Amplitude Modulation

Amplitude Modulation

1. AM Transmitter

   • basic block diagram
   • block diagram of Low-level AM DSBFC transmitter
     • RF carrier oscillator
       • crystal controlled oscillator
       • most common circuit
     • Buffer amplifier
       • 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
     • Modulator
       • Emitter/ collector modulator
     • modulating signal source
       • music transmission = acoustical transducer
     • Preamplifier
       • linear voltage amplifier with high input impedence
       • raise the amplitude of source signal to usable lable
       • produce min. nonlinear distortion & adding little thermal noise
     • modulating signal driver
       • simplify the info signal to an adequate level
     • linear intermediate power amplifier
       • required for low level transmitters
       • maintain symmetry in AM envelope
     • Compiling network
       • matches the output impedance of final power to transmission & antenna
   • block diagram of High-level AM DSBFC transmitter
     • 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
       • same as low-level except for the addition of power amplifier signal power must higher than low-level TX
     • RF oscillator
       • same as low TX
       • RF carrier undergoes additional power amplification prior to the modulator stage
     • Buffer amplifier
       • same as low TX
       • RF carrier undergoes additional power amplification prior to the modulator stage
     • RF oscillator
       • same as low TX
       • RF carrier undergoes additional power amplification prior to the modulator stage

2. TUNED CIRCUIT

   • TUNING : used when it is desired to pass only a specific band or channel
     • Inductors & capacitors that resonate at specific frequencies
     • Contained virtually in all communications equipment
   • Reactive Component
     • Capacitor
       • 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
     • Inductor
       • 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
     • Resistor
       • 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
   • Tuned circuits & resonance
     • Series resonance circuit
       • Inductance, capacitor, resistor in series
     • Resonance occur when inductive and capacitance reactance are equal * The resonant frequency (fr) =
     • Parallel resonance circuit
       • Inductance and capacitor in parallel
     • Referred as LCR or RLC circuits

3. FILTER

   • 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.
     • Advantages: Gain, no inductors, easy to tune, isolation and easier impedance matching
   • RC Filter
     • 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
   • 5 basic filter
     • 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
   • LC Filter
     • 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.

4. OSCILLATOR

   • 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
       • WEIN BRIDGE
         • Untuned RC phase shift oscillator that uses both positive and negative feedback
       • PHASE SHIFT
         • 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°.
     • LC Oscillators
       • 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
     • Relaxation Oscillators
       • Circuits that produce a continuous signal of some type without the need of an input
       • Can be used to generate nonsinusoidal waveforms

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

   • Generating SSB
     • Filter 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
     • Phasing Method
       • 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
   • Demodulation SSB
     • 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)
   • DSBSC
     • 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
   • DSB & SSB Demodulation
     • 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

6. AM reception

   • RF section
     • first stage of receiver
     • functions: detecting, band limiting & amplifying the received RF signals
   • Mixer/converter section
     • down-convert the received RF frequencies to intermediate frequencies
   • IF section
     • amplification & selectivity
   • AM detector
     • demodulates the wave & convert it to the original information signal
   • Audio section
     • implifies the recovered information
   • conventional DS-AM receiver
     • 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)

7. AM receiver

   • coherent (synchronous Rx)
     • frequencies generate in Rx used for demodulation
   • Non coherent (asynchronous Rx)
     • 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
   • TRF
     • RF stage
     • detector stage
     • audio stage

8. Reciever parameters

   • evaluate the ability of receiver performance
   • dynamic range
   • fidelity
   • insertion loss, IL
   • noise
   • selectivity
   • sensitivity
   • bandwidth improvement

9. Modulation

   • 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
     • Modulating signal amplitude < Carrier signal amplitude, Vm < Vc
   • AM Expression
   • AM Spectrum and Bandwidth
   • Modulation Index
     • Definition: The amount of amplitude modulation in a signal
   • AM in Frequency Domain
   • AM Spectrum
   • AM Power Distribution
   • AM Current

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