FUNDAMENTALS OF COMMUNICATION SYSTEMS TE2016

Lab Experiment No.3

Angle Modulation

Dr. Alejandro Aragón Zavala

It is time now to examine how a carrier can be modulated by an information signal

(message) if its frequency is varied in proportion to this message. This is known as

frequency modulation and by now you should be familiar with this technique.

We will perform various experiments here modifying a sinusoidal carrier with both an

analogue (FM) and digital (FSK, BPSK) messages. For this, the frequency and phase of

the carrier is to be modified. More sophisticated digital modulation schemes will be

analysed later on in the course.

Software simulations

1. Generate a sinusoidal information signal in the audio frequency range, about 8

kHz, using MATLAB or Simulink.

2. Perform frequency modulation of this audio signal using a sinusoidal carrier of

95.5 MHz, and a frequency deviation of 75 kHz.

3. Observe the signals in both time and frequency. Print the graphs and draw your

conclusions after varying both the carrier and the frequency deviation values.

This will give you an idea of narrowband and wideband FM, depending on the

frequency modulation index you use.

4. Now, using two different frequencies, one for mark (logic 1) and the other one

for space (logic 0), generate an FSK signal using an 8 kHz random pulse train.

Show waveforms in time and frequency.

5. For the same pulse train, design a BPSK modulator and also show time and

frequency waveforms.

6. Report all your observations and findings.

Hardware

We will build an FM modulator and demodulator using the principles discussed in class.

For FM modulation, we will use the LM566, which is a VCO (Voltage Controlled

Oscillator). For FM demodulation, a commercial LM565 PLL (Phased Locked Loop) is

used.

1. Using the LM566, build an FM modulator as shown in Figure 1. The centre

carrier frequency of the circuit is set by the values of and , as follows:

where

For testing purposes, we will fix this carrier frequency to . Therefore, if we fix to be , which is a commercial value for resistance,

then:

To perform a fine adjustment of the carrier frequency, is made of a fixed

resistor of and a potentiometer of , as indicated in Figure 1. In this

way, we make sure that:

Use a 1 Vp-p, 2 kHz modulating audio signal. The modulating signal can vary

the carrier frequency over nearly a 10:1 range, making very large frequency

deviations possible. The deviation is linear with respect to the input amplitude

over the entire range.

Figure No.1 FM modulator using LM566

2. Notice that you will produce either a square or a triangular wave rather than a

sine wave at the output of the circuit built in 1 and shown in Figure 1. An active

LPF (Low Pass Filter) is required in order to smooth this triangular carrier, and

produce a sinusoidal one. Figure 2 shows a typical active LPF designed with the

popular op-amp LM741.

NE566

6 8

5

7 1

4

3

Modulating

signal FM out

V+

R2

R1

R3

1.5 k

10 k

C1

1 nF

0.1 nF

12 V

Figure No.2 Active LPF for FM modulator

The values of resistance and capacitance are:

Having a carrier frequency of 11.11 kHz, design your LPF filter according to

the schematic shown in Figure 2, and connect it as suggested to your FM

modulator of Figure 1.

3. Test your circuit by examining the signals in time (oscilloscope) and frequency

(spectrum analyser).

4. Using the LM565, build an FM demodulator as indicated in Figure 3, making

sure you keep component lead lengths as short as possible. The loop is set to

free-run at 100 kHz by C104 and the series combination of R107 and R106.

Potentiometer R106 allows adjustment of the free-running frequency, which

should be the same as the carrier or centre frequency of the FM transmitter. The

FM signal is coupled into the number 1 reference input of the phase detector

(pin 2) through C103. Since the PLL is operating from a single supply (+15 V),

resistors R102 and R103 are used as a voltage divider to split the power supply

in half. C102 is an RF bypass for the bias point, and R104 and R105 serve to

isolate the two phase detector inputs.

C1

741

+12 V

-12 V

7

6

4

2

3

-

+

To pin 4,

NE566

R1 R2

C2

0.001 F

Sinusoidal

FM carrier

Figure No.3 FM demodulator using LM565

The VCO control voltage of the loop on pin 7 of the LM565 contains two

components. One is the DC level corresponding to the “average” frequency

going into the PLL from the FM modulator, and the other is the AC level that is

actually the detected information signal. This AC signal arises because of the

PLL’s self-correcting action; as the transmitter deviates up or down in

frequency, the PLL attempts to force the VCO to follow this frequency exactly

by varying its control voltage. Thus, the control voltage is a copy of the original

information signal.

Components C106, C107 and R108 form the loop filter, which sets up the loop

operating parameters (capture range, damping ratio, natural frequency)

appropriately for demodulation of an FM signal.

The signal at pin 7 of the LM565 contains some carrier frequency in addition to

the AC and DC levels just discussed; components R109, C108, R110, and C109

form a low-pass filter to remove any traces of 100 kHz carrier signal. The final

output is AC coupled by C110, leaving only demodulated information at the

output.

5. Generate an FM signal using the signal generator with the following

characteristics:

a. Message type: sinusoidal

b. Carrier frequency: 100 kHz

c. Frequency deviation: 10 kHz

d. Message frequency: 5 kHz

6. Apply power to the demodulator circuit of Figure 3, but do not connect its FM

input to anything yet. First, adjust R106 so that the free-running frequency VCO

output on pin 4 (square wave) of the LM565 PLL is 100 kHz (the actual carrier

frequency you are using).

7. Now connect the FM output of the signal generator to your demodulator, and

observe the time and frequency domain signals in the oscilloscope. Vary the

information frequency and see how the PLL responds. Change the carrier

frequency and make sure you also adjust the R106 potentiometer, and see the

effect. Increase and decrease the frequency deviation and check the result in the

demodulated output. Draw your conclusions.

8. Could you build an FM modulator using the circuit LM565? Include in your

report how you would perform this, showing connection diagrams.

Equipment

You will again use the TIMS equipment to modulate and demodulate signals using FM,

along with a signal generator.

1. Generate FM using the TIMS equipment and the VCO module, as seen in class

and shown in Figure 4. Use a TIMS module to generate a variable audio signal,

which spans from 100 Hz up to about 8 kHz. Notice that the VCO allows you to

choose between a HI (high) and a LO (low) carrier frequency. Try both and see

the differences. Make sure the GAIN red indicator is never ON, otherwise

reduce the gain – this will indicate overloading of the controlling voltage and

therefore malfunctioning of the VCO module.

Figure No.4 FM modulator using a VCO with TIMS

2. Observe the signals in time and frequency, obtain plots and draw your

conclusions. Vary the carrier and information frequency as well as the

amplitudes of the information signal and carrier to check how the modulation

index is varied.

3. Repeat steps 1 and 2 to generate:

a. FSK

b. BPSK

4. Report your observations and make sure you take enough pictures of it!

Optional: To demodulate FM, you will use a PLL which you will build according

to Figure 5 using various TIMS modules. You will need to use a MULTIPLIER, a

TUNEABLE LPF and a VCO modules. Since we only have one VCO module

available for use, generate an FM signal using the signal generator, observing the

time and frequency plots using the digital oscilloscope.

Figure No.5 FM demodulation using TIMS

VCO FM output Audio signal

8 kHz

X LPF

VCO

FM input Message out

Adjust the gain and centre frequency of your TUNEABLE LPF by using the GAIN

and TUNE knobs. The way to do it is you examine the spectrum using the digital

oscilloscope in FFT mode, and moving the TUNE knob until you remove all

undesired harmonics. That will be your cutoff frequency.

Try different information signals and see if your demodulator can recover them all.

You might need to use as VCO your circuit in Figure 1 for this.

Draw your conclusions.