Lab Report #3
Principles of Communication Systems
Hataf Bin Atif
FA17-BEE-041
BEE-5A
Lab#3 Voltage Control Oscillator on LabVolt: VCO-LO and VCO-HI
Objectives:
 At the completion of this exercise, you will be able to
 Generate the low and high frequency signals using VCO-LO and VCO-HI modules of the LabVolt
 Use an oscilloscope to make signal measurements.
Pre Lab:
The two voltage-controlled oscillators (VCOs) on the circuit board are the VCO-LO circuit block (452 kHz or 1000 kHz) and
the VCO-HI circuit block (1455 kHz). These oscillators provide the high frequencies that are necessary for transmitting
and receiving the low-frequency message signal.
LabVoltTM Analog Communication Module
Procedure A - Oscillator Circuit Blocks:
In this procedure section, you will examine and use the VCO-LO and VCO-HI circuit blocks. The VCO-LO circuit block is an
oscillator circuit that performs three functions: it can output a 1000 khz signal, a 452 khz signal, or an FM signal. The
VCO-HI circuit block outputs a 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.
2. On the VCO-LO circuit block1 set the potentiometer knob completely counter clockwise (CCW). Does this pot
adjust the VCO- LO frequency or the VCO-LO amplitude?
Observation: This potentiometer controls the amplitude of the signal. Upon setting the knob counter-clockwise,
the amplitude is at its minimum.
3. Set the NEGATIVE SUPPLY knob on the left side of the base unit completely CCW. Does the NEGATIVE SUPPLY
knob adjust the VCO-LO frequency or the VCO-LO phase?
(NOTE: To avoid confusion, you might want to write VCO-LO and VCO-HI on labels made from masking tape. Place the
labels below the NEGATIVE SUPPLY (VCO-LO) and POSITIVE (VCO- HI) SUPPLY knobs)
Observation: Upon adjusting this potentiometer, the frequency of the signal is adjusted. The top of the knob is used for
controlling the frequency, while the lower knob is used to fine-tune this frequency.
4. Set the oscilloscope channel 1 to 100 mV/DlV 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.
(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.)
5. With channel 1 set to 100 mV/DIV, how many vertical oscilloscope graticule divisions will equal a 300 mVpk-pk
signal?
Observation: Since one vertical division shows 100mV, a signal of 300mV will be consist of three division.
6. Slowly turn the VCO-LO potentiometer knob clockwise (CW) until the output signal on channel 1 is 300 mVpk-pk·
(NOTE: To adjust the VCO-LO frequency, adjust the NEGATIVE SUPPLY knob on the base unit).
7. While observing the signal on channel 1, slowly turn the NEGATIVE SUPPLY knob about a quarter turn CW. Use
the top portion of the NEGATIVE SUPPLY knob for fine adjustments, and use the bottom portion for coarse
adjustments. Did the VCO-LO frequency increase or decrease?
Observation: Turning the knob clockwise results in an increase in the frequency.
8. With a sweep setting of 0.5 μs/DIVI how many horizontal oscilloscope graticule divisions equal one cycle of a
1000 kHz signal?
Observation: Since one horizontal division shows 0.5μs, and the time period of 1MHz wave is 1μs, it can be deduced that
one cycle of the signal generated would be of 2 vertical division, as shown in the figure in point 9.
9. Adjust the NEGATIVE SUPPLY knob so that the VCO-LO output frequency is 1000 kHz.
10. Remove the two-post connector from the 1000 kHz terminals and insert it in the 452 kHz terminals. Adjust VCOLO potentiometer knob to set the output signal on channel 1 to 200 mVpk-pk·
11. With a sweep setting of 0.5 μs/DIV, how many horizontal oscilloscope graticule divisions equal 1 cycle of a 452
kHz signal?
Observation: It can be seen in the figure that one cycle of the ~452KHz wave generated is of almost 4 divisions. It can
also be calculated mathematically by the given formula:
Division: 1/425KHz/500ns = 4.42478
12. Adjust the NEGATIVE SUPPLY knob so that 1 cycle of the channel 1 signal equals 4.425 divisions, which sets the
VCO-LO frequency at 452 kHz.
13. Locate the VCO-HI circuit block on the ANALOG COMMUNI- CATIONS circuit board, and set the potentiometer
knob, which adjusts the amplitude completely CCW
(NOTE: To adjust the VCO-HI frequency, adjust the POSITIVE SUPPLY knob on the base unit).
14. Set oscilloscope channel 1 to 100 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 dip to a ground terminal on the circuit board.
15. Slowly turn the VCO-HI potentiometer knob CW until the output signal on channel 1 is 300 mVpkpk·
16. While observing the signal on channel 1, slowly turn the POSITIVE SUPPLY knob about a quarter turn CW. Use
the top portion of the NEGATIVE SUPPLY knob for fine adjustments and the bottom portion for coarse
adjustments. Did the VCO-HI frequency increase or decrease?
Observation: Turning the knob clockwise results in a decrease
in frequency.
17. Set the oscilloscope sweep to 0.2 μs/DIV. How many
horizontal oscilloscope graticule divisions equal one cycle of
a 1455 kHz signal?
Observation: At this time sweep and frequency, one cycle of the
signal is plotted on 3.5 horizontal division.
18. Adjust the POSITIVE SUPPLY knob so that one cycle of
channel 1 signal covers 3.436 divisions. This adjustment set
the VCO-HI frequency at 1455 kHz.
Critical Analysis / Conclusion:
In this lab, we were introduced to Analog Communication Module (LabVolt 91018). We learnt how to generate an AC
signal with a frequency of choice. The module consisted of multiple circuit blocks, out of which, we only used two;
VCO-LO and VCO-HI. VCO-LO was used to generate a signal at a range with median 1000 KHz, and 452 KHz. It can
also be used to generate FM signals. The VCO-HI block was capable of generating signals at a range with median
1455 KHz. We also used potentiometers to control the amplitude of the signal.
Lab Assessment
Pre Lab
In-Lab
Performance
Results
Viva
/1
/2
/1
/2
/10
/5
PostLab
Data
Presentation
Data
Analysis
Writing Style
/4
/4
/4
/4
Instructor Signature and Comments: