by one and display the corresponding peak level in the PeakPercent1 column as well as the Result window. After finishing the last test frequency, it will go back to the first test frequency again and repeat this process until the plan is stopped manually. During this process, you should adjust the output level and input gain using the Windows Volume Control and Recording Control such that the maximum peak level among all test frequencies is close to 100% without any clipping. You may have to change the Rr value if the peak levels are too low.

Having set the desired test tone level, press the Start/Stop button to stop the plan.

Step2---Test with DUT (Zsc)

Keep the test tone reference level intact, then choose “Step 2-Test with DUT”. The DUT in this step is in fact Zsc. The value of Rr must be changed in this step. In the example, we changed it to 202_[P2+P1] (=202 kΩ).

Once the test plan is started, it will generate each test frequency one by one and display the corresponding Zsc value measured in the Zsc column as well as the Result window. After finishing the last test frequency, it will go back to the first test frequency again and repeat this process until the plan is stopped manually. The output level and input gain set in the previous step must be maintained in this step.

You should press the File Save button beside “Zsc” to save the calibration result as default.

Save, Save As, Open a Zsc calibration file

The File Open button beside “Zsc” can be used to load a sound card impedance file (*.zsc).

The File Save button beside “Zsc” is used to save the current calibration data to the default sound card impedance file named default.zsc. The LCR Meter refers to this file for the value of Zsc by default. You can use the File Open button to load a non-default Zsc calibration file if necessary.

The File Save As button beside “Zsc” is used to save the current calibration data to a specified sound card impedance file.

The path and name of the current sound card impedance file in use, if any, is displayed below the above three buttons.

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

To start the actual measurement, you need to choose “Zsc” in the Zsc combo box. Then, a locked dedicated test plan named LCRdefault.dtp will be automatically loaded (see figure below). By factory default, this plan will measure the Zx values at 1000 Hz only.

Then 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 2_[P1+P1] (=2 kΩ) for Rr and the peak level at the test frequency (1000 Hz) was set to 97.02%.

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 200 kΩ resistor with a tolerance value of 1% for Zx, and the measured value was 199.484 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.

If Zx is a capacitor with the same impedance value at 1000 Hz, then the capacitance would be 797.831 pF.

If Zx is an inductor with the same impedance value at 1000 Hz, then the inductance would be 31.749 H.