Digital Signal Processing Reference
In-Depth Information
5,0
4,0
3,0
2,0
1,0
0,0
1,00
0,50
0,00
-0,50
-1,00
1,00
0,50
0,00
-0,50
-1,00
Sine-carrier
Bit pattern
(bipolar)
2-PSK
Sinus-carrier
Bit pattern
(bipolar)
2-PSK
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
ms
Tim e domain
Tim e domain
S pectrum
Spectrum
Freq. domain
Freq. domain
Bit pattern
B it
pattern
Bessel-LP
B
esse
l-LP
bipol ar
b
ipo
l
ar
Sine carrier
Sine
c
arrier
Multiplic.
Bessel-LP
Multiplic.
Bessel-LP
Sine carrier
Sine carri
er
3
2
1
0
-1
-2
-3
3
2
1
0
-1
-2
-3
3
2
1
0
-1
-2
-3
1,5
1,0
0,5
0,0
-0,5
-1,0
-1,5
0,8
0,6
0,4
0,2
0,0
0,40
0,30
0,20
0,10
0,00
0,40
0,30
0,20
0,10
0,00
0,40
0,30
0,20
0,10
0,00
2-PSK-Spectrum
Demodulation by
multiplying with carrier
0
25
50
75
10
0
250
500
750
1000
1250
ms
Hz
Illustration 260:
Phase Shift Keying 2-PSK
Phase shift keying is the result of the multiplication of a bi-polar signal with the sine carrier. This automa-
tically leads - as can clearly be seen above -t o phase shift keying. The Illustration in the centre shows a
modulation and demodulation circuit for phase shift keying with DASYLab. An arbitrary sequence of bit
patterns is band-limited by a lowpass, transformed into a bipolar signal and multiplied by the carrier. The
pre-filtered signal clearly shows the phase steps. In contrast to 2-ASK no carrier is recognizable in the
spectrum, the total transmission energy is used by that part of the signal which carries the information.
Demodulation is carried out by multiplying the incoming signal by the sinusoidal carrier which can be
reconstructed using a PLL or a Costa's loop. Aggregate and differential signals are - as with AM - shown
in the spectrum. The source signal is reconstructed by lowpass filtering.
Frequency Shift Keying 2-FSK
2-FSK allocates two different frequencies to the two states of the bit pattern. Illustration
210 shows signals, a block diagram and the DASY
Lab
-system (2-FSK with two different
states).
Additionally, the input bit pattern is inverted. This branching means that only one signal
at a time is "high", whereas the other one is always "low" at that moment. Both bit patterns
are then filtered and finally multiplied by the frequencies f
1
and f
2
. Thus only one frequen-
cy at a time is - in line with the bit pattern - switched on. The sum is the 2-FSK signal.
