Measuring Resistance

Here we use the sound card to measure the resistance of a 10-ohm non-inductive resistor. You must have a dual channel, duplex sound card to perform this measurement. A pre-measured, non-inductive resistor is required as a reference, although a resistor that deviates about 5% will give reasonable results. Select the reference so that the voltages at both inputs of the sound card are above the noise floor of the sound card for the duration of the test. Also, do not swamp the sound card output with an impedance that is too low. For our purposes a 100-ohm resistor will be a reasonable reference.

We will use "32768_MLS_Impedance_Measurement.process" to perform the measurement. This process ships with the release version of this product. It consists of four modules. The first is the signal generator, which generates a 32768 length MLS stimulus to excite the resistor. Second is the SoundIO module, which plays the stimulus and records the response of the resistor. Third is the Oscilloscope module, which allows us to view the time domain response of the resistor. Finally is the Spectrum Analyzer, which performs an FHT/ FFT on the time domain data and allows us to view impedance vs. frequency and phase vs. frequency graphs.

1. Measure the value of the reference resistor using a precision ohmmeter. Our reference was 100.77 ohms.

2. Wire the resistors as shown in Figure 1. Use short low resistance wiring. Note that sound card Speaker Outputs have more output drive than their respective Line Outputs.

Figure 1: Resistance Measurement Process Calibration Wiring

3. Open "32768_MLS_Impedance_Measurement.process" from the applications File…Open… menu.

4. Select your sound card in the In: and Out: combo boxes in the SoundIO modules Sound Device Select group.

5. Press the Open Mixer button the SoundIO modules Options group. Select Options… Properties… Choose your sound card from the Mixer Device and press the Recording radio button in the Adjust Volume for group. Press the OK button.

6. Deselect all Record Control mixer paths except the Line In. Adjust the Line In mixer slider to its one-quarter setting and equalize its balance slider.

7. Select Options… Properties… Press the Playback radio button in the Adjust Volume for group. Press the OK button.

8. Mute all Playback mixer gain settings except the Volume Control and the Wave Out. Equalize the Volume Control and the Wave Out mixer balance sliders. Adjust the Volume Control and the Wave mixer sliders to their one-quarter settings

9. Press the applications Run button. You should be able to see the MLS sequence in the oscilloscope module as shown in Figure 2. If a SoundIO “No data in record buffer” message appears first check that your wiring conforms to Figure 1. If it is correct, increase the mixers Playback Volume Control and Wave Out sliders and Recording Line controls to 50%.

Figure 2: MLS Sequence in the Oscilloscope Module

10. If all three controls are at maximum you may reduce the level at which the sound card triggers. When in Record/Play mode, the SoundIO module sends a record buffer to the sound card that is 1.4 longer than required. This is to compensate for various system delays. It then scans the buffer for the first level that is greater than the trigger level. It then marks this point as the beginning of the record and returns the remainder of the record (up to the number of samples required for the selected FFT size) to the application. This is the record that the modules processes and sends to subsequent modules. Trigger level is expressed in terms of percentage full scale. Select 10 from the Trigger Level (%F.S.) combo box in the SoundIO modules Trigger Parameters group. Press the Run button and check the oscilloscope display again. You can reduce this value to as low as 1%. This corresponds to 1% of the sound card full-scale output. You can estimate the length of the buffer that is sent to the sound card for a given FFT Size from the equation below.

If you know the full scale output voltage of your sound card, you can estimate the level that causes the SoundIO module to trigger from the equation below. Sound cards have a typical input swing ranging from +0.5 to +2.0 volts.

11. Once you have a valid trigger, adjust the Play Control and the Wave sliders so that the signal in the oscilloscope display is not clipped. The

SoundIO modules Peak Levels should be below 90%.

11. Press the Calibration button in the SoundIO module. The Calibration dialog box will open.

12. Select Frequency from the Calibration Type Select: combo box in the Calibration Status group box. Select Vpeak from the Input Cal. Meas. Type Sel: combo box. Select MLS from the Freq. Cal. Type Sel: combo box.

13. Press the Run button and wait for the hour glass cursor to disappear. If a “No data in record buffer” message appears increase the mixers Playback Volume Control and Wave Out or Recording Line sliders.

14. Check the both Apply check boxes in the Frequency Response Calibration group box. The calibration dialog should look as in Figure 3.

Figure 3: Calibration Dialog Box after Calibration

15. Press the Save button in the Calibration dialog box.

16. Select a calibration file from the Save Calibration File dialog box and press the Save button.

17. Press the OK button in the Calibration dialog box.

18. Now rewire the circuit as shown in Figure 4.

Figure 4: Resistance Measurement Process Test Wiring

19. Select Log20 in the Spectrum Analyzers XAxis Scale Selection combo box. Select |Z| in the Spectrum Analyzers YAxis Scale Selection combo box. Select 2ohms/Div vertical scale using the Dn button YAxis group. Enter the value of the reference resistor wired between Ch1 Line-In and Ch2 Line-In in the Ref1: edit box in the spectrum analyzer. Ours is 100.77 ohms.

Figure 5: Spectrum Analyzer Impedance Measurement Setttings

20. Press the Run button on the application toolbar and let the process run for a few cycles. Observe the upper trace in the spectrum analyzer. It should be almost flat line from about 20 Hz to 20kHz as shown in Figure 5. Press the left mouse button to get the measurement at the frequency of interest. The spectrum analyzer returns10.13 ohms at 1Khz which is less than 0.1 ohm from its true value which measured in at 10.067 ohms

What We Do

Sonic beacon produces electrical and acoustical data acquisition and analysis software for the Windows operating system.

About Us

Sonic beacon is a Canadian organization founded in 1997. It is located in Pakenham Ontario which is near Ottawa, Canada. Ottawa is also home to the National Research Council of Canada's anechoic chamber, a key facility for acoustics testing. The chamber has been instrumental in the development of Canadian loudspeakers, hearing aids and microphone arrays.

News and Events

July 8, 2009: Sonic beacon Version 1.1.0.6 released. Data-Logger can now save in .FRD and .ZMA file format. Data-Logger and Spectrum Analyzer can perform .FRD and .ZMA clipboard copy transactions. Signal generator can output signal complements on each sound card channel to allow bridging in many consumer audio interfaces. Module status bar shows current data type, FFT size and sample rate.