1. Modulation Techniques
Lecture 7:
Modulation Techniques
ASK, FSK, PSK ,BPSK and QPSK
1st semester 1
By: Elham Sunbu

2. OUTLINE
 ASK
 FSK
 PSK
 BPSK
 QPSK

3. WHY DO WE NEED DIGITAL-TO-ANALOG CONVERSI2 ON?!
The medium/channel is band pass, and/or
Multiple users need to share the medium.

4. MODULATION OF DIGITAL DATA3
Modulation –process of converting digital data or a low-pass analog to band-pass (higher-frequency) analog signal
Digital-to-analog modulation.
Analog-to-analog modulation.
Carrier Signal – aka carrier freq. or modulated signal - high freq. signal that acts as a basis for the information signal
information signal is called modulating signal
bandpass channel
freq

5. MODULATION OF DIGITAL DATA (C4ONT.)
Digital-to-Analog – process of changing one of the characteristic
Modulation
of an analog signal (typically a sinewave) based on the information in a digital signal
sinewave is defined by 3 characteristics (amplitude, frequency, and phase) digital data (binary 0 & 1) can be represented by varying any of the three
application: transmission of digital data over
telephone wire (modem)
Types of Digital-to-Analog Modulation

6. MODULATION OF DIGITAL DATA: A5 SK
ASK –strength of carrier signal is varied to represent binary 1 or 0
both frequency & phase remain constant while amplitude changes
commonly, one of the amplitudes is zero
Is this picture, from the textbook, entirely correct?!
demodulation:
only the presence or absence of a sinusoid in a given time interval needs to be determined
advantage: simplicity
disadvantage: ASK is very susceptible to noise interference –
noise usually (only) affects the
amplitude, therefore ASK is the modulation technique most affected by noise
application: ASK is used to transmit digital data over optical fiber

7. Modulation of Digital Data: ASK6
Example
[ ASK ]
vd(t)
vc(t)
vASK(t)
How does the frequency spectrum of vASK(t) look like!?

8. Modulation of Digital Data: ASK7

9. MODULATION OF DIGITAL DATA:8 FSK
FSK – frequency of carrier signal is varied to represent binary 1 or 0
peak amplitude & phase remain constant during each bit interval
 2
demodulation:
demodulator must be able to determine which of two possible frequencies is present at a
given time
FSK is less susceptible to errors than ASK – receiver looks for specific frequency changes
over a number of intervals, so voltage (noise) spikes can be ignored
advantage:
disadvantage:
application:
FSK spectrum is 2 x ASK spectrum
over voice lines, in high-freq. radio transmission, etc.

10. MODULATION OF DIGITAL DATA: FSK9
Example
[ FSK ]

11. MODULATION OF DIGITAL DATA: FSK10

12. MODULATION OF DIGITAL DATA: P11 SK
PSK – phase of carrier signal is varied to represent binary 1 or 0
peak amplitude & freq. remain constant during each bit interval
example: binary 1 = 0º phase, binary 0 = 180º ( rad) phase
 PSK is equivalent to multiplying carrier signal by +1 when the information is 1, and by -1 when the
information is 0
demodulation:
demodulator must determine the phase of received sinusoid with respect to some reference phase
PSK is less susceptible to errors than ASK, while it requires/occupies the same bandwidth as ASK
more efficient use of bandwidth (higher data-rate) are possible, compared to FSK !!!
more complex signal detection / recovery process, than in ASK and FSK
advantage:
disadvantage:

13. MODULATION OF DIGITAL DATA: P12 SK
Example
[ PSK ]
vd(t)
vc(t)
vPSK(t)

14. BINARY PHASE SHIFT KEYING (BPSK)
 In BPSK, the symbol mapping table encodes bits (bn) 1 and 0 to transmission symbols (an) 1 and –1, respectively
 Every Tb seconds the modulator transmits one of the two carrier bursts that corresponds to the information bit being a 1 or 0
0  t  Tb 18

15. QUADRATURE MODULATION SCHEMES
 In BPSK the phase of the carrier burst is shifted 0 or 180 degrees every pulse or symbol interval depending upon the information sequence. Thus each modulated carrier pulse transmits 1 bit of information
 If, on the other hand, the modulation scheme can use phase shifts of 45, 135, 225, or degrees, each modulated carrier pulse transmits 2 bits of information. This technique is called Quadrature Phase Shift Keying (QPSK)
 Using QPSK, we can double the data rate over the same channel bandwidth.
 QPSK is one of the modulation methods in the family known as Quadrature modulation schemes which are widely used, including in cellular and cable modem applications 11

16. THANK YOU