y 1.0 Amplitude Modulation And Demodulation

2.0 Learning Outcomes:

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

FACULTY OF ELECTRONIC AND COMPUTER ENGINEERING

BENC 3751

COMPUTER ENGINEERING LAB 1

Lab session 3

FREQUENCY MODULATION

Department of Telecommunication Engineering

Sem 1, 2012/2013

1.0 Title: Frequency Modulation

2.0 Learning outcomes:

After completing this lab, students should be able:

1. to produce an FM signal using ED-2950 Electronic Communication Trainer

2. to calculate the frequency deviation of the FM signal.

3. to explain the working principle of the voltage control oscillator (VCO)

3.0 List of Equipments and Components:

1. ED-2900 Power Console (0~4 Vdc variable and +15V/-15V dc)

2. Module- type 2950A – signal source

3. Oscilloscope

4. Function Generator

4.0 Safety:

1. Do not touch any exposed wires or sockets.

2. Ensure that the equipments are cleaned on a regular basis.

5.0 Theory:

Frequency modulation (FM) is a process in which the carrier frequency is varied by the

amplitude of the modulating signal (i.e. intelligence signal). The FM signal can be

expressed by the following equation:

[ ]∫+== dttSktfAtAtx fcccFM )(2cos))(cos()( πθ (1)

If )2cos()( tfAtS mm π= , then

⎥⎦

⎤⎢⎣

⎡+= )2sin(2

2cos)( tffAk

tfAtx mm

mfccFM π

ππ (2)

[ ])2sin(2cos tftfA mcc πβπ +=

where

)(tθ = instantaneous modulated frequency

cf = carrier frequency

mf = modulating frequency

β = modulation index

The frequency of FM signal )(txFM may be expressed as

)2cos()(21 tffftdtdf mmc πβθ

π−== (3)

From (3), we can find that the FM frequency deviation from the center frequency of the

carrier occurs when the intelligence amplitude is varied.

A Voltage Controlled Oscillator (VCO) is an oscillating circuit whose output frequency

changes in direct proportion to an input voltage. It is used to perform direct frequency

modulation on signals. VCO has a center frequency, fc and the input (control) voltage

m(t) modulates the instantaneous frequency around this center frequency.

Figure 1: The block diagram of the VCO

6.0 Procedures:

1. For the priliminary procedures, set the variable attenuator to 0 dB and input

sensitivity control B to maximum (x0.1).

2. Apply the signal source of 100 Hz and 10 Vp-p to the ED-2950A module.

3. Set the frequency control C to 465 KHz.

4. Observe the changes of the FM signal by varying the signal source amplitude

and record the maximum frequency deviation. Comment your results.

5. Reset the input to 10 Vp-p. Observe the changes of the FM signal by reducing

the signal source frequency up to 0.1 Hz and comment your results.

6. Describe the function of the input sensitivity control B and frequency control C of

the ED-2950A.

7. Explain your findings if the output is measured at various fixed attenuator.

VCO

Center frequency, fc

Frequency deviation constant, fd

y(t) m(t)