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
To Modulator
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 shortterm frequency variations.
CARRIER DRIVER
a linear amplifier that amplifies the carrier signal to an adequate level to sufficiently drive the 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.
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.
RF CARRIER
- With high-level transmitter, the RF carrier undergoes additional power amplification prior to the modulation stage.
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
BI BW ( RF ) BW ( IF )
e) Insertion Loss (IL) is the ratio of the output power of a filter to the input power for frequencies that fall within the filters passband.
IL(dB) 10 log
Pi Po
f) Equivalent Noise Temperature (Te) is an indication of reduction in the signal to noise ratio as signal propagates through a receiver.
Pn Te ( F 1) KB
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.
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.
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.
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.
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.
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.
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
