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
Sinus carrier
Bit pattern
2-ASK
2-ASK
Bit pattern
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
Time domain
Spectrum
Freq. domain
B i t pattern
Bessel-LP
Si nusträger
Abs. value
LP filter
6
5
4
3
2
1
0
-1
7, 5
5, 0
2, 5
0, 0
-2,5
-5,0
-7,5
5, 5
4, 5
3, 5
2, 5
1, 5
0, 5
3, 5
3, 0
2, 5
2, 0
1, 5
1, 0
0, 5
0, 0
Bit pattern (filtered)
2, 50
2, 25
2, 00
1, 75
1, 50
1, 25
1, 00
0, 75
0, 50
0, 25
0, 00
2, 50
2, 25
2, 00
1, 75
1, 50
1, 25
1, 00
0, 75
0, 50
0, 25
0, 00
Spectrum of the
filtered bit pattern
2-ASK- signal
Absolute value
Spectrum of the
modulated
2-ASK-signal
Bit pattern (receiver)
0
25
50
75
100
125
150
175
200
225
250
275
0
100
250
400
550
700
850
ms
Hz
Illustration 259:
Amplitude Shift Keying 2-ASK
Illustration top: In principle, 2-ASK does not differ from classical amplitude modulation AM with regard
to circuits. Instead of a continuous signal, a discrete bit pattern is modulated which only chooses between
two states.
Illustration bottom: this example is much more realistic. At first the bit pattern is band limited using a
suitable (BESSEL) filter. The signal does not have any steps and cannot change faster than the highest
frequency it contains. The modulated or “keyed” signal has a continuous curve. 2-ASK is a double-side-
band AM with carrier (see bottom right).
Here demodulation is conducted in a conventional way using rectification (or formation of absolute value)
prior to BESSEL lowpass filtering. Thanks to the phase linearity of the filter the pulse form - the bit pat-
tern - is hardly distorted. The original bit pattern can easily be reconstructed. For this purpose the basic
pulse rate of the binary pattern needs to be extracted from the received signal. The components which are
needed are a PLL (Phase Locked Loop, see Illustration 181), a comparator (with discriminator potential)
and a Sample & Hold circuit.
A considerable amount of transmission energy is used by the carrier which does not contain any informa-
tion. This means that the distortion liability of the information transmitted is relatively high. The following
Illustration shows that phase shift keying 2-PSK is a preferable method. It is, however, hardly used
anymore because quadrature phase shift keying is a much more efficient method which guarantees a much
higher transmission rate.
