Basic Scalar Measurements
A LV Player VI showing measurement of AC, DC, and AC+DC
This demo generates both AC and DC signals, on which you can make various types of AC and DC measurements. An AC (Alternating Current) signal is one that is constantly changing, such as a sine wave. A DC Signal (Direct Current), does not change or only changes extremely slowly.
Experiment 1: DC Measurements: Try to change the Peak Amplitude of the Sine Wave. Notice that this has no effect on the DC measurement, because the sine wave is symmetrical about DC and hence has a 0 average value. You can see this if you hit the Show Graph Button. Hide Graph again. Now change the DC Voltage and verify that the measured value actually changes.
Experiment 2: Now change the Measurement to AC RMS. This removes the DC component from the measurement. Try changing the DC Voltage to verify that it has no effect on the AC measurement. Now change the Peak Amplitude of the 10 Hz signal. Notice that the Measured Voltage changes. Note that the AC RMS voltage is peak voltage of the sine wave divided by the square root of two.
Experiment 3: Experiment with different AC Signal Types. Note that for the square wave, the peak and AC RMS voltage are the same.
Experiment 4: AC + DC Measurements: Here the "sum" of the AC and DC signals is measured. So what is 1 + 1? Well when you deal with RMS voltage measurements, 1+1 does not equal 2 (At least not in volts!) With the Sine wave at a Peak Amplitude of 1.4142 (remember this equals 1 V RMS) and the DC voltage at 1.00 V select AC + DC RMS. The result is 1.4142. This is a doubling of the power, but only an increase by a factor equal to the square root of two for the voltage. This is because RMS measurements always are based on the power carried by the signal. The values at each instance in time is Squared, these values (which are the power values) are then summed up giving the Mean value and finally the Square root is taken to get us back to volts. This is why it is called Root Mean Square (RMS).
Experiment 5: Now try a sine wave with a peak amplitude of 5.6569 (RMS = 4 V) and a DC voltage of 3 V DC.
Select an AC measurement and verify that the AC voltage indeed is 4.0000 V. Now try to figure out what the 3 volts DC plus the 4 Volts AC gives: 3 squared plus 4 squared gives 9 + 16 = 25, and the square root is 5. Verify that the instrument measures 5 Volts for AC + DC RMS. This again illustrates that for RMS measurements, voltages are summed as power values, i.e. squared and added under the square root sign.
Experiment 6: Try to change the No. of Samples. Since you don't an integral number of periods in your measurement, it will be wrong! In real life, you either need to have an integral number of periods, have a long averaging time, or best of all, use the Extra Single Frequency function which is new in LabVIEW 6i and overcomes many of these problems!
Experiment 7: For all of these measurements, you may feel you are measuring "in the dark". This is because you cannot see the waveform of the signal you are dealing with. This is also the case with traditional voltmeters. But with virtual instrumentation, you can display what you want. Hit the Show Graph button and you will see an oscilloscope graph come to life. Now repeat the above experiments, and see how much easier it is to know what you are doing when you can also see the waveform.
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