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Sonic beacon Loudspeaker Test Software

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 Measuring Complex Impedance

Here we measure the complex impedance of a 1uF polypropylene capacitor and then check its deviation from the rated value using the equation for capacitive reactance.

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First we measured the capacitor with a precision LC meter. It came in at 1.017uF. A 994W resistor will be used as the reference impedance. This resistor was measured with a precision ohmmeter. We will use "32768_MLS_Impedance_Measure.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 DUT. Second is the SoundIO module, which plays the stimulus and records the response of the capacitor. Third is the Oscilloscope module, which allows us to view the time domain response of the capacitor. 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.

You must have a dual channel, duplex sound card to perform this measurement.

Later a table showing the measured impedance results using three different values of reference resistance will be developed. This will illustrate some of the problems that may be encountered in the transition from constant current to constant voltage measurement. 

 

1.       Measure the value of the reference resistor using a precision ohmmeter. Our reference was 994.482 ohms.

2.       Wire the circuit as shown in Figure 1. Use short low resistance wiring. Note that many sound cards Speaker Outputs have more output swing than their respective Line Outputs.

image003

Figure 1: Capacitance Measurement Process Calibration Wiring

3.       Open “32768_MLS_Impedance_Measurement.process” from the applications File…Open… menu. Press OK if a “No Compatible Calibration File Present” message box appears.

4.       Open the FFT Options dialog from the applications Options…FFT… menu and ensure the FFT Size is 32768 in the Frequency Resolution group. Press OK in the FFT Options dialog box. Press OK when the “No Compatible Calibration File Present” message box appears.

5.       Select 4 in the SoundIO modules Repeat Sequence: combo box. This will provide some MLS pre-excitement for the sound card and device under test. This will help to stabilize high frequency phase calibration, which can be erratic, depending on where the sound card triggers. 

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

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

8.       Select the Record Controls Options… Properties… menu. Press the Playback radio button in the Adjust Volume for group. Press the OK button.

9.       Mute all Playback mixer gain settings except Master Volume and Wave. Set both volume controls to one quarter and equalize their balance settings.

10.   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 or Recording Line controls.

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. Check the Trig Level (%F.S.) combo box in the SoundIO Trigger Parameters group. If it is greater than 20 enter 10 in the edit control. 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.

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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.

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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 (as in Figure 2).

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Figure 2: MLS Sequence in the Oscilloscope Module

11.   You now need to calibrate the frequency response curve of the sound card if you do not already have a valid calibration file loaded for the input and output devices selected in the SoundIO module. Level and Latency calibration are not required for complex impedance measurement. Press the Calibration button in the SoundIO module. The SoundIO Module Calibration dialog box will open.

12.   Select Frequency from the Calibration Type Select: combo box in the Calibration Status group box. Select MLS from the Signal Type combo box in the Frequency Calibration group box.

13.   Press the Run button and wait for the Frequency Calibration Complete status message to appear. If a “No data in record buffer” message box appears Press the Open Mixer button and increase the applications output level slider. The SoundIO modules input device level may also be increased by right clicking on the Windows Task Bar Sound icon and selecting the Recording and Levels and adjusting the devices slider. Frequency calibration may be restarted by pressing the Run button. If successful, the calibration dialog should look as in Figure 3. The Calibration Progress bar may not update in certain versions of Windows.

 

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Figure 3: SoundIO Module Calibration Dialog Box After Calibration

12.   Press the OK button in the calibration dialog and select Yes when the “Calibration Parameter Has Changed Save To Process File” message box appears.

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

 

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Figure 4: Capacitance Measurement Process Test Wiring

14.   Select |Z| from the spectrum analyzers Y-Axis Select combo box and change the Y-Axis Scale to 1000 ohms/div. Enter the exact value of the reference resistor wired between Ch1 Line-In and Ch2 Line-In in the Ref1: edit box in the spectrum analyzer. Check the Apply Freq Cal. Checkbox in the Spectrum Analyzers Options group. Change the smoothing factor to 7 in the Smth combo box of in the spectrum analyzer. This will help smooth the low frequency results.

15.   Press the Run button on the application toolbar and observe the trace in the spectrum analyzer. The impedance and phase appear in channels one and two respectively. Pressing the right mouse button at the desired frequency will show individual values. For example at 1004 Hz we obtain a value of 157.0W. This is out by 1.12W (0.724%) from the calculated value of 155.87W at that frequency.

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Figure 5: Measured Impedance Results in Spectrum Analyzer

Below is a table showing the measured impedance results using three different values of reference resistance (101W, 994W and 10,467W respectively). The process was re-calibrated for each new reference resistor.

Using a 101W reference we see that low frequency data seems unreliable. This is because the voltage on both sound card inputs is almost equal out to about 1KHz, because the impedance of the capacitor is much larger than that of the resistor. In this case even a small amount of sound card noise can cause a very large error in the apparent measurement.

Using a 10,467W reference we see that high frequency data seems unstable. This is because the voltage at the Channel 2 sound card input is almost equal to zero because the impedance of the capacitor is much lower than that of the resistor. In this case even a small amount of sound card noise can cause a very large error in the apparent measurement

 

Reference

20Hz

50Hz

100Hz

500Hz

1Khz

5KHz

10KHz

Calculated

7,825W

3,129 W

1,565 W

313 W

157 W

31 W

16 W

101 W

8,069 W

2,751 W

1,633 W

313 W

158 W

32 W

15 W

994 W

6,788 W

3,074 W

1,557 W

310 W

156 W

32 W

16 W

10467 W

7,492 W

2,985 W

1,623 W

312 W

162 W

31 W

17 W

 

Below is a table showing the same measurements done with a 994W reference and a process FFT Size of 8192. This translates to a spectral resolution of 10.77 Hz. We would expect low frequency measurements to be inaccurate because of the decreased resolution. This can be seen as an increased raggedness in the low frequency portion of the curve

 

Figure 6: Measured Impedance Results in Spectrum Analyzer at 8192 FFT Size

Reference

20Hz

50Hz

100Hz

500Hz

1Khz

5KHz

10KHz

Calculated

7,825W

3,129 W

1,565 W

313 W

157 W

31 W

16 W

994 W

7,886 W

2,770 W

1,533 W

315 W

157 W

33 W

15 W

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 that provides a free set of virtual instruments that are useful for measuring the time and frequency domain responses of audio components using a personal computer. It is located in Pakenham Ontario which is near Ottawa, Canada.

News and Events

March 23, 2014: 32 and 64 Bit Versions of sonic beacon (1.1.0.9) released. Tested on Microsoft Windows 8 (64 Bit), Microsoft Window 7 Home Premium (64 Bit) and Windows XP 32 Bit Professional.

Copyright © 2007 sonic beacon. All rights reserved.