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.
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.
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.
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 Volumefor
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.
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.
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 (as in Figure
2).
13. Press the New
Button. Select Auto from the Calibration Type Select: combo
box. Select Vpeak from the Input Cal. Meas Type Sel: combo box. Select
MLS from the Freq. Cal. Type Sel: combo box.
14. Press the Calibration Run
button and wait for the hour glass cursor to disappear. Because Auto
calibration does not calibrate Input Signal Levels its values
should be unity. Check both Apply check boxes in Frequency
Calibration group box. The calibration dialog should look as in
Figure 3.
Figure 3: Calibration Dialog Box
After Calibration
15. Press the OK button
in the calibration dialog and select Yes when the “Calibration Parameter Has Changed Save To
Process File” message box
appears.
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.
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
