PC221 Analog Electronics I
AC Measurements
Objectives
- To review the use of the function generator.
- To introduce measurement of simple AC circuits.
Equipment
- digital multimeter, bench power,
function generator, oscilloscope
Background
Analyzing circuits with both DC and AC components is slightly
more complicated than analyzing DC circuits. One reason is
because of the challenge involved in measuring AC quantities.
Amplitude, Peak-to-peak, or RMS?
A critical issue in AC measurements is
terminology; values can be described in at least 3 different
ways. You always need to know which is being used.
AC voltages can be given as
- amplitude
- peak-to-peak
(i.e. twice the amplitude)
- RMS
The last one, RMS, is the most tricky because it depends on the
shape of the waveform.
Question: How would the RMS for a triangle wave be
related to the RMS of a sawtooth wave? Be prepared to explain
this.
Most digital meters have separate scales for DC and AC
measurements.
Procedure
For the following test, make sure the coupling for the
oscilloscope channel(s) is set to DC.
To test whether the digital meter measures RMS or amplitude,
do the following:
- Connect the signal generator to the oscilloscope and the
digital meter.
- Read the information about measuring AC
voltage with a digital multimeter.
- Produce a 1 kHz, one volt sine wave with no DC offset and
record the offset and amplitude from the oscilloscope
in Table 1.
Since the offset is midpoint voltage of
the wave, it is
the average of the voltages at the top and
bottom of the wave.
Since the amplitude is half of the peak-to-peak value, it is
the difference between
the voltages at the top and bottom of the wave, divided by
two.
- Measure the voltage using both the DC and AC settings on the
meter. What does this tell you?
Table 1: Voltage Measurements (no offset) |
Shape |
Digital Meter |
Oscilloscope |
Units |
|
DC |
AC |
Offset |
Amplitude |
|
Sine |
|
|
|
|
|
Square |
|
|
|
|
|
Triangle |
|
|
|
|
|
- Repeat the measurements for the other shapes listed in
Table 1.
- Read the information about measuring
frequency with a digital multimeter.
- Measure the frequency of the waveform using both the meter
and the oscilloscope.
Table 2: Frequency Measurements |
Shape |
Digital Meter |
Oscilloscope |
Units |
Sine |
|
|
|
Square |
|
|
|
Triangle |
|
|
|
- Adjust the DC offset on the signal generator, and see how
the DC and AC meter readings change.
- Now change the coupling for the oscilloscope to
AC. Adjust the DC offset on the signal generator,
and see what happens to the signal on the oscilloscope.
For a specific value of offset, fill in Table 3.
Table 3: Voltage Measurements (with offset) |
Offset (volts) |
|
Shape |
Digital Meter |
Oscilloscope |
Units |
DC |
AC |
Offset |
Amplitude |
Sine |
|
|
|
|
|
Square |
|
|
|
|
|
Triangle |
|
|
|
|
|
Can you measure the amplitude of the signal in AC mode? Can
you measure the offset in AC mode?
It is critical that you understand
what AC and DC coupling do. Specifically, you need to
understand that it is wrong to say that you should use DC
coupling for measuring DC voltages and AC coupling for AC
voltages. DC coupling is always a better choice as long as
it makes
the AC portion of the
signal large enough to measure on the screen.
Switch the coupling back to DC.
- Once you feel you know how to convert between meter
readings and oscilloscope readings, switch the waveform; first
to
a square wave and then to a triangle wave to see how that
affects
the meter readings.
Make sure to keep the waveform the same size, so that
the amplitude and peak-to-peak measurements should stay the
same.
- Sketch the waveforms for a sine wave, a triangle wave, and
a square wave of the same amplitude and write what the digital
meter reads for each on the sketch, on both the DC and
AC settings.
- What do your results tell you about using a digital meter
for AC signals when you don't know what the waveform looks
like? Why is an oscilloscope a better choice?
- Does the AC reading on your digital meter display
amplitude,
peak-to-peak, or RMS voltage?
For a sine wave, how would you convert an RMS reading to
amplitude?
For a sine wave, how would you convert an amplitude reading to
RMS?
Explain your findings to the
lab
instructor.