Digital modulation : The process by which digital symbols are transformed into waveforms that are compatible with the characteristic of the channel.

In BASEBAND MODULATION

waveform usually take the form of shaped pulses.

In BANDPASS MODULATION

the shaped pulses modulate a sinusoid called a carrier wave

Why modulation ?

For radio transmission the carrier is converted to (EM) field for propagation to the desired destination .

The size of antenna depends on the wavelength

If more than one signal utilizes a signal channel modulation may be used to separate the different signals as frequency- division multiplexing.

f

C

Compare between coherent detection and non-coherent detection ?

Coherent detection : when the receiver exploits knowledge of the carrier’s phase to detect the signals .

Non-coherent detection : when the receiver doesn't need the knowledge of the carrier’s phase to detect the signals .

PSK Phase shift keying

ASK Amplitude shift

keying

FSKFrequency shift

keying

Digital Modulation

QAM Quadratrue amplitude

modulation

ASK modulation That represents different digital data as variation of the amplitude

of carrier .

The number of symbol is M and M=2^K , K = number of bits .

If K=1 the M=2 , we say that the type of digital modulation is 2ASK.

If K=2 the M=4 , we say that the type of digital modulation is 4ASK.

Mi

Tt

twT

tEtS o

ii

,....,2,1

0

)cos()(2

)(

2ASK

4ASK

ASK modulation ( On-off-keying)(OOk)

ASK modulator by using 2206 IC The oscillation frequency of XR2206 IC is determined by resistors R1&R and

the resistor located at pin 7 and the resistor R at pin 8. Its oscillation frequencies are:

If R= , then the frequency f equal zero .

Assume that when the input is 5 V , the output frequency is f1,

When the input is 0 V , the output frequency is f.

RCf

CRf

2

1&

2

1

1

1

Circuit diagram of ASK modulator by using 2206 IC

ASK modulator by using MC 1496

ASK Demodulation

Asynchronous ASK Detector

Circuit diagram of ASK asynchronous detector

Synchronous ASK Detector

We utilize MC 1496 balanced modulator to design the square-law detector.

Advantage of ASK: simplicity.

Disadvantage of ASK : ASK is very susceptible to noise interference –noise usually (only) affects the amplitude, therefore ASK is the modulation technique most affected by noise.

Application of ASK: ASK is used to transmit digital data over optical fiber.

For analog communication system the basic SNR is the ratio between average signal power and average noise Power .

The basic SNR Parameter for Digital communication Systems is (Eb/No) , Eb = bit energy and can described as signal power times the bit Tb .

No is noise power spectral density and can described as noise power N divided by bandwidth W.

The bit rate is Rb = 1/ Tb

b

b

R

W

N

S

N

E

0

Why used (Eb/No) ?

Symbol have finite duration and non periodic.

Average power at this time goes to zero , Power is not useful parameter for digital communication.

We measure the performance of digital communication system by using Probability bit error PB Vs (Eb/No) .

The smaller required (Eb/No) is the more efficient in the detection process for a given probability of error .