Laboratory Exercise #5
Frequency Modulation
College of Engineering and Architecture
Department of Electronics Engineering
ECE 078
Principles of Communication Systems
Semester II, SY 24-25
Laboratory Exercise #5
Frequency Modulation
Presented by
03-2324-032313
Benitez, Asheir Clerk P.
03-2324-034478
Curimao, Axel Ross T.
03-2324-035659
Gagute, Paul Benedict B.
03-2324-033899
Ramos, Darold Lawrence P.
BSECE2-01
Presented to
Engr. Stephanie M. Macanlalay
Instructor
March 9, 2025
Laboratory Exercise #5
Frequency Modulation
I. Objectives
1. Perform the frequency modulation process.
2. Study the frequency modulation.
II. Conceptual Framework
[1] Frequency modulation is a process of changing the frequency of a carrier wave in
accordance with the slowly varying base band signal.
[2] Difference between AM and FM Amplitude modulation and frequency modulation are used
to transmit data using the method of modifying a carrier signal. The main difference between
both modulations is that in frequency modulation, the carrier wave frequency is modified as per
the transmit data.
III. Materials and Procedures
Materials:
1. 5 V, 400 kHz AC Source
2. 2 V, 20 kHz AC Source
3. 12 V DC Source
4. BC 108 Transistor
5. 1 uF Capacitor
6. 10 uF Capacitor
7. 1 uH Inductor
8. 6.8 kΩ Resistor
9. 22 kΩ Resistor
10. 680 Ω Resistor
11. 4.7 kΩ Resistor
Procedures:
[3]
1. The circuit connection is made in the simulator (PROTO).
2. The DC power supply is connected to the collector of the transistor.
3. Set the input signal fm as 20 kHz and 2 V sinusoidal signal.
4. Set the carrier signal fc as 400 kHz and 5 V sinusoidal signal.
5. The frequency modulated output is taken from the collector of the transistor through the
inductor with capacitor and inductor in parallel to resonate the frequency.
6. Measure the frequency of the FM output signal with F1 and F2 (using F = 1/T).
7. Take a screenshot of input signals (fc), modulating signals (fm), and obtained FM output
waveforms.
Laboratory Exercise #5
Frequency Modulation
Figure 3.1: FM Modulation Circuit
IV. Observations / Output
Figure 4.1: Modulating Signal Waveform with Time Step of 500 ns
Figure 4.2: Carrier Signal Waveform with Time Step of 250 ns
Laboratory Exercise #5
Frequency Modulation
Figure 4.3: Modulated Signal FM Waveform with Time Step of 1 µs
V. Data and Results
A. Values from the experiment from the Simulation App (PROTO)
Table 5.1
2V 20 kHz 50 µs
12 V 44.65 kHz 44.65 kHz 22.4 µs 22.4 µs
VI. Analysis and Discussion
[4] Generation of sinusoidal signal: A demodulator generates a sinusoidal signal of
frequency fm. Step 2. Modulation of the frequency: The sinusoidal signal is supplied to a PID
controller in which provides a modulated frequency (fc+Δf). Step 3. Output of signal: The
modulated frequency is provided as an output through an oscillator, generating the FM signal.
[5] General equation:
fm = [0.8m | 2 × 10³ t + 8 sin 400πt |]
= Ac sin (2π fc t + β sin 2π fm t)
Ac = 10, fc = 10⁸, β = 5, fm = 200
Δf = β fm
= 5 × 200 = 1000 Hz
Laboratory Exercise #5
Frequency Modulation
The Carson’s BW rule defines the approx. BW requirements of a system comprising of a Sc that is
freq modulated by a broad spectrum of frequencies
CBR = 2 (Δf + fm)
Radioband is from 88 to 108 MHz. It has a BW of 200 kHz and 100 possible bands to choose from.
fm = 7.5 kHz, Δf = 75 kHz
m = Δf / fm = 75 / 7.5 = 10
VII. Conclusion
Through simulation in PROTO we were successful in executing and observing the
frequency modulation (FM) process. The output indicated how the frequency of the carrier
wave varies with the input signal and how bandwidth is influenced by critical parameters such
as frequency deviation and modulation index but we did not solve for them rather we
calculated the amplitude, frequency, and time period. Using Carson's rule of bandwidth, we
confirmed the theoretical concepts and ensured the efficiency of FM in communication
systems.
VIII. References
[1] ECE 078: Principles of Communication Systems LABORATORY Exercise #5, page 1
[2] Difference Between AM And FM - Introduction | What is Amplitude Modulation
[3] ECE 078: Principles of Communication Systems LABORATORY Exercise #5, page 3
[4] Using a PID Controller to Generate Frequency Modulation (FM) Signals | Zurich Instruments
[5] Laboratory Report Cover Sheet | PDF | Frequency Modulation | Modulation
