Modulation of Digital Data

Digital Signal Analog Signal

Modulation of Digital Data

1. Digital-to-Analog Conversion2. Amplitude Shift Keying (ASK)3. Frequency Shift Keying (FSK)4. Phase Shift Keying (PSK)5. Quadrature Amplitude Modulation (QAM)6. Bit/Baud Comparison7. Modems

Digital-to-analog modulation

Types of digital-to-analog modulation

Aspects to digital-to Analog conversion

Bit Rate / Baud Rate

Bit rate is the number of bits per second. Baud rate is the number of signal units per second. Baud rate is less than or equal to the bit rate.

Bit rate is important in computer efficiency Baud rate is important in data transmission.

Baud rate determines the bandwidth required to send signal

Baud rate = bit rate / # bits per signal unit

Modulation Techniques

Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK)

Phase Shift Keying (PSK)

Amplitude Shift Keying (ASK)

Amplitude Shift Keying (ASK) is a form of modulation that represents digital data as variations in the amplitude of a carrier wave.

The amplitude of an analog carrier signal varies w.r.t modulating signal, keeping frequency and phase constant.

The level of amplitude can be used to represent binary logic 0s and 1s.

Amplitude Shift Keying (ASK)

BasebandData

ASK modulatedsignal

A cos ct A cos ct0 0

We can think of a carrier signal as an ON or OFF switch.

In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation and hence the name given.

Amplitude Shift Keying (ASK)

•The strength of the carrier signal is varied to represent binary 1 and 0.•Frequency and phase remains the same.•Highly susceptible to noise interference.

PRO’s of ASK: It is Simple and Easy to implement. Transmission and Receiving of such signals using

components becomes easy.

CON’s of ASK: Inefficient Modulation Techniques. Sensitive to noise. Susceptible to sudden gain changes.

They are used mainly for radio They are used mainly for radio frequency.frequency.

Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK)

Frequency Shift Keying (FSK) is a form of modulation that represents digital data as variations in the frequency of a carrier wave.

The frequency of an analog carrier signal varies w.r.t modulating signal, keeping amplitude and phase constant.

Frequency of the carrier is varied to represent digital data (binary 0/1)

Peak amplitude and phase remain constant.

Frequency Shift Keying (FSK)

BasebandData

FSK modulatedsignal

f1 f1f0 f0

where f0 = A cos(c-)t and f1 = A cos(c+)t

We can think of a carrier signal as an ON or OFF switch.

In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation and hence the name given.

Frequency Shift Keying (FSK)

Frequency of the signal during each bit duration is constant, and its value depends on the bit (0 or 1); both peak amplitude and phase remain constant.

FSK avoids most of the problems from noise.

Frequency Shift Keying (FSK)

PRO’s of FSK: It is Easy to implement. It has better noise immunity than ASK. It is not too sensitive to varying attenuation on the

channel. Simple implementation possible for low bit-rates.

Frequency Shift Keying (FSK)

CON’s of FSK: The high bandwidth requirement is the major

disadvantage. The FSK is not preferred for the high speed modems

because with increase in speed, the bit rate increases. Therefore FSK is extensively used in low speed

modems having bit rates below 1200 bits/sec.

They are used mainly for high radio They are used mainly for high radio frequencyfrequency

(3 to 30MHz)..

Phase Shift Keying (PSK)

Phase Shift Keying (PSK) is a form of modulation that represents digital data as variations in the phase of a carrier wave.

The phase of an analog carrier signal varies w.r.t modulating signal, keeping amplitude and frequency constant.

Phase of the carrier is varied to represent digital data (binary 0/1)

Peak amplitude and Frequency remain constant.

Phase Shift Keying (PSK)

BasebandData

Binary PSK modulatedsignal

s1 s1s0 s0

where s0 = -A cos ct and s1 = A cos ct

We can think of a carrier signal as an ON or OFF switch.

In the modulated signal, logic 0 is represented by the absence of a carrier, thus giving OFF/ON keying operation and hence the name given.

Phase Shift Keying (PSK)

Binary Phase Shift Keying (BPSK) demonstrates better performance than ASK and FSK. If we start with a phase of 00 to represent binary 0, then we can change the phase to 1800 to send binary 1.

Phase Shift Keying (PSK)

The phase of the signal during each bit duration is constant, and its value depends on the bit (0 or 1).

Two different phases (00 and 1800 ) are used. Constellation or phase-state diagram shows the relationships by illustrating only the phases.

Phase Shift Keying (PSK)

PRO’s of PSK: It is much more robust than ASK It not vulnerable to noise, which changes amplitude of

the signal.

CON’s of PSK: Minimum bandwidth required for PSK transmission is

the same as that required for ASK transmission.

Maximum bit rate in PSK transmission is much greater than that of ASK.