communication systems lab 2 - fm

THE UNIVERSITY OF THE WEST INDIES ST. AUGUSTINE, TRINIDAD & TOBAGO, WEST INDIES

FACULTY OF ENGINEERING Department of Electrical & Computer Engineering

B. Sc. in Electrical & Computer Engineering

ECNG 2005

Laboratory and Project Design III

Communication Systems

Laboratory Exercise 2

Frequency Modulation



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Contents

1. General Information ................................................................................................................ ii

2. Lab Learning Outcomes .......................................................................................................... 3

3. Pre-Lab .................................................................................................................................... 3

3.1. Required Reading Resources ------------------------------------------------------------------- 3

3.2. Pre-Lab Exercise --------------------------------------------------------------------------------- 4

3.2.1. Background theory: Concepts of FM. ........................................................................... 4

3.2.2. Practical analysis of Frequency modulation: ............................................................... 5

3.2.3. Detection of FM: .......................................................................................................... 5

3.2.4. Concept of alternate forms of FM generation:.............................................................. 5

4. In-Lab Exercise........................................................................................................................ 6

4.1. Procedure------------------------------------------------------------------------------------------ 8

4.1.1. Practical 1: Concepts of Frequency Modulation and Generation by Direct Oscillator Frequency Shift........................................................................................................................ 8

4.1.2. Practical 2: Measuring Deviation, Bandwidth and Bessel Null and Carson’s Rule... 12

4.1.3. Practical 3: Demodulation of an FM signal using a Phase Locked loop.................... 15

5. POST -LAB ........................................................................................................................... 17

5.1. Questions on Practical 1:-----------------------------------------------------------------------17

5.2. Questions on Practical 2:-----------------------------------------------------------------------19

5.3. Questions on Practical 3------------------------------------------------------------------------21

5.4. General questions -------------------------------------------------------------------------------22

Table of Figures

Figure 1 - Selection of Frequency Modulation Exercise------------------------------------------------ 7

Figure 2 – Teknikit FM Practical 1 Selection ------------------------------------------------------------ 8

Figure 3 - Teknikit FM Practical 1 Connection Diagram Selection ----------------------------------- 9



ECNG 2005 Lab and Project Design III

http://myelearning.sta.uwi.edu/Semester II 2009 / 2010

1. GENERAL INFORMATION

Lab #: 2 Name of the lab: Frequency Modulation Lab weighting: 5 % Estimated total

study hours1: 6 hrs

Delivery mode: Lecture Online Lab Other

Venue for the lab: Electronics Laboratory

Lab dependencies The theoretical background to this lab is provided in ECNG 2001 Theoretical content link: http://myelearning.sta.uwi.edu/login/index.php

Recommended prior knowledge and skills:

To undertake this lab, students should be able to: • Recall basic Frequency modulation principles • Use an oscilloscope

Course Staff Position/Role E-mail

Phone

Office Office

Hours Mark Lessey Tutor [email protected] Ext. 3191 Room 335

3rdFloor Block 1

Wednesdays 12-1

Kizzy Lee Demonstrator Ext. Electronics Lab

Riyad Omar Demonstrator Ext. Electronics Lab

1 Estimate includes teaching time, study time, and student preparation time for classes and labs.

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http://myelearning.sta.uwi.edu/

http://myelearning.sta.uwi.edu/login/index.php

mailto:[email protected]

CMS Lab 2: Frequency Modulation

2. LAB LEARNING OUTCOMES

Upon successful completion of the lab assignment, students will be able to:

1. Understand the concepts of frequency modulation and the term ‘deviation’. 2. Understand the generation of a frequency modulated signal by direct oscillator

frequency shift. 3. Investigate the spectrum and bandwidth of a frequency modulated signal and to

appreciate the use of Bessel’s functions in determining the spectrum. 4. Use Carson’s rule for the determination of bandwidth and to understand the

operation of a phase locked loop.

3. PRE-LAB

Due Date: Submission procedure: Submit to Demonstrators/Tutor at start of laboratory session

Estimated time to completion: 1hr

3.1. Required Reading Resources

http://myelearning.sta.uwi.edu : Log in, go to ECNG 2001: 2009/2010 Course Notes: Topic 4: Angle Modulation:

Frequency Modulation: Notes.pdf; Slides.ppt Demodulation: Notes.pdf; Slides.ppt

http://myelearning.sta.uwi.edu : Log in, go to My Courses: ECNG 2005: Laboratory and Project Design III 2009/2010 → Labs related to ECNG 2001: Communication Systems→ Lab Resources




3.2. Pre-Lab Exercise

Please remember to make use of the additional material posted on the site for your reference.

In this Pre-Lab, you will reinforce basic concepts of FM generation and detection.

3.2.1. Background theory: Concepts of FM.

1. Considering the FM generation process, describe the impact of the modulating signal

on the carrier signal in terms of the carrier’s frequency, phase and amplitude.

2. State the relationship between frequency and phase.

3. State the phase, Φ, of the signal v = V1 sin(ω t), where v is the instantaneous voltage

and V1 is the maximum voltage amplitude.

4. State the mathematical expression used to evaluate the peak value of deviation of the

modulating signal.

5. Develop an expression for instantaneous carrier frequency in an FM system given

that the unmodulated carrier and the modulating signal are both sinusoidal i.e. vc= Vc

sin(ωC t) and vm= Vmsin(ωm t) where ‘c’ denotes carrier and ‘m’ denotes modulation.

6. Expand the result obtained above, rearranging in terms of the carrier and modulating

frequencies. Explain the significance of the terms in the new expression.

7. Define the term modulation index of an FM signal.

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3.2.2. Practical analysis of Frequency modulation:

1. In theory an FM signal can have an infinite number of sidebands. However in

practice a simplifying rule is applied to calculate the approximate FM bandwidth.

State the rule and justify its use.

2. State the relationship between bandwidth and deviation in an FM system.

3. Briefly describe how FM bandwidth changes with modulation indexβ .

4. Briefly describe how the power contained in the carrier frequency component of an

FM signal varies with modulation depth. Support you answer with appropriate

diagrams demonstrating the behavior in the frequency domain.

5. How does the spectrum of an FM signal compare to that of AM signals for small FM

modulation index values? Illustrate your answer graphically.

3.2.3. Detection of FM:

1. Explain why AM detectors cannot be directly used to demodulate an FM signal.

2. Briefly describe the Slope Detection process and state its disadvantages.

3. Explain the operation of a Quadrature Detector.

4. Explain the operation of a Phase Locked Loop (PLL).

5. State the operational limits of the Phase Locked Loop.

3.2.4. Concept of alternate forms of FM generation:

1. Briefly explain how frequency modulation is achieved by a phase modulator

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4. IN-LAB EXERCISE

Allotted completion time:

3 hours

Required lab Equipment:

1) Modulation and Coding Principles Workboard 53-230.

2) One (1) PC with Modulation and Coding Principles 53-230 software.

3) One (1) Teknikit Console 92-300 Docking station with power and

USB data cables attached.

Before performing the practical exercise follow the instructions indicated below

Set the screen resolution to the maximum by

o Right clicking on the desktop, and going to properties.

o Select the “Settings” tab.

o Set the screen resolution to its highest value.

Ensure the Feedback Teknikit Console 92-300 board is connected and switched on before opening the software.

Open Modulation and Coding Principles software by going to:

Start → Programs →Feedback Instruments→ Discovery 3→Modulation and Coding Principles 53-230→ Modulation and Coding Principles 53-230 Course content.

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Select the Frequency Modulation exercise.

Figure 1 - Selection of Frequency Modulation Exercise

Follow the instructions provided and answer the post-lab questions. Take note of all observations.

Practical exercises are provided as follows:

• Practical 1: Concepts of Frequency Modulation and Generation by Direct Oscillator Frequency Shift.

• Practical 2: Measuring Deviation, Bandwidth and Bessel Null and Carson’s Rule.

• Practical 3: Demodulation of an FM signal using a Phase Locked loop.

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4.1. Procedure

4.1.1. Practical 1: Concepts of Frequency Modulation and Generation by Direct Oscillator

Frequency Shift.

1. Ensure the Teknikit Console is powered on and Click on practical 1.

Figure 2 – Teknikit FM Practical 1 Selection

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2. Click the “Make Connections” icon.

Figure 3 - Teknikit FM Practical 1 Connection Diagram Selection

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3. Follow the steps until all the connections are completed on the board. NOTE: Do

not make Connection 5 until later instructed. Have the circuit checked by the

TA before you proceed.

4. Close the “Make Connections” window to return to the practical 1window.

5. Set the modulation source to minimum using the Signal Level Control.

6. Open the oscilloscope and the spectrum analyser. The icons for these are located

below the “Make Connections” icon on the practical 1 window.

7. Observe the unmodulated carrier (monitor point 1) waveform on the oscilloscope

(the blue trace) and on the spectrum analyser (orange trace).

8. Increase the modulation level, using the Signal Level Control, and note the sidebands

on the spectrum analyser.

9. Using the oscilloscope screen (do not maximize), adjust the timebase to see only one

cycle of the modulation signal and the individual cycles of the carrier. Small

frequency changes in time with the modulation should be visible.

10. Using the cursor, measure the carrier period at both positive and negative modulation

peaks. Record the values obtained in section 5.1.1 of the post-lab.

11. Calculate the two frequencies and record their values in section 5.1.2.

12. Observe the waveform on the oscilloscope and answer question 5.1.3 in the post lab

script.

13. Open the spectrum analyser and note that there are multiple sidebands for the single-

frequency modulating signal.

14. Open the frequency counter and measure the frequency of the modulating signal.

Record the value in section 5.1.4 of the post lab script.

15. Measure the spacing of the sidebands on the spectrum analyser and complete section

5.1.5 in the post lab script.

16. Referring to the make connections diagram for practical 1, Remove connection 3

and add Connection 5. Have the circuit checked by the TA before you proceed.

17. On the Modulation and Coding Principles Workboard, set the Function Generator

block to sine and Fast. You may need to adjust the small silver dial to achieve the

correct setting.

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18. Use the Signal Level Control to set the amplitude of the modulation signal to

maximum.

19. Vary the frequency of the modulation source using the Frequency control knob on

the Function Generator block of the workboard and note the effect on the spectrum

analyser.

20. Based on the observation above answer section 5.1.6 and 5.1.7 of the post lab script.

21. Close the Practical 1 window, power off Teknikit console and remove all the

connections made on the workboard.

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4.1.2. Practical 2: Measuring Deviation, Bandwidth and Bessel Null and Carson’s Rule.

In this practical, FM deviation is measured using a DC voltage to drive the modulator. This value is confirmed by the Bessel null method using AC modulation.

1. Switch on the Teknikit Console and click Practical 2 in the “Discovery 53-230

Modulation and Coding principles Frequency Modulation” window.

2. In the practical 2 window, click ‘Make Connections’ icon.

3. Complete all connection steps on the workboard as instructed in the “Make

Connections” window by clicking ‘Next’ after each step performed.

4. Using the Signal Level Control, set the modulation deviation to maximum.

5. Open the oscilloscope, spectrum analyser, frequency counter and voltmeter.

6. On the workboard set the Function Generator to sine and Fast.

7. Adjust the Frequency control on the Function Generator block and use the

frequency counter to set the modulation frequency to approximately 10 kHz.

8. Set the voltmeter to AC-pp and measure the peak to peak amplitude of the sine

wave. Record the value obtained in section 5.2.1 in the post-lab script.

9. Refer to the ‘Make connections’ diagram for practical 2 and remove connection 2

from the workboard. Add connection 5 so that the modulation source is now the DC

source. Have the circuit checked by the TA before you proceed.

10. Set the voltmeter to DC and adjust the DC source voltage to negative one-half the

AC peak to peak value that was measured. This sets the carrier frequency to that

obtained with the AC modulating source at its peak negative value.

11. Using the mouse move the frequency counter probe (yellow probe on the block

diagram) to the carrier output (point 1 on the block diagram).

12. Measure the frequency of the carrier output and record the value in section 5.2.2 in

the post-lab script.

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13. Set the DC source voltage to positive one-half the AC peak to peak value and

measure the carrier frequency. Record the new value of the carrier output in section

5.2.3 in the post-lab script. The frequency change can be seen on the oscilloscope

and the spectrum analyser.

14. Referring to the make connections diagram for practical 2, remove connection 5 and

replace connection 2 on the workboard so that the modulation source is again the

Function generator.

15. Move the frequency counter probe (yellow probe) back to the modulation point

(monitor point 2 on the block diagram).

16. Set the Modulation Frequency to approximately 10 kHz, then slowly increase the

frequency and observe the spectrum analyser display.

17. Observe the individual sidebands and the carrier on the spectrum analyser and

answer section 5.2.4 in the post-lab script.

18. There should be a modulation (source) frequency between 15 kHz and 20 kHz where

the carrier component disappears. Adjust the modulation frequency carefully so that

the carrier component almost disappears, and answer section 5.2.5 in the post-lab

script.

19. Calculate the deviation using the Bessel Null method (see post-lab section 5.2.5) and

record the answer in section 5.2.6 of the post-lab script.

20. Using the value obtained above for the calculated deviation answer section 5.2.7 of

the post-lab script.

21. Now use Carson’s rule to calculate the bandwidth for this combination of modulating

frequency and deviation. Record the value obtained in section 5.2.8 in the post-lab

script.

22. On the spectrum analyser use the cursors to measure the occupied bandwidth and

answer section 5.2.9 in the post-lab script. Note: Bandwidth is measured at the -3dB

points. To be absolutely certain of the measurement points, please consult the

TA

23. Calculate the control sensitivity of the VCO using: S= 2 /Vp-p, where S is the

sensitivity, is the deviation and Vp-p is the peak to peak magnitude of the AC

modulation. Record the value in section 5.2.10 of the post-lab script.

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24. Close the Practical 2 window and power off the Teknikit console.

25. Remove connections from the board.

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4.1.3. Practical 3: Demodulation of an FM signal using a Phase Locked loop.

1. Switch on the Teknikit Console and click on Practical 3 in the “Discovery 53-230

Modulation and Coding principles Frequency Modulation” window.

2. In the practical 3 window, click the ‘Make Connections’ icon.

3. Complete all connection steps on the workboard as instructed in the “Make

Connections” window by clicking next after each step performed.

4. Set the Function Generator on the workboard to Fast and sine output. This

provides the modulation source for the FM generator.

5. Open the frequency counter and use the Frequency control on the Function

Generator to set the frequency of the modulation to 8 kHz.

6. Open the voltmeter and use the Signal Level Control to set the AC p-p modulation

amplitude to 0.2 volts. Set the DC Source control to about half scale to provide a DC

offset to the Loop Filter of approximately zero volts.

7. Set the Loop filter Compensation to Fast.

8. Open the oscilloscope and the spectrum analyser and note that the output is FM.

9. By using the value calculated for VCO voltage sensitivity and the current modulation

amplitude of 0.2 volts peak to peak, calculate the deviation. Record the answer in

section 5.3.1 of the post lab script.

10. Move the Channel 1 oscilloscope probe (blue probe on block diagram) to the output

of the multiplier (monitor point 3). Observe that the demodulator reproduces the

modulating signal. If the display is unstable then adjust the DC Source control to

lock the loop.

11. Increase the timebase of the oscilloscope and you should be able to see twice the

carrier frequency component.

12. Move the oscilloscope Channel 1 probe (blue) to the loop filter output (monitor point

4) and note that the high frequency component has been removed.

13. Use the spectrum analyser to observe the signal after the loop filter.

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14. Using the spectrum analyser to observe the signal after the post detection filter

(monitor point 5 on the block diagram) answer section 5.3.2 in the post-lab script.

15. Monitor the loop filter output using the oscilloscope. Using the Signal Level

Control, increase the modulating signal amplitude.

Note that if the deviation is too wide, the loop unlocks at peaks of deviation.

16. Using the Signal Level Control, return the modulation voltage to 0.2 volts peak to

peak.

17. Using the Function generator Frequency control on the workboard, adjust the

modulation frequency until the demodulator output decreases. (Observed on the

oscilloscope). Note that this effect is due to the loop filter bandwidth.

18. Close the Practical 3 window and power off Teknikit board. Remove all connection

wires from the workboard.

Proceed to complete the post-lab

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5. POST -LAB

Record the numerical values obtained and answer the relevant questions.

5.1. Questions on Practical 1:

5.1.1. Time between positive peaks = . Time between negative peaks = .

5.1.2. Frequency of positive peak= . Frequency of negative peak= .

5.1.3. At or between modulating signal peaks, is there any change in the amplitude of the

modulated signal when viewed on the oscilloscope?

5.1.4. Frequency of modulating signal= .

5.1.5. Spacing of sidebands= .

How does the spacing of the sidebands you obtained compare to the modulating

frequency?

Since the spectrum analyzer trace depicts the power contained in the signal under observation, briefly explain the significance of the peaks observed.

5.1.6. Describe what is observed on the spectrum analyser when the modulating frequency is

varied while the modulation source’s amplitude is at maximum.

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5.1.7. How does the bandwidth of the FM signal relate to the deviation at low modulating

frequencies?

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5.2. Questions on Practical 2:

5.2.1. Sine wave peak value = .

5.2.2. Carrier frequency at DC source voltage = +half AC peak to peak value = .

5.2.3. Carrier frequency at DC source voltage = -half AC peak to peak value = .

5.2.3.1. The frequency difference between steps 5.2.3 and 5.2.2 represents the frequency

change produced by the modulator over the amplitude range of the sinusoidal

modulating signal. Based on this, state the modulator’s deviation for this modulating

signal range.

5.2.4. From observations of the spectrum analyser display, explain any changes in carrier

frequency power level.

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5.2.5. State the modulation source frequency, fm, required to reduce the power contained in

the carrier frequency component until it disappears. fm= .

In practical FM systems, the concept of the Bessel Null (which is not covered in

ECNG 2001) is often used to determine the deviation of an FM transmitter. The

Bessel form of the FM expression can be used to observe the phenomenon by

plugging appropriate values into the expression and graphing the results. A major

observation is that as FM modulation index ß increases, the carrier frequency

component of the FM signal fades to an insignificant power level (null) at values of ß

termed Bessel Nulls. The first null, called the first Bessel Null, occurs around ß =

2.405, which is what you have observed in this step. Note that nulls also occur at ß =

5.52, 8.655, and other values. The points at which nulls occur are a feature of all FM

systems.

You must now calculate in the next question, noting that the null observed in this step

is the first Bessel Null for this system, the deviation of the FM transmitter. Note that

405.2==mffΔβ in this case.

5.2.6. The deviation using Bessel null method= .

5.2.7. Compare the value of deviation you measured from the DC method to that obtained

using the Bessel’s null method.

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5.2.8. The calculated bandwidth obtained using Carson’s rule= .

5.2.9. Compare the calculated bandwidth using Carson’s rule to the measured bandwidth on

the spectrum analyser.

5.2.10. Using appropriate units, state the control sensitivity for the VCO. .

Would this VCO be suitable for inputs such as microphones, the output dynamic ranges

of which are typically on the order of a few hundred microvolts into millivolts? Briefly

explain.

5.3. Questions on Practical 3

5.3.1. The calculated deviation= .

5.3.2. From the observations on the spectrum analyser before and after the post detection

filter explain the function of the post detection filter in the demodulation process.

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5.4. General questions

4.4.1 Explain why the carrier frequency component of the modulated waveform varies with modulation in FM, while this is not so for AM. Support your answer using the mathematical expression for a standard FM signal.

4.4.2 Is it possible to overmodulate an FM signal? Explain.

End of LAB 2:

Frequency Modulation

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communication systems lab 2 - fm

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