digital modulation

Modulation

Modulation

In this section, we will study about

• What modulation is ?• Categories of modulation (Analog and Digital)• Types of Modulation (Analog and Digital)

6.1 Modulation

• Modulation is a process of suppressing low frequency information signal on a high frequency carrier signal.

OR Modulation is a process of modifying the any of the characteristics (amplitude, frequency, phase) of

high frequency carrier in accordance with low frequency information signal.

• Remember, for modulation require two signals, high frequency carrier and low frequency information signal (also called modulation signal).

• The resultant wave achieved known as “modulated signal”. i.e. information (modulating signal) + carrier = modulated signal.

• Why there is need of modulation?• Is there any substitute of modulation?

6.1 Modulation

• Remember for modulation, there is no change in modulating signal, only the characteristics of carrier signal are changed always.• Why?• The carrier wave is usually sine wave.• Information signal may be of any type (either analog or digital).

6.2 Categories of Modulation

Categories of Modulation

Analog ModulationInfo = Analog

Carrier = Analog

6.2 Categories of modulation

Remember that while considering categories of modulation

• In either type of modulation (analog or digital), carrier is of analog format.• Why not digital carrier? • The only difference between both categories is type of information (modulating) signal. i.e. analog modulation = analog information signal digital modulation = digital information signal• Also remember in modulated wave, the resultant change is in the respective characteristics of high frequency carrier.

6.3 Types of Modulation

6.3.2 Digital modulation

• Basically we will study three main types of digital modulation. a) Amplitude Shift Keying (ASK)b) Frequency Shift Keying (FSK)c) Phase Shift Keying (PSK)

a) Amplitude Shift Keying (ASK)

• The simplest digital modulation technique.• a binary information signal directly modulates amplitude of an analog carrier.• Similar to amplitude modulation except there are two output amplitude possible only.• Mathematically can be expressed as VASK(t) = [ 1 + Vm(t)] [ (Ac/2) cos(ωct) ]

Where• VASK(t) = Amplitude Shift Keying Modulated wave• Vm(t) = Amplitude of binary modulating (information) signal• (Ac/2) = Amplitude of un-modulated high frequency carrier• ωc = 2πfc = un-modulated high frequency carrier.

Vm (t) = logic 1 = +1V VASK(t) = [ 1 + Vm(t)] [ (Ac/2) cos(ωct) ] = [ 1 +1] [ (Ac/2) cos(ωct) ] = [2] [ (Ac/2) cos(ωct) ] = [Ac cos(ωct) ]

Vm (t) = logic 0 = -1V

VASK(t) = [ 1 + Vm(t)] [ (Ac/2) cos(ωct) ] = [ 1 - 1] [ (Ac/2) cos(ωct) ] = [0] [ (Ac/2) cos(ωct) ] = 0


Conclusion:

• As information signal is of binary format, only two voltage levels occur at input (either +1V or -1V)• As a result, we get only two voltage levels at output (either [Ac cos(ωct) ] or OV)• because of these two output voltage levels, the carrier is either in “ON” or “OFF” state.• that’s the reason why ASK is also known as “ON-OFF Keying (OOK)”


ASK with more than two input levels

b) Frequency Shift Keying (FSK)

• Simple and low performance type of digital modulation.• a binary information signal directly modulates frequency of an analog carrier.

• Mathematically can be expressed as VFSK(t) = Vc cos[ 2π (fc + Vm(t)Δf)t ]

Where• VFSK(t) = Frequency Shift Keying Modulated wave• fc = Analog carrier central frequency• Vc = peak analog carrier amplitude• Vm(t) = binary input (modulating) signal• Δf = peak change (shift) in the analog carrier frequency

Vm(t) = logic 1 = +1V

VFSK(t) = Vc cos[ 2π (fc + Vm(t)Δf)t ] = Vc cos[ 2π (fc + 1.Δf)t ]

= Vc cos[ 2π (fc + Δf)t ]

Vm(t) = logic 1 = -1V

VFSK(t) = Vc cos[ 2π (fc + Vm(t)Δf)t ] = Vc cos[ 2π (fc - 1.Δf)t ] = Vc cos[ 2π (fc - Δf)t ]


We observed that in FSK• carrier wave continuously shifted by an amount specified as Δf.• when logic 1 input is applied, the carrier wave modulated itself to a frequency known as “mark frequency (fm)”.• when logic 0 input is applied, the carrier wave modulated itself to a frequency known as “space frequency (fc)”.• the overall spectrum of FSK will be observed as


• PSK is another form of angle modulated (later define in upcoming slides) constant amplitude digital modulation.• PSK is an M-ary digital modulation scheme• a binary information signal directly modulates phase of an analog carrier.• Simplest form is “Binary phase shift keying” (BPSK)• with BPSK, two phases are possible for carrier wave.• one phase represents a logic 1 and other a logic 0.

• as the input digital signal changes state (i.e. from 1 to 0 or 0 to 1), the phase of the output carrier shifts between two phases (that are sepereated by 180o .

c) Phase Shift Keying (PSK)

Remember in PSK,• When information binary signal (modulating signal) goes low or 0 state, then output carrier undergoes 180o shift with reference to original carrier for that particular bit slot.

• When information binary signal (modulating signal) goes high or 1 state, then output carrier undergoes 0o shift with reference to original carrier for that particular bit slot. • PSK modulated output wave will be shown here

c) Phase Shift Keying (PSK)

CATEGORIES OF PHASE SHIFT KEYING

Phase Shift Keying follows M-ARY coding

•BPSK (Binary Phase Shift Keying)• QPSK (Quaternary Phase Shift Keying)• 8 PSK

Constellation Diagram (BPSK)

Balance Ring Modulator

QPSK Transmitter

Constellation Diagram (QPSK)

Digital Modulation Techniques

digital modulation

Education