Contents

Introduction ( P1-P4).

Frequency modulation .(P5-P7)

Frequency demodulation (P8-P14)

FM using simulink implementation (P15 – P 17)

To transmit or to receive a signal , you cant do that with the signal itself , too many signals will overlap each other. So that we need a carrier to do the mission, too many kinds of carriers can be used . AM, FM, PM …

The frequency modulation (FM) is a technique that conveys information over a carrier wave by varying its instantaneous frequency . Compared with the Amplitude modulation (AM) in which the amplitude of the carrier varied according to the information while the frequency keeps constant .

Introduction

This is how the FM and the AM carriers carry the information .

The Baseband signal

AM carrier

FM carrier

Matlab work:

First we put the carrier frequency at 100 Hz and the period as shown in the figure

Then we plot the time domain and the frequency domain figures of the baseband signalThe baseband frequency is 1 Hz

The wave form and the magnitude spectrum

The baseband signal is tow pulses at 1 and -1 Hz

To modulate a NBFM signal we first Integrate the baseband signal then we modulate it using DSB-sc

Modulation:

Now to modulate the signal we have to integrate the baseband signal using (cumsum) order in matlab

Then we plot wave form and the magnitude spectrum of the modulated signal

Matlab work:

The wave form and the magnitude spectrum of the modulated signal

Here we can see how is the frequency change according to the baseband signal

There are tow methods to demodulate the signal -

The first method is the direct differentiation method :

(using the op-amp as differentiator )

Demodulation:

The second method is the slope detection method (vco) :

( tank circuit and envelop detector -Voltage-controlled

oscillator )

Matlab work:

The demodulated signal is the differentiation of the modulated signal

Then we plot wave form and the magnitude spectrum of the demodulated signal

The wave form and the magnitude spectrum of the demodulated signal

Now its obvious that the envelop is shown we can use envelop detector

Envelop detector:

To detect the baseband signal we use an envelop detector as shown

RC should be in the range :

1/Fc < RC < 1/B

The demodulated signal and the envelop of the baseband signal

The envelop ( baseband signal ) = sin (2*pi*t)

Simulink implementation:

First we draw the circuit as shown in the figure

The baseband signal

The output of the VCO

The FM signal

The demodulated signal ( ready for envelop detection )