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AMPLITUDE MODULATION1. DEFINING AM

• A carrier frequency whose amplitude is varied in proportion to the instantaneous amplitude of a modulating voltage

2. GENERATING THE AM

nonlinear device: diode or transistor biased in its nonlinear region

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3. ANALYSIS OF THE AM WAVE

tfVv ccc 2sin

tffVm

tffVm

tfVv sccscccc )(2cos2

)(2cos2

2sin

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4. Different Carriers and AM

Carriers are spaced at 20 kHz, beginning at 100kHz. Each carrier is modulated by a signal with 5kHz bandwidth. Is there interference from sideband overlap?

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5. MODULATION INDEX AND SIGNAL POWER

minmax

minmax

VV

VV

V

Vm

c

m

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Moduiation Index and Power

R

V

R

V

R

VP cccarrc 2

)2/( 222

42

2m

R

VPP cUSBLSB

)2

1()2

1(2

222 mP

m

R

VP c

CT

21

P

P 2

c

T m )1(2

C

T

P

Pm

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Current Calculations

21

2m

I

I

c

T

ExampleA carrier of 1000 W is modulated with a resulting modulation index of 0. 8. What is the total power?

What is the carrier power if the total power is 1000 W and the modulation index is 0.95?

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6. SUPPRESSED CARRIERS AND SINGLE SIDEBAND (SSB)

6.1 Signal Magnitudes and Ranges

decibel (dB)

O

I

P

PdB log10

O

I2O

2I

V

Vlog20

V

V10logfor volts)( dB

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The dB gain is

Ratio Power Voltage

(dB) (dB)

1 0 0

10 10 20

100 20 40

1,000 30 60

10,000 40 80

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dB REFERENCE VALUES

• dBm (reference is 1 mW),

• dBV (reference is 1 V),

• dBW (reference is 1 W).

• Example:

A signal of 200 mW could be expressed in dBm as

An audio amplifier might have an output of 7 dBW. This corresponds to

A signal of 8.2 V corresponds to

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6.2 Double Sideband Suppressed Carrier (DSBSC)

When the carrier is reduced, this is called double-sideband suppressed-carrier AM, or DSB-SC. If the carrier could somehow be removed or reduced, the transmitted signal would consist of two information-bearing sidebands, and the total transmitted power would be information

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• For a 100-W total available power, compare the power in the sidebands when modulation is standard AM with a modulation index of 100%, versus a suppressed carrier design where 90% of the carrier power is suppressed. How many times greater is the sideband power in the suppressed carrier case?

• A 500-W DSB-SC system with 100% modulation suppresses 50% of the carrier, and the suppressed carrier power goes to the sidebands. How much power is in the sidebands, and how much is in the carrier? By how many dB has the sideband power increased?

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6.3 Single-Sideband (SSB)

• suppressing the carrier and one of the sidebands

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6.4 Filtering the SSB LSB or USB • Dual Conversion: up-converting the mod

ulating frequency twice and selecting the upper or lower sideband for transmission.

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7. Filtering the SSB LSB or USB

Their quality or "Q" factor: A filter is said to have a high Q if it selects or rejects a narrow range of frequencies compared with its centre frequency. Q is defined as centre frequency fo/3dB bandwidth.

Type of the filters

LC filters ceramic mechanical crystal

Order of Qs

200 2000 10,000 50,000

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Dual Conversion

up-converting the mod ulating frequency twice and selecting the upper or lower sideband for transmission.

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8. VESTIGIAL SIDEBAND

• In VSB, a portion (or vestige) of the lower sideband is transmitted along with the full carrier power and upper-sideband content.

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AM: Features and Drawbacks:

the AM signal is greatly affected by noise

impossible to determine absolutely the original signal level

conventional AM is not efficient in the use of transmitter power

AM is useful where a simple, low-cost receiver and detector is desired

The noise is either not a problem or can be tolerated in the ap plication.