Download Experiment 2b: TVS, Simulation using Scope

Salt Lake Community College
Electrical Engineering Department
EE 1010
AC Time Varying Signals and RC Filters (Simulation) Lab
Original 12/31/2009 by Harvey Wilson
Latest revision 6/22/2013 by Harvey Wilson
Introduction
Many signals vary continuously or temporarily verses time. The concepts and
tools of the previous experiments need to be adapted to be useful for these changed
conditions. The Oscilloscope will be introduced. Two types of time varying signals will
be considered: continuous sinusoids and temporary transients.
Objectives
1. Gain confidence in simulation.
2. Recognize measurement conditions where an Oscilloscope may be useful.
Equipment

Multisim circuit simulator
Pre-Lab
1. (None)
Experiment
1. Continuous Sinusoids
a. Continuous Sinusoids are the same shape (average magnitude and frequency) for a
long time. Examples of sinusoids include a single note played on a musical
instrument (violin), a whistle sound, etc.
b. The complex math associated with time varying signals will be introduced in other
classes, is beyond the scope of this class, and will not be covered here. Some
observations will be noted, but not thoroughly explained in this class.
c. Consider a sine wave of 1.41 Volts magnitude and 15.9 k Hertz frequency connected
across a 10nF Capacitor in series with a 1k Ohm Resistor. The voltage across each
component, Vc and Vr, not calculated here, is measured with an AC meter as 1.0
Volt magnitude.
d. Notice that, at the frequency given, the voltage magnitudes are each 1 Volt, but the
total voltage magnitude is not 2, but is 1.41 Volts. We will use the Multisim simulator
to observe these voltage values. The meters included in Multisim display RMS rather
than peak magnitude AC values, but the source (V2) shows peak rather than RMS
(that is what the “free” simulator does), so the AC voltage source will be adjusted
such that the meter monitoring the source will display the intended 1.41 Volts value.
e. Double-click on the figure below to begin a simulation using Multisim. Then do as
directed by lab instructor to estimate the result of voltages and resistors and
capacitors connected in various ways.
Page 1 of 5
Notice that about 1 volt across C1
added to about 1 volt across R2
does NOT add up to about 2 volts
measured across the source!
XMM1
V1
12V
R3
9.1kohm
XMM3
R1
1.0kohm
C3
C1
1.0nF
10nF
V2
2V 15.8kHz 0Deg
XMM2
R2
1.0kohm
C4
C2
100nF
10nF
f.
g. Double-click on each of the XMM symbols, then position the meter displays near the
respective meters. If needed, change each meter to measure AC voltage.
h. Use “Place Text” to add your name and comments on the simulator screen for each
required test condition or circuit before you copy resulting image using “Print
Screen”, and paste results in a document.
i. Try a capacitor and a resistor in series and also connected to a sine wave voltage
source, as given, to learn if voltage distribution is affected by frequency. Use 15.8
kHz as a reference.
j. Try changing the voltage source frequency up to 158 kHz, then down to 1.58 kHz to
observe the result of increasing or decreasing frequency on the voltage across the
capacitor. Is the capacitor voltage affected by frequency?
2. Continuous Sinusoids with RC Filters
a. The frequency at which the voltage across the capacitor is the same magnitude as
the voltage across the in-series resistor is the cross-over or break or ½ power
frequency. That is an important frequency measurement for filters. The RC (Resistor
Capacitor) circuit just tested is known as a high-pass filter because the voltage
magnitude across the output resistor is greater (passes easier) for frequencies
higher than the cross-over frequency.
b. Try making a low-pass filter by swapping the positions of the capacitance and
resistance such that the source voltage passes through a resistor, then capacitor,
then to ground. This way the capacitor is the output circuit element. The RC circuit is
known as a low-pass filter because the voltage magnitude across the output
capacitor is greater (passes easier) for frequencies lower than the cross-over
frequency. (Did you try three frequencies?)
c. Try two resistors in series (no capacitor) and also connected to a sine wave voltage
source to learn if voltage is distributed according to the ratio of resistances and
independent of frequency. (Did you try three frequencies?)
d. Try making a voltage divider of this ratio: 2:1 using only two capacitors. Is this
voltage divider ratio independent of frequency in the range of 1.58 kHz to 158 kHz?
Please describe what you observe.
e. The next few tests may be skipped if you do not want the challenge.
Page 2 of 5
f.
Try making a voltage divider of this ratio: 10:1 using the three capacitors. Is this
voltage divider ratio independent of frequency in the range of 1.58 kHz to 158 kHz?
Please describe what you observe.
g. Try making a voltage divider of this ratio: 10:1 using the two capacitor values from
the previous experiment in parallel with a set of two resistors you selected in the DC
experiments. This voltage divider would be similar to the voltage divider made by
anyone who uses a 10X probe with an oscilloscope. Is this voltage divider ratio
independent of frequency in the range of 1.58 kHz to 158 kHz? Please describe what
you observe.
h. End of “may be skipped”.
i. The next set of simulations will include a DC source connected in series with the AC
source before the revised source is connected to any load resistor(s) or capacitor(s).
Refer to the next figure.
Notice that the addition of the
12V DC did not change the AC voltage
magnitude across any AC meter.
XMM1
V1
12V
R3
9.1kohm
XMM3
R1
1.0kohm
C3
C1
1.0nF
10nF
V2
2V 15.8kHz 0Deg
XMM2
R2
1.0kohm
C4
C2
100nF
10nF
j.
k. The DC voltage does not show up across the meters while measuring AC voltage,
but does show up primarily across the series capacitor when measuring DC voltage.
This means that the combination of AC with DC voltages is changed to only AC after
passing through a capacitor – the capacitor “blocks” the DC voltage from the circuit
that follows. This can be a useful effect, or a bother, depending on what the
designer wants the circuit to do.
l. Ideal AC meters measure AC frequencies from near 0 Hz (almost DC) to way higher
than any frequency our lab equipment can produce (about 5megHz maximum). But
our lab meters are not that good. So, how good are they – what range of
frequencies will each type of our lab meters measure when set to measure AC? We
will make these measurements in a later experiment.
m. Again,
n. would you want to advertise that feature?
3. Continuous Sinusoids Observed on an Oscilloscope
a. One measurement instrument which displays graphically signals which vary over
time is called an oscilloscope or “scope”. The scope display will assist in
understanding and measuring time varying signals.
b. The next set of simulations will include a scope. Refer to the next figure while making
as many notes as you need to be able to use and understand a scope in your
simulation. Double-click on the figure to start the simulation. Next double-click on the
Page 3 of 5
XSC1 symbol and each of the XMM symbols, and then position all displays where
you want them (or as instructor shows you). Last double-click on the V2 symbol and
position as shown. Run and observe a scope display.
XSC1
G
XMM1
V1
5V
A
R1
B
100kohm
XMM3
V2
XMM2
2V 100Hz 0Deg
R2
C1
100kohm
100nF
c.
d. The above circuit has 5 volts DC source added to a 2 volts peak (4 volts peak to
peak) magnitude AC sine wave source. The AC meters only display the AC RMS
(not peak) voltages. The scope shows the voltages verses time. Note that the “B”
display on the scope (red color) shows the voltage divider effect on the DC source
(½ of 5 is 2.5) combined with the low-pass filter effect on the AC source (smaller and
delayed). Note that the “A” display on the scope (green color) shows the combination
of the DC and AC sources (4 volts peak to peak with a 5 volt offset).
e. The other type of time varying signal is the temporary transient.
4. Temporary Transients
a. Temporary Transients are irregular in shape or are momentary in duration or both.
Examples of transients are thunder and speech.
b. Temporary signals are not easily measured or displayed on the DC or AC meters we
have use in class up to this point. We will use a scope which can capture and hold
signals which wiggle for only a short time. Scopes are complicated, take hours to
master, and are the preferred instrument for many of the measurements in EE
classes, including EE1010.
c. One common temporary transient signal encountered in EE experiments is the
exponential wave.
d. The next simulation is different from the previous simulation only in the nature of the
AC source – it is a rectangular-shape wave (square wave). Run and observe a scope
display. How meaningful are the AC meter displays for this circuit?
Page 4 of 5
T
XSC1
G
XM M 1
V1
5V
A
R1
B
T
10 0kohm
XM M 3
V3
XM M 2
- 2 V 2V 100 Hz
R2
C1
10 0kohm
10 0nF
e.
f.
Try various changes as instructed by lab instructor and keep good notes.
Conclusions
Write a conclusion in your notebook. Mention any problems that you
encountered in this lab and how you overcame them.
Page 5 of 5