FREQUENCY SHIFT KEYING (FSK)
Table of Contents
Intro
The modulated frequency of the carrier represents the digital information. FSK is bandwidth inefficient and power efficient, not susceptible to amplitude distortions and often used in telecommunication-systems like DECT or GSM, but the spectrum is wide. Variations like Gaussian- FSK or Minimum Shift Keying (MSK) reduce the bandwidth in a way that the signal still can be reconstructed. Since the envelope of the signal is constant nonlinear and thererfore cheaper power transmitters can bes used.
BFSK
The simplest form is Binary-FSK (BFSK) which uses a set of two frequencies, 
for a binary zero and 
for a binary one. This can be seen as a combination of two On/Off-Keying signals where always just one signal is present at a time slot. Therefore the spectrum is a combination of these signals and FSK needs at least twice the bandwidth of ASK.
for a binary zero and for a binary one. This can be seen as a combination of two On/Off-Keying signals where always just one signal is present at a time slot. Therefore the spectrum is a combination of these signals and FSK needs at least twice the bandwidth of ASK. disp('Binary FSK');
init;
x=['1';'0';'1']; % bit sequence;
plot_SK(x,'type','FSK');
Note: There are no steady states in the constellation diagram that show a digital state of an FSK-signal. It is possible to indicate the symbols´ phases moving clockwise or counterclockwise relative to the unmodulated carrier, but typically the constellation diagram ist not used for FSK signals.
Modulator and Demodulator
A BFSK modulator switches in accordance to the digital input signal between two oscillators, each of them running at a different frequency. A Charge-Pump PLL can be used for demodulation. The quiescent frequency 
of the PLL is between the frequencies of the oscillator signals and 
.
of the PLL is between the frequencies of the oscillator signals and . A reception of a logical low causes the PLL to decrease its VCO frequency and the phase detector outputs negative values, a reception of a logical one produces positive values at the phase detector´s output. A Schmitt trigger creates a binary output shape of this signal.
file2sim='FSK_MODEM';
if (do_sim)
init_FSK;
convert_model(file2sim); % create HTML-version of simulinkfile
disp(get_link('-> FSK in Simulink',file2sim));
[TX_signal,carrier,MOD_signal,RX_signal,t]=sim_model(file2sim);
figure
plot_MOD(t,TX_signal,carrier,MOD_signal,RX_signal)
end
pic_file=strcat(file2sim,pic_ext,'.jpg');
if exist(pic_file,'file')
disp('FSK in Simulink - block diagram');
figure
image(imread(pic_file))
end
MFSK
If a symbol is coded by a set of binary digits of the length n, there are possible 
different symbols. In M-FSK modulation each of these symbols is represented by a particular frequency of the carrier.
different symbols. In M-FSK modulation each of these symbols is represented by a particular frequency of the carrier. disp('MFSK');
x=['000';'100';'111']; % bit sequence;
plot_SK(x,'type','FSK');
