Introduction - Theoritical preliminary study - Simulation under Simulink

Download (french + english) : recepteur.zip (189ko)

Introduction
 



  The system of FSK broadcasting covers, in France, the waveband [88MHz,108MHz]. The bandwidth B of the modulated signal vary between 200kHz and 250kHz and the signal’s bandwidth is b=15kHz ; it corresponds to human voice’s spectrum.

In this project, we’re going to try to demodulate such signals. So we want to realise a system that allows reception of a great number of messages m1(t), …,m2(t) (stations) modulated to carrier frequencies of different values, spaced of a constant quantity D(f). The system of reception covers a waveband B.

Introduction - Theoritical preliminary study - Simulation under Simulink

Theoretical preliminary study
 



  The block diagram presented in figure 1 represents the classic reception system of a message mk(t) of bandwidth bk modulated to a frequency fk.

Figure 1 System of reception

To receive several messages, it would be theoretically necessary that the receiver contains so much demodulators that different carrier frequencies. It’s easy to conceive this solution is very quickly impracticable as soon as the number of stations is important. To solve this difficulty, there exists a technique which instead of adapting the demodulator to the frequency of the signal to modulate, adapts the signal to a demodulator permanent. This operation is allowed by returning all signals to receive, around a permanent frequency, told intermediate frequency. This technique allows therefore to use the same circuit of demodulation. It is the heterodyne receiver.

In a first time we study messages on the canal of transmission having undergone a modulation of amplitude without carrier frequency. Results will be transposed to the FSK modulation by the way.
Expression of the modulated signal: The spectrum density in power of the signal is then : After the local oscillator, the spectrum density contain four different parts : figure2 ; we need a bandpass filter to obtain a modulated signal of permanent frequency fi : figure 3.
 

Figure 2

Figure 3



Signal enters then in the demodulator. This demodulator is conceived to function to the intermediate frequency fI.


We choose fI = fosc- fk to demodulate the signal modulated to the frequency fk. Indeed after the local oscillator the spectrum of the signal is around frequencies fosc - fk and fosc + fk.

When a signal is received around the carrier frequency f1 we just put fosc such sort that fosc - f1=fI.
Note : If fI is badly chosen fosc - fk can be in the waveband [fosc - fk , fosc + fk] and therefore there can be interferences with one other station.
We will take fI in the middle of the waveband [88MHz,108MHz].

If we make a synchronous and coherent demodulation of the signal s3(t), we obtains in exit of the receiver the signal by using for the filter 4 a lowpass filter.

The interest of a such system of reception is with a demodulator only, it is possible to receive messages modulated to different carrier frequencies. You only need to vary fosc (a potentiometer would allow to realise this operation).

 

Introduction - Theoritical preliminary study - Simulation under Simulink

Simulation under Simulink
 





We used here, modulators and demodulators in transposed band of the communication toolbox of Simulink. All elements are typical digital because in the next part we will use an acquisition card that contains an analogue-digital converter. We will be therefore brought to work on digital signals.

Figure 4


The figure 4 shows the model we realised under Simulink to simulate an heterodyne receptor.
For the simulation we used values of sampling and modulation frequency weaker than those used in practice. We have undertaken the simulation by using a sinusoidal source. Moreover, to facilitate the visualisation, we took a modulation frequency relatively weak (close to the bandwidth of the signal). Thus we can observe on the same graph the source, the result of the modulation and the demodulated signal :

Figure 5


On the figure 5, we can notice that the signal at the exit of the system corresponds to the initial signal with a small delay time to the time of reply of the demodulator.