Block Diagram of a Low Level AM Transmitter

LOW LEVEL AM TRANSMITTER

- used for low power, low capacity system such as wireless intercoms, remote

control units, pager and short range walkie-talkies.

ELEMENTS OF A LOW-LEVEL AM TRANSMITTER

MODULATING SIGNAL

SOURCE – an acoustic transducer, such

as microphone, magnetic tape

or CD disk, that is need for

voice or music transmission.

PREAMLIFIER – a sensitive, CLASS A

linear voltage amplifier with

high input impedance. Its

function is to raise the

amplitude of the source

signal to a usable level while

producing minimum nonlinear

distortion and adding as little

thermal noise as possible.

MODULATING

SIGNAL DRIVER – also a linear amplifier

that amplifies the information

signal to an adequate level to

sufficiently drive the modulator.

To Modulator

RF CARRIER

OSCILLATOR – crystal controlled

oscillators that will produce

the high frequency carrier

signal.

BUFFER AMPLIFIER – a low gain, high input

impedance linear amplifier (EF

amplifier). Its function is to isolate

the oscillator from high power

amplifiers. The buffer provides a

relatively constant load to the

oscillator, which helps to reduce the

occurrence and magnitude of short-

term frequency variations.

CARRIER DRIVER – a linear amplifier that

amplifies the carrier signal to

an adequate level to sufficiently

drive the modulator.

To Modulator

MODULATOR – collector or emitter

modulators where AM

modulation takes place. A

circuit that changes the

amplitude of the carrier signal

in respond to the variations of

the modulating signal.

INTERMEDIATE AND

FINAL STAGE – either a linear CLASS A or

CLASS B push-pull amplifier. This is

required with low level transmitter to

maintain symmetry in the AM

envelope.

ANTENNA

COUPLING NETWORK – matches the output

impedance of the final

power amplifier to the

transmission line and

antenna.

Block Diagram of a High Level AM Transmitter

MODULATING SIGNAL

- With high-level transmitter, the power of the

modulating signal must be high that is why additional

power amplifier is used. Note that the carrier is at full

power at the point in the transmitter where

modulation occurs, therefore, it is required to have a

high amplitude-modulating signal to produce 100%

modulation.

RF CARRIER - With high-level transmitter, the RF carrier

undergoes additional power amplification prior to the

modulation stage.

MODULATOR – drain-, plate- or collector-modulated class C

amplifier. It provides the circuitry necessary for

modulation to occur. It is the final power amplifier

stage and it is a frequency up converter.

TRAPEZOIDAL PATTERN

– used for observing the modulation characteristics of AM transmitters.

Parameters used to evaluate the ability of a receiver to successfully

demodulate an RF signal

a) Selectivity is the ability of the receiver to accept a given band of frequencies

and reject all others.

b) Sensitivity is the minimum RF signal level that can be detected at the input

to the receiver and still produce a usable demodulated information signal.

c) Dynamic Range is the input power range over which the receiver is useful.

d) Bandwidth Improvement (BI) is the noise ratio achieved by reducing the

bandwidth

)(

)(

IFBW

RFBWBI

e) Insertion Loss (IL) is the ratio of the output power of a filter to the input

power for frequencies that fall within the filter’s passband.

Po

PidBIL log10)(

f) Equivalent Noise Temperature (Te) is an indication of reduction in the

signal to noise ratio as signal propagates through a receiver.

)1( FKB

PnTe

AM SUPERHETERODYNE RECEIVER

-It provides a greater gain , better selectivity and better sensitivity than

other receiver configurations.

-Heterodyne means to mix two frequencies together in a nonlinear device

or to translate one frequency to another using nonlinear mixing.

- Major Edwin Howard Armstrong, a US radio engineer, developed

superheterodyne tuning for reception over a very wide spectrum of radio

frequency in 1936.

Block Diagram of an AM

Superheterodyne Receiver

ELEMENTS OF AN AM SUPERHETERODYNE RECEIVER

PRESELECTOR - a broad-tuned bandpass filter with an

adjustable center frequency that is tuned to the

desired carrier frequency.

- provides enough initial bandlimiting to

prevent a specific unwanted radio frequency called

image frequency from entering the receiver.

- reduces noise bandwidth of the receiver

and provides initial step toward reducing the

overall receiver bandwidth to the minimum

bandwidth requirement to pass the information

signal.

RF AMPLIFIER - determines the sensitivity of the

receiver. It sets the signal threshold.

- provide a greater gain, improved

image-frequency rejection, better signal to

noise ratio and better selectivity.

LOCAL OSCILLATOR - oscillator circuit that provides a local oscillator frequency.

When the fLO is tuned above the RF, it is called high side

injection or high beat injection. When the fLO is tuned below

RF, it is called low-side injection of low-beat injection. Tracking

is the ability of the local oscillator to oscillated above or below

the selected RF carrier by an amount equal to the intermediate

frequency throughout the entire radio-frequency band.

MIXER/ CONVERTER SECTION - a nonlinear device whose purpose is to convert radio

frequencies to intermediate frequencies. Heterodyning takes

place in this stage, and RF is down converted to intermediate

frequency. Although carrier and sidebands frequencies are

translated to IF, the shape of the envelope and the original

information signal remains unchanged. The most common IF

used in AM broadcast band receivers is 455KHz.

IF SECTION

- consists of five or six IF amplifiers in series and bandpass

filters. Most of the receiver gain and selectivity is achieved in the IF

section.

DETECTOR - convert the IF signals

back to the original source

information. It can be a

single diode demodulator or

as complex a phase-locked

loop or balanced modulators.

AUDIO SECTION - comprises of several

cascaded audio amplifiers

and one or more speakers…

The number of audio

amplifiers depends on the

audio signal power desired.

Electronic Communication Systems Through Advanced by W. Tomasi

Communication Electronics by L. Frenzel

Lecture Notes in Principles of Communication by A.H. Ballado and M.M. Sejera

Electronic Communication Systems by

G. Kennedy