Digital Signal Processing Reference
In-Depth Information
Multiplier
Inverter
x
Adder
-1
Bit pattern
+
Multiplier
Sinus f
2
2-FSK
x
Sinus f
1
Ti me do ma i n
Spectrum
Fr eq. do ma i n
Multi plier
Bessel-LP
Bit pattern
Invert outp.
Addition
Sinus f1,f2
Time domain
Frequency domain
6, 00
4, 25
2, 50
0, 75
-1,00
6
4
2
0
6
4
2
0
7, 5
5, 0
2, 5
0, 0
-2,5
-5,0
-7,5
7, 5
5, 0
2, 5
0, 0
-2,5
-5,0
-7,5
7, 5
5, 0
2, 5
0, 0
-2,5
-5,0
-7,5
1, 0
0, 8
0, 6
0, 4
0, 2
0, 0
1, 0
0, 8
0, 6
0, 4
0, 2
0, 0
1, 0
0, 8
0, 6
0, 4
0, 2
0, 0
3, 0
2, 5
2, 0
1, 5
1, 0
0, 5
0, 0
2, 50
2, 00
1, 50
1, 00
0, 50
0, 00
3, 5
2, 5
1, 5
0, 5
Bit pattern
Filtered bit pattern
Filtered inverted bit pattern
FSK f
1
FSK f
2
2-FSK signal
0
25
50
75
100
125
150
175
200
225
250
0
50
100
200
300
400
500
600
700
ms
Hz
Illustration 261:
Frequency Shift Keying (2-FSK)
The DASYLab circuit additionally inverts the incoming bit pattern. Both signals - the inverted and the
original bit pattern - are then lowpass filtered. Only one of the two signals is different from zero. Multipli-
cation of each signal by the frequencies f
1
or f
2
.results in two ASK signals of different frequencies, only one
of which is present at a certain time. The sum of both signals is the 2-FSK signal.
The frequency domain (bottom) shows that the two frequencies need to be relatively wide apart from each
other so that the two signals do not overlap. This method is therefore not applied where a particularly
efficient transmission on a band-limited channel is aimed at.
Signal space
Discrete states of sinusoidal oscillations are allocated to the discrete states of the bit
pattern with regard to amplitude, phase and frequency. We will see that combinations of
all three possibilities are generally used with the particularly efficient digital modulation
method. This results in a kind of APFSK (amplitude-phase-frequency-shift keying).
