Measuring Resistance
Here we use the computers 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. AC'97 and USB sound cards generally have very poor output drive on
the line outputs. For our purposes a 100-ohm resistor will be a
reasonable reference (around 150mW if the sound card can output 4 Volts
maximum).
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 good ohmmeter. Our reference was 100.77
ohms.
2. Wire the resistors as
shown in Figure 1. Use short low resistance wiring. Note that older sound
card Speaker Outputs have more output drive than their respective Line
Outputs. However most AC'97 and USB sound cards may only have line level
outputs so choose a reference resistor equal to or above 100 ohms.
Figure 1: Resistance Measurement Process Calibration Wiring
3. If you are running
Windows 7 or Vista you must disable all sound effects applied to your
sound card. Right click the sound icon on the windows task bar and select
Playback devices from the popup menu that appears. Select the
sound card you intend to use from the "Select a playback device
below to modify its settings" listbox and press the Properties
button. Select the Enhancements tab and check the Disable all
Sound effects checkbox. Press the OK button.
Figure 2 Windows 7/Vista Playback Enhancements Disabled
4. Open " C:\Users\Public\Documents\Sonic
Beacon\Sonic Beacon\32768_MLS_Impedance_Measurement.process"
from the applications File…Open… menu.
5. Select your sound
card in the In: and Out: combo boxes in the SoundIO modules
Sound Device Select group. Sonic Beacon can detect and work with
multiple sound cards.
6. You need to adjust
the level of your selected sound card recording path. If you are running
Windows 7 or Vista right click the sound icon on the windows task bar and
select Recording devices from the popup menu that appears. Double
click the selected sound card in the Sound dialog box Recording
tab. Select the Levels tab and adjust the slider to its
one-quarter setting. Press the OK button. If you are running XP or below;
select the Levels tab 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. 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. You need to adjust
the output level of your selected sound cards playback path. If you are
running Windows 7 or Vista select the Playback tab of the Sound
dialog box and double click the selected sound card. Select the Levels
tab and adjust the Line In to its 25% setting. Otherwise if you are using
XP or lower select Options… Properties… Press the Playback
radio button in the Adjust Volume for group. Press the OK
button. 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
8. Now press the
applications Run button. You should be able to see the MLS
sequence in the oscilloscope module as shown in Figure 2. The amplitude
of the oscilloscopes signal may vary depending on the sound card. 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% (as detailed above).
Figure 2: MLS Sequence in the Oscilloscope Module
9. 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.
10. Once you have a valid trigger, adjust the Play
Control (Playback) and the Wave (recording) sliders so that
the signal in the oscilloscope display is not clipped. The SoundIO
modules Peak Levels must be below 95%.
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 resistance
measurement. 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 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.
Figure 3: Calibration Dialog Box after Calibration
14. Press the Save button in the
Calibration dialog box.
15. Select a calibration file from the Save
Calibration File dialog box and press the Save button.
16. Press the OK button in the
Calibration dialog box.
17. Now rewire the circuit as exactly as shown
in Figure 4.
Figure 4: Resistance Measurement Process Test Wiring
18. Make sure that the Apply Frequency Checkbox
in the Spectrum Analyzers Options Group is selected or the frequency
calibration performed above will not take effect.
19. Select Log20 in the Spectrum
Analyzers XAxis Scale Selection combo box. Select |Z| in the
Spectrum Analyzers YAxis Scale Selection combo box. 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. Select a YAxis
Scale that is guaranteed to show the impedance of your resistor. We
selected 2ohms/Div vertical scale using the Dn button YAxis
group because our unknown resistor was marked as 10 ohms and the maximum
YAxis Scale value is 16 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. The text in
the status bar at the bottom of each module will turn green as each
module processes the data. Wait until all modules have processed the
data. 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
Figure 6: Spectrum Analyzer Impedance
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