AM & FM Modulation for Comunication Systems

In this paper,I will compare AM and FM modulation in matlab program. AM stands for amplitude modulation where the amplitude of the carrier is modulated as per the message signal. The other aspects of the carrier wave such as frequency phase etc. remain constant. On the other hand, FM means frequency modulation and in it only frequency of the carrier wave changes while amplitude, phase etc. remain constant. 

1.Frequency Modulation: Frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave. The amount of frequency variation is proportional to the Modulation index. A major advantage of FM as compared  with AM, is improved Signal-to-noise ratio (SNR). The improvement depends on modulation level and deviation. For typical voice communications channels, improvements are typically 5-15 dB.

The following points about frequency modulation (FM) may be noted carefully:

 (a) The frequency deviation of FM signal depends on the amplitude of the modulating signal.

(b) The centre frequency is the frequency without modulation or when the modulating voltage is zero.

 (c) The audio frequency (i.e. frequency of modulating signal) does not determine frequency deviation.

2.Amplitude Modulation: When the amplitude of high frequency carrier wave is changed in accordance with the intensity of the signal, it is called amplitude modulation. In amplitude modulation, only the amplitude of the carrier wave is changed in accordance with the intensity of the signal.

The following points are worth noting in amplitude modulation:

1: The amplitude of the carrier wave changes according to the intensity of the signal.

2: The amplitude variations of the carrier wave is at the signal frequency fs.

3: The frequency of the amplitude modulated wave remains the same i.e. carrier frequency fc.

Comparison of FM and AM Modulation

Type	FM	AM
Frequency range of working	88-108MHz	540-1650 KHz
Bandwidth consumption	80KHz	30KHz
Circuit Complexity	Complicated  circuitry	Older Process:simple circuitry
Wavelength	FM waves lower than AM	AM waves higher than FM
Signal Quality	Good : Easy to filter for noice	Not Good:Difficult filter for noice
Stands for	FM stands for Frequency Modulation	AM stands for Amplitude Modulation
Modulating differences	In FM, a radio wave known as the "carrier" or "carrier wave" is modulated in frequency by the signal that is to be transmitted. The amplitude and phase remain the same.	In AM, a radio wave known as the "carrier" or "carrier wave" is modulated in amplitude by the signal that is to be transmitted. The frequency and phase remain the same

Matlab Implementations

FM:

%FM signal generation

fm=10;

fc=40;

mi=1;%modulating index

t=0:0.001:1;

m=sin(2*pi*fm*t); %message

subplot(3,1,1);

plot(t,m);

xlabel('Time');

ylabel('Amplitude');

title('Message Signal');

grid on;

c=sin(2*pi*fc*t); %carrier

subplot(3,1,2);

plot(t,c);

xlabel('Time');

ylabel('Amplitude');

title('Carrier Signal');

grid on;

y=sin(2*pi*fc*t+(mi.*sin(2*pi*fm*t)));

subplot(3,1,3);

plot(t,y);

xlabel('Time');

ylabel('Amplitude');

title('Modulated Signal of FM');


AM:

%AM signal generation

Ac=1;

Am=1;

fc=40;

fm=10;

m=1;%modulating index

t=0:0.001:1;

y1=sin(2*pi*fm*t); % message signal

y2=sin(2*pi*fc*t); % carrier signal

eq=(1+m.*y1).*(Ac.*y2);

subplot(311);

plot(t,y1);

xlabel('Time');

ylabel('Amplitude');

title('Message signal')

subplot(312)

plot(t,y2);

xlabel('Time');

ylabel('Amplitude');

title('Carrier signal');

subplot(313);

plot(t,eq);

plot(t,eq,'r');

xlabel('Time');

ylabel('Amplitude');

title('Modulated signal of AM');

Conclusion

In this paper,I compared FM and AM modulation in matlab.There are some differences between them;

The frequency of the information signal may change over time. But the frequency of the carrier with high frequency is fixed. The high frequency of the carrier wave allows the information to be transmitted at a more smooth and distant distance to be transmitted at the same time. The most important difference between AM and FM broadcast is actually  from here. Namely; 'Parasite' is a more serious problem in AM receivers, as the majority of man-made or natural 'noise' is in the form of signals exhibiting amplitude variation. So FM broadcast is better quality .

The second difference is that the AM signal is played with the amplitude of the signal, so the received signal is very weak in some places. The AM receiver therefore needs to have  the ability to troubleshoot these weaknesses in the signal, ie, the additional electronic components that are needed for it. Since the amplitude remains constant in the FM broadcast, this is no problem.

Preparing by Emre AKKAYA.