AM Modulation - created from Mind Map

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AM Transmitter

TUNED CIRCUIT

FILTER

OSCILLATOR

Single Side Band (SSB) & Double Side Band Surpressed Carrier (DSBSC)

AM reception

AM receiver

Reciever parameters

Modulation

basic block diagram

block diagram of Low-level AM DSBFC transmitter

block diagram of High-level AM DSBFC transmitter

RF carrier oscillator

Buffer amplifier

Modulator

modulating signal source

Preamplifier

modulating signal driver

linear intermediate power amplifier

Compiling network

crystal controlled oscillator

most common circuit

low gain, ihigh- input impedence linear amplifier

isolate the oscillator from high power smplifiers

provides constant load to oscillator which helps to reduce occurrence & magnitude

Emitter/ collector modulator

music transmission = acoustical transducer

linear voltage amplifier with high input impedence

raise the amplitude of source signal to usable lable

produce min. nonlinear distortion & adding little thermal noise

simplify the info signal to an adequate level

required for low level transmitters

maintain symmetry in AM envelope

matches the output impedance of final power to transmission & antenna

provides the circuitry necessary for modulation to occur

it's the final power amplifier

it's a frequency up-converter

can radiated from antenna & propagated through free space

Modulating signal source

RF oscillator

Buffer amplifier

RF oscillator

same as low-level except for the addition of power amplifier signal power must higher than low-level TX

same as low TX

RF carrier undergoes additional power amplification prior to the modulator stage

same as low TX

RF carrier undergoes additional power amplification prior to the modulator stage

same as low TX

RF carrier undergoes additional power amplification prior to the modulator stage

TUNING : used when it is desired to pass only a specific band or channel

Reactive Component

Tuned circuits & resonance

Inductors & capacitors that resonate at specific frequencies

Contained virtually in all communications equipment

Capacitor

Inductor

Resistor

Coil and capacitor together form tuned or resonant

Oppose voltage charges across it

AC circuit : charges & discharges

Current lead the applied voltage

Capacitive reactance (Xc): opposition to alternating current

Also called a coil or choke

Inductance: Varying applied voltage and current causes a voltage to be self-induced into the coil winding.

Current lag the applied voltage

Magnetic field is produced around the coil when a current passed through

Inductive reactance (XL): Continuous and constant opposition to alternating current

Low frequencies: Nearly pure resistance

High frequencies: Resistor’s leads have inductance

Resistor’s lead inductance + Stray capacitance = Complex RLC circuits

Film resistors minimize thermal effect noise

Series resonance circuit

Resonance occur when inductive and capacitance reactance are equal * The resonant frequency (fr) =

Parallel resonance circuit

Referred as LCR or RLC circuits

Inductance, capacitor, resistor in series

Inductance and capacitor in parallel

A frequency-selective circuit which pass certain frequencies and reject others

Passive filters: Resistors, capacitors, and inductors that do not amplify

Active filters: Use amplifying devices such as transistors and operational amplifiers.

RC Filter

5 basic filter

LC Filter

Advantages: Gain, no inductors, easy to tune, isolation and easier impedance matching

Combinations of resistors and capacitors: Achieve a desired frequency response

Mostly the low-pass or high-pass type

RC coupling circuit: a high-pass filter because the ac input component is developed across the resistor while dc voltage is blocked by a capacitor

Low pass filter Only pass frequencies below a critical (cutoff) frequency

High pass filter Only pass frequencies above the cutoff frequency.

Band pass filter Pass frequencies over a narrow range between lower and upper cutoff frequencies

Band reject filter Reject or stop frequencies over a narrow range between lower and upper cutoff frequencies

All pass filter Pass all frequencies over a desired range but have a predictable phase shift characteristic

Combinations of inductors and capacitors to achieve a desired frequency response

Types of LC Filter

Butterworth: The Butterworth filter effect has maximum flatness in response in the passband and a uniform attenuation with frequency.

Bessel (Thomson): Provides the desired frequency response (i.e., low-pass, bandpass, etc.) but has a constant time delay in the passband.

Chebyshev: Has extremely good selectivity, and attenuation just outside the passband is very high, but has ripple in the passband.

Cauer (Elliptical): Produces greater attenuation out of the passband, but with higher ripple within and outside of the passband.

Convert DC to AC without the need for an AC input source

An electronic circuit that generates a periodic waveform on its output without an external signal source

The feedback oscillator relies on a positive feedback of the output to maintain the oscillations

Types of Oscillators

RC Oscillators

LC Oscillators

Relaxation Oscillators

WEIN BRIDGE

PHASE SHIFT

Untuned RC phase shift oscillator that uses both positive and negative feedback

Each of the three RC circuits in the feedback loop can provide a maximum phase shift approaching 90°.

Oscillation occurs at the frequency where the total phase shift through the three RC circuits is 180°.

Normally used in oscillators that requires higher frequencies of oscillation

Types of LC feedback oscillators: Colpitts, Clapp, Hartley, Armstrong, and crystal-controlled oscillators

Uses an LC circuit in the feedback loop to provide the necessary phase shift and to act as a resonant filter that passes only the desired frequency oscillation

Circuits that produce a continuous signal of some type without the need of an input

Can be used to generate nonsinusoidal waveforms

Generating SSB

Demodulation SSB

DSBSC

DSB & SSB Demodulation

Filter Method

Phasing Method

Modulation signal applied to the audio amplifier

Amplifier's output fed to one input of balanced modulator

Crystal oscillator provide corner signal that applied to balanced modulator

Output = double side band (DSB) signal

SSB signal produced by passing DSB signal through highly selective band pass filter

Use phase-shift technique which causing one of the side bands to cancel out

Uses two balanced modulators which eliminate carrier

Carrier oscillator are applied to the upper balanced modulator along with the modulating signal

Carrier and modulating signal signal both shifted in phase 90 degree and applied to the balanced modulator

Phase shifting causes one side band to be cancelled out when two modulator output are added together

To recover original signal from IF SSB signal : 1.SSB must be frequency-shifted down to its original range of base band frequencies using product detector 2.Product detector mixes it with output of Beat Frequency Oscillator (BFO)

Used to ensure discrete carrier signal is surppressed

Carrier level is reduced to the lowest practical level which are compressed

In DBS-SC, the wave carrier is not transmitted , so much of the power is distributed between side bands

AM wave without carrier, thus reduce power waste and have 100% efficiency

Recovering intelligence both side bands, the surppressed carrier at the receiver must be inserted

Product detector is used to recover modulating signal

Any balanced modulator can be used as a product detector to demodulate SSB signal

To demodulate DSB-SC signal must be multiplied with the carrier signal

RF section

Mixer/converter section

IF section

AM detector

Audio section

conventional DS-AM receiver

first stage of receiver

functions: detecting, band limiting & amplifying the received RF signals

down-convert the received RF frequencies to intermediate frequencies

amplification & selectivity

demodulates the wave & convert it to the original information signal

implifies the recovered information

receive, amplify & demodulate the AM wave

able to band limiting the total radio-frequency spectrum to a specific desired band of frequencies (called tuning the receiver)

coherent (synchronous Rx)

Non coherent (asynchronous Rx)

TRF

frequencies generate in Rx used for demodulation

frequencies used for demodulation. Completely independent from the Transmitter's carrier frequency.

called envelope detector

superheterodyne

RF section

IF section

audio amplifier section

audio detector section

mixer/converter section

RF stage

detector stage

audio stage

evaluate the ability of receiver performance

dynamic range

fidelity

insertion loss, IL

noise

selectivity

sensitivity

bandwidth improvement

Definition: The process of changing the amplitude of relatively high-frequency carrier signal in proportion with the instantaneous value of the modulating signal (information).

AM Envelope

AM Expression

AM Spectrum and Bandwidth

Modulation Index

AM in Frequency Domain

AM Spectrum

AM Power Distribution

AM Current

Modulating signal amplitude < Carrier signal amplitude, Vm < Vc

Definition: The amount of amplitude modulation in a signal