AMPLITUDE MODULATION (AM)DOUBLE SIDEBAND (DSB)

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MATRIX NUMBER

Double-sideband suppressed carrier

(DSB-SC)

AMPLITUDE MODULATION (AM)

Double-sideband Full Carrier (DSB-FC)

Single-sideband (SSB)

Vestigial Sideband (VSB)

DOUBLE SIDEBAND FULL CARRIER(DSB-FC)

• Also called the ordinary Amplitude Modulation (AM)

Definition:Process of changing amplitude of carrier signal (high frequency) follows instantaneous amplitude value of information signal (low frequency)

AM modulation is one type of modulation –  Easy, cheap, low-quality–  Used for AM receiver and CBs (citizen bands) –  Generally high carrier frequency is used to modulate the voice signal(300 – 3000 Hz)

DOUBLE SIDEBAND FULL CARRIER(DSB-FC)

AMPLITUDE MODULATION

Modulated SignalModulating Signal

High Frequency Carrier

Figure 1.1: Block Diagram of Amplitude Modulation

Features: • Low Frequency• Changes Amplitude

Features: • High Frequency• Constant Amplitude

Features: • Carrier Amplitude follows

instantaneous amplitude of information signal

• Frequency and phase did not change

• Produce 3 frequencies spectrum

- Carrier Frequency - Upper Sideband - Lower Sideband

Mathematical Expression

No Modulation

No Modulation

fm fc

AM DSB-FCenvelope

CarrierPlusmodulation

Carrieronly

(Em)Modulating signal

(Ec)Carrier

[Eam (t)]Modulated wave

Em = Em sin[2fmt]

Ec = Ec sin[2fct]

The Modulated Wave Expression

Mathematical Expression of Modulated Wave

Carrier frequency signal (volts)

Lower side frequency signal (volts)

Upper side frequency signal (volts)

AM FREQUENCY SPECTRUMDSB-FC

Figure 1.2: Frequency spectrum of an AM DSBFC wave

𝑩=𝟐 𝒇 𝒎

AMPLITUDE MODULATION INDEX+𝑉𝑚𝑎𝑥=𝐸𝑐+𝐸𝑚 +𝑉𝑚𝑖𝑛=𝐸𝑐−𝐸𝑚

−𝑉𝑚𝑎𝑥=−𝐸𝑐−𝐸𝑚 −𝑉𝑚𝑖𝑛=𝐸𝑐+𝐸𝑚

𝐸𝑚=12(𝑉𝑚𝑎𝑥−𝑉𝑚𝑖𝑛)

𝐸𝑐=12(𝑉𝑚𝑎𝑥+𝑉𝑚𝑖𝑛)

𝑚=𝐸𝑚

𝐸𝑐

𝐸𝑢𝑠𝑓 =𝐸𝑙𝑠𝑓 =𝐸𝑚

2=1

4(𝑉𝑚𝑎𝑥−𝑉𝑚𝑖𝑛)

AM VOLTAGE SPECTRUMDSB-FC

Figure 1.3: Voltage spectrum of an AM DSBFC wave

AM POWER DISTRIBUTION

𝑃𝑐=(0.707𝐸¿¿𝑐)2

𝑅=

(𝐸¿¿𝑐 )2

2𝑅¿¿

𝑃𝑢𝑠𝑏=𝑃 𝑙𝑠𝑏=𝑚2𝑃𝑐

4

𝑃 𝑡=𝑃𝑢𝑠𝑏+𝑃 𝑙𝑠𝑏+𝑃𝑐

Figure 1.4: Power spectrum of an AM DSBFC wave

AM TRANSMITTERS

Low-Level AM DSB-FC Transmitter

High-Level AM DSB-FC Transmitter

LOW-LEVEL AM DSB-FC TRANSMITTER

Figure 1.5: Low-Level AM DSBFC Transmitter

Functions

Functions The preamplifier  (linear voltage amplifier with high input impedance)• To raise source signal amplitude to a usable level with minimum non linear distortion

and as little thermal noise as possible Modulating signal driver  (linear amplifier)• Amplifies the information signal to an adequate level to sufficiently drive the

modulator

RF carrier oscillator• To generate the carrier signal

The buffer amplifier  (low-gain, high-input impedance linear amplifier)• To isolate the oscillator from the high-power amplifiers

The coupling network • Matches output impedance of the final amplifier to the transmission line/antenna

The intermediate and final power amplifiers• Required with low-level transmitters to maintain symmetry in the AM envelope

HIGH-LEVEL AM DSB-FC TRANSMITTER

Figure 1.6: High-Level AM DSBFC Transmitter

Functions

Functions Modulating signal is processed similarly as in low-level transmitter except for the addition of power amplifier• To provide higher power modulating signal necessary to achieve 100%

modulation (carrier power is maximum at the high-level modulation point)

Same circuit as before for carrier oscillator, buffer and driver but with addition of power amplifier The modulator circuit has three primary functions:• Provide the circuitry necessary for modulation to occur• It is the final power amplifier• Frequency up-converter: translates low-frequency information signals to

radio-frequency signals that can be efficiently radiated from an antenna and propagated through free space

DOUBLE SIDEBAND SUPPRESSED CARRIER

(DSB-SC)

• In DSBFC the Frequency carrier are suppressed so there is no carrier.

Figure 1.7: A frequency-domain display of a DSBSC signal

WHY???

DOUBLE SIDEBAND SUPPRESSED CARRIER

(DSB-SC)

• The real information is contained within the sidebands. • One way to overcome this problem is simply to suppressed the carrier.

• Since the carrier does not provide any useful information, there is noreason why it has to be transmitted. • By suppressing the carrier, the resulting signal is simply the upper andlower sidebands (referred to as a Double sideband suppressed carrier, DSBSC).

• The benefit is no power is wasted on the carrier and that the power saved can be put into the sidebands.

DIFFERENCES BETWEEN DSBFC AND DSBSC

DSBFC DSBSC

With Carrier Without Carrier

Wasted power Power saved (No wasted power)

APPLICATION Analogue TV systems: to transmit color information. For transmitting stereo information in FM sound broadcast at very high

frequency One important application of DSB is the transmission of color information

in a TV signal. CB radio (Citizen Band Radio) TV broadcasting Air traffic control radios Garage door opens keyless remotes DSB-SC is a technique used in electronic communication, most commonly

for transmitting information via a radio carrier wave. DSB-SC used in stereo transmission of FM radio. Two way radio communications.

CONCLUSIONDouble-sideband suppressed-carrier transmission (DSB-SC) is transmission in which

frequencies produced by amplitude modulation (AM) are symmetrically spaced above

and below the carrier frequency and the carrier level is reduced to the lowest practical

level, ideally being completely suppressed. In the DSB-SC modulation, unlike in AM

DSB-FC, the wave carrier is not transmitted; thus, much of the power is distributed

between the sidebands, which implies an increase of the cover in DSB-SC, compared

to AM DSB-FC, for the same power used. DSB-SC transmission is a special case of

double-sideband reduced carrier transmission. It is used for radio data systems.

THE END