University of Jordan
Electrical Engineering Department
EE 429
Communications Lab
EXPERIMENT 1
LAB EQUIPMENT FAMILIARIZATION
Lab Supervisors: Dr. Mohammed Hawa
Dr. Ahmad Mustafa
Lab Engineer: Eng. Reem Al-Debs
Prepared By: Dr. Mohammed Hawa
EXPERIMENT 1
LAB EQUIPMENT FAMILIARIZATION
PROCEDURE A - CIRCUIT BOARD FAMILIARIZATION
In this PROCEDURE section, you will examine and use the two oscillator circuit blocks in the
ANALOG COMMUNICATIONS circuit board.
An oscillator circuit is an electronic circuit that produces a repetitive signal, in this case a
sinusoidal wave. A special oscillator, called a Voltage-Controlled Oscillator (VCO), is an
oscillator circuit that is designed in such a way its output frequency is controlled by an input
DC voltage. This allows us to change the output frequency of the oscillator by simply changing
its input voltage. It also allows a VCO to work as an FM modulator since the frequency of the
output carrier can be controlled by the voltage of the modulating message signal.
The ANALOG COMMUNICATIONS circuit board contains two VCO circuit blocks. They are
called VCO-LO (produces low frequencies) and VCO-HI (produces high frequencies).
The VCO-LO circuit block can output a sinusoidal signal in the 452 kHz range or in the
1000 kHz range. It can also be used to output an FM signal. The VCO-HI circuit block, on the
other hand, outputs a sinusoidal signal in the 1455 kHz range.
1. Locate the VCO-LO circuit block on the ANALOG COMMUNICATIONS circuit board.
Insert a two-post connector in the 1000 kHz terminals as shown in Figure 1-21.
2. Set the oscilloscope channel 1 to 10 mV/DIV and set the sweep to 0.5 μs/DIV. Connect the
channel 1 probe to OUT on the VCO-LO circuit block, and connect the probe ground clip to a
ground terminal on the circuit board (Figure 1-21). Set Trigger Source to CH1.
NOTE: Whenever you make oscilloscope measurements or observations, be sure to connect the
probe’s ground clip to a ground terminal on the circuit board.
Two-post
connector
VCO-LO
potentiometer
knob
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3. On the VCO-LO circuit block, set the potentiometer knob completely clockwise (CW) and
then counterclockwise (CCW). Does this potentiometer adjust the VCO-LO frequency or the
VCO-LO amplitude?
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4. Set the NEGATIVE SUPPLY knob (which is located on the top left side of the base unit)
completely CW and then completely CCW. Does the NEGATIVE SUPPLY knob adjust the
VCO-LO frequency or the VCO-LO amplitude?
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NOTE: The probe used in this experiment is x10 probe, which means that vertical readings on
the oscilloscope should be multiplied by 10 to obtain the correct reading. For example a
reading of 100 mV on the oscilloscope is actually 1 V. Time (horizontal) readings are not
affected, though.
5. With channel 1 set to 10 mV/DIV, and VCO-LO generating a sinusoidal signal that is
300 mVpk-pk, calculate how many vertical oscilloscope graticule divisions will equal the
300 mVpk-pk?
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6. Slowly turn the VCO-LO potentiometer knob until the output signal on channel 1 is
300 mVpk-pk.
7. With a sweep setting of 0.5 μs/DIV, calculate how many horizontal oscilloscope graticule
divisions equal one cycle of a 1000 kHz signal?
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NOTE: To adjust the VCO-LO frequency, adjust the NEGATIVE SUPPLY knob on the base
unit. This knob controls the amount of input DC voltage to the VCO. Use the top portion of the
NEGATIVE SUPPLY knob for fine adjustments, and use the bottom portion for coarse
adjustments.
8. Adjust the NEGATIVE SUPPLY knob so that the VCO-LO output frequency is 1000 kHz.
Does the signal shown on the oscilloscope match the signal shown in Figure 1-22 or not?
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9. Remove the two-post connector from the 1000 kHz terminals and insert it in the 452 kHz
terminals. Adjust the VCO-LO potentiometer knob to set the output signal on channel 1 to
300 mVpk-pk.
10. With a sweep setting of 0.5 μs/DIV, calculate how many horizontal oscilloscope graticule
divisions equal 1 cycle of a 452 kHz signal?
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11. Adjust the NEGATIVE SUPPLY knob so that the VCO-LO frequency is 452 kHz. Does the
signal shown on the oscilloscope look like the signal shown in Figure 1-23 or not?
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12. Locate the VCO-HI circuit block on the ANALOG COMMUNICATIONS circuit board, and
set its potentiometer knob (which adjusts the amplitude) completely CCW.
NOTE: To adjust the VCO-HI frequency, adjust the POSITIVE SUPPLY knob, which is located
on the top right corner of the base unit.
13. Set oscilloscope channel 1 to 10 mV/DIV and set the sweep to 0.5 μs/DIV. Connect the
channel 1 probe to OUT on the VCO-HI circuit block, and connect the probe ground clip to a
ground terminal on the circuit board (Figure 1-24).
14. Slowly turn the VCO-HI potentiometer knob CW until the output signal on channel 1 is
300 mVpk-pk.
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15. While observing the signal on channel 1, slowly turn the POSITIVE SUPPLY knob in the
CW direction. Did the VCO-HI frequency increase or decrease?
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16. Set the oscilloscope sweep to 0.2 μs/DIV. Calculate how many horizontal oscilloscope
graticule divisions equal one cycle of a 1455 kHz signal.
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17. Adjust the POSITIVE SUPPLY knob completely CW and then completely CCW. Use the top
portion of the POSITIVE SUPPLY knob for fine adjustments and the bottom portion for coarse
adjustments. What is the range of frequencies that the VCO-HI on your kit can provide?
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PROCEDURE B – THE OSCILLOSCOPE
This PROCEDURE section aims at reminding you of some of the main features of the
oscilloscope. Such concepts will be helpful to you as you execute the different experiments of
this lab.
An oscilloscope (see Figure 1) is a measurement device designed to measure voltages. Unlike a
voltmeter, however, an oscilloscope does not display a single number. Instead, it displays
signals (voltages) that are functions of time. Such a signal shape allows you to measure certain
signal parameters, such as its frequency, period, peak-to-peak voltage, DC offset value, phase
shift, etc.
The oscilloscope consists of four subsystems (Figure 1): Horizontal subsystem controls,
Vertical subsystem controls, Trigger subsystem controls, CRT subsystem controls.
Figure 1.
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1. On your oscilloscope, identify the four different subsystems mentioned above. List all the
knobs and controls you can see within each subsystem? Also list the possible settings for each
such control or knob? Notice that your answer might vary depending on the type of
oscilloscope you are using.
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2. Locate the VCO-LO circuit block on the ANALOG COMMUNICATIONS circuit board.
Insert a two-post connector in the 1000 kHz terminals as shown in Figure 1-21.
3. Set the oscilloscope’s “Volts/Division” knob for Channel 1 to 20 mV/DIV (this knob is
located within the Vertical subsystem), and set the sweep (i.e., “Seconds/Division” knob) to
0.5 μs/DIV (this knob is located within the Horizontal subsystem). Connect the channel 1
probe to the output (OUT) of the VCO-LO circuit block, and connect the probe ground clip to a
ground terminal on the circuit board (Figure 1-21).
4. Set the Vertical Mode to CH1, and set the Trigger Source (in the Trigger subsystem) also to
CH1 (i.e., the signal on channel 1 is triggered internally by its own signal).
5. While observing the signal on the oscilloscope, turn the VCO-LO potentiometer knob
completely in the CW direction. This will give you maximum peak-to-peak voltage in the
output signal. Also, adjust the NEGATIVE SUPPLY knob to get an output signal frequency of
exactly 1000 kHz.
6. Turn the “Volts/Division” knob for channel 1 in the CW and then CCW directions. How
does that affect what you see on the oscilloscope?
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7. Now, turn the “Seconds/Division” knob in the CW and then CCW directions. How does
that affect the signal you see on the oscilloscope?
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PROCEDURE C – OSCILLOSCOPE TRIGGER
The oscilloscope is suitable for displaying signals that are periodic (i.e., repeat themselves in
time), such as sinusoidal, triangular and square wave signals.
The way a periodic signal is displayed on the screen of an oscilloscope is through a Trigger
subsystem, which is used to display a portion of the signal over and over again by successive
sweeps that start at corresponding points on that periodic waveform. This operation is
schematically illustrated in Figure 2.
Oscilloscope
Screen
Trigger
Level
Periodic Signal being displayed by the Oscilloscope
Sweep starting points
Figure 2.
To get a stable diagram of a periodic signal on the oscilloscope’s screen, each sweep on the
screen should start at a similar point of the periodic signal. This is why the signal used to
trigger the sweeping system should have the exact same frequency as the signal to be
displayed (or at least exact multiples of the signal’s frequency). Otherwise, you will not be able
to see a stable signal on the screen.
Figure 3 shows both an un-properly triggered (unstable) signal and a properly triggered
(stable) signal both being displayed on the oscilloscope screen.
Figure 3.
The LEVEL knob in the Trigger subsystem controls the voltage at which the trigger starts the
successive sweeps. The SLOPE button in the Trigger subsystem is used to specify whether the
signal should have a positive or negative slope at the sweep start.
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The source of the trigger signal (Trigger Source) is chosen with a set of switches on the
oscilloscope. You may choose as your source of trigger the signals applied to the vertical
channels (by clicking on CH1 or CH2 buttons), or you may choose as your source of trigger the
50 Hz sinusoidal signal on power lines (by clicking on the LINE button), or you may choose as
your source of trigger the signal applied to the EXT input connector (by clicking on the EXT
button).
Choosing the Trigger Source to be CH1 or CH2 is the more common option, and the one you
will often use in this lab. You may choose to use CH1 or CH2 for triggering, independently of
which channel you are actually displaying on the oscilloscope’s screen. There is also an ALT
sweep, which selects the trigger from whichever channel is supposed to be displayed next on
the screen.
8. Make sure that channel 1 is set to 20 mV/DIV and that the sweep is set to 0.5 μs/DIV. Also
make sure that VCO-LO output is connected to the oscilloscope’s channel 1 probe, and the
Trigger Source is set to CH1. Is the signal you see now on the oscilloscope’s screen a stable
signal or not? Explain why it is stable, or why it is not stable. Note: If you get an unexpected
result, try adjusting the LEVEL knob in the Trigger subsystem.
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9. Set the Trigger Source to EXT. Use a BNC cable to connect the oscilloscope external trigger
port (EXT port) to the output of the VCO-LO block. Is the signal you see on the oscilloscope a
stable signal or not? Explain? Note: The EXT port is located on the bottom right corner in some
oscilloscopes, and on the rear panel in some other oscilloscopes.
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10. Now connect the channel 2 probe to the output of the VCO-HI circuit block, and connect
the probe ground clip to a ground terminal on the circuit board. Set the Vertical Mode to
DUAL (some oscilloscopes call this ALT mode). This will display both signals (channel 1 and
channel 2) on the oscilloscope simultaneously. Set the Trigger Source to CH2.
11. Slowly turn the VCO-HI potentiometer knob CW until the output signal on channel 2 is
400 mVpk-pk, and slowly turn the POSITIVE SUPPLY knob until the signal’s frequency is 1455
kHz?
12. Which of the two signals: VCO-LO or VCO-HI is stable? Explain?
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13. Switch the Trigger Source to EXT. Which of the two signals: VCO-LO or VCO-HI is stable?
Explain?
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14. What is the only Trigger Source setting that will ensure that both signals VCO-LO and
VCO-HI are stable at the same time? Explain?
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15. Keep the Trigger Source at EXT, and use a BNC cable to connect the oscilloscope external
trigger port to the output of the function generator. Set the function generator output to
roughly 1200 kHz and its voltage to around quarter turn of the amplitude signal on the
function generator. [You do not need to measure these values; rough values are acceptable].
Which of the two signals: VCO-LO or VCO-HI is stable now? Why?
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16. Disconnect all wiring from the ANALOG COMMUNICATIONS circuit board.
PROCEDURE D – OSCILLOSCOPE COUPLING
For each of the vertical inputs (channels 1 and 2), the Oscilloscope provides three possible
coupling choices: GND, DC and AC. This allows you to select either a ground signal, direct
(DC) coupled signal or a capacitive (AC) coupled signal based on your needs.
17. Set the oscilloscope channel 1 to 50 mV/DIV and set the sweep to 0.5 ms/DIV, the Vertical
Mode to CH1, and the Trigger Source to CH1. Connect the channel 1 probe to the function
generator output.
18. Set the coupling of channel 1 to GND. Adjust the "Vertical Position" knob of channel 1 on
the oscilloscope so that the beam is in the middle of the screen. Now set the coupling of
channel 1 to DC.
19. Pull the "Offset" knob of the function generator outward and turn it to half way through.
This will add zero offset to the output signal of the function generator. While observing the
signal on the oscilloscope, adjust the function generator output signal to 2 Vpk-pk and 1 kHz
frequency.
20. Now slowly turn the "Offset" knob of the function generator further in the CW direction
and notice what happens to the signal on the oscilloscope. Explain your observations.
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21. While the offset knob is completely in the CW direction, set the coupling of channel 1 to
AC. What happens to the signal shown on the Oscilloscope?
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22. What circuit element does the oscilloscope use in AC coupling to block DC signals (offsets)?
Explain.
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