Amplitude Modulation Principles

Basic Principles of Amplitude Modulation

• Modulator circuits vary carrier amplitude with modulating signals, producing AM, DSB, and SSB transmission methods.
• The basic equation for an AM signal is V_AM = V_csin(2πf_ct) + (V_msin(2πf_mt))(sin(2πf_ct)).
• The first term, V_csin(2πf_ct), is the sine wave carrier.
• The second term, (V_msin(2πf_mt))(sin(2πf_ct)), represents the product of the sine wave carrier and modulating signals.
• Amplitude modulation voltage is produced by multiplying the carrier by the modulating signal, then adding the carrier.
• if a circuit's gain is a function of (1 + m sin 2πf_mt), the expression for the AM signal is V_AM = A(V_C), where A is the gain or attenuation factor.
• The product of the carrier and modulating signal can generated by using a nonlinear component such as a diode.
• A square-law function varies in proportion to the square of the input signals; diodes give a good approximation and can also be achieved with bipolar and field-effect transistors (FETs).
• Diodes and transistors that deviate from a pure square-law function produce third, fourth, and higher-order harmonics – intermodulation products.
• Intermodulation products are easy to filter out.
• Tuned circuits filter out the modulating signal and carrier harmonics, leaving only the carrier and sidebands.

Amplitude Modulators

• There are two types of amplitude modulators: low-level and high-level modulators.
• Low-level modulators generate AM with small signals that must be amplified before transmission.
• High-level modulators produce AM at high power levels, typically in the final amplifier stage of a transmitter.

Low-Level AM: Diode Modulator

• Diode modulation consists of a resistive mixing network, a diode rectifier, and an LC tuned circuit.
• The carrier and modulating signals are applied to separate input resistors.
• This resistive network linearly mixes the two signals (algebraically added).
• A diode passes half cycles when forward biased.
• The coil and capacitor repeatedly exchange energy, causing an oscillation or ringing at the resonant frequency.

Low-Level AM: Transistor Modulator

• Transistor modulation Includes a resistive mixing network, a transistor, and an LC tuned circuit.
• The emitter-base junction acts as a diode and nonlinear device.
• Amplification and modulation occur as base current controls a larger collector current.
• The LC tuned circuit oscillates to generate the missing half cycle.

Low-Level AM: PIN Diode Modulator

• Variable attenuator circuits using PIN diodes produce AM at VHF, UHF, and microwave frequencies.
• PIN diodes are designed for use at frequencies above 100 MHz.
• When forward-biased, PIN diodes act as variable resistors.
• Attenuation by PIN diode circuits changes with the modulating signal's amplitude.

Low-Level AM: Differential Amplifier

• Differential amplifier modulators have high gain, good linearity and can be 100 percent modulated.
• The output voltage is balanced or differential when taken between the two collectors.
• The output can also be single-ended, taken from either collector to ground.
• The modulating signal is applied to the base of a constant-current source transistor.
• The modulating signal impacts the emitter current and the circuit's gain, resulting in AM.

High-Level AM

• High-level modulation varies the voltage and power in the final RF amplifier stage of the transmitter.
• This results in high efficiency in the RF amplifier stage with overall very good performance.

High-Level AM: Collector Modulator

• The collector modulator is a linear power amplifier that steps up modulating signals to a high-power level.
• A modulating output signal couples through a modulation transformer to a class C amplifier.
• The modulation transformer's secondary winding connects in series with the collector supply voltage of the class C amplifier.

High-Level AM: Series Modulator

• A series modulator achieves high-level modulation without needing the large modulation transformer used in collector modulators.
• It improves frequency response, but is not efficient.
• A series modulator replaces the modulation transformer with an emitter follower.
• The modulating signal is applied to the emitter follower.
• The emitter follower is in series with the collector supply voltage.
• The collector voltage varies with the amplified audio modulating signal.

Amplitude Demodulators

• Demodulators or detectors are circuits that accept modulated signals to recover the original modulating information.

Amplitude Demodulators: Diode Detector

• On positive alternations of the AM signal, the capacitor charges rapidly to the peak value of pulses passed by the diode.
• When the pulse voltage goes to zero, the capacitor discharges into the resistor.
• The time constant of the capacitor and resistor is long compared to the period of the carrier.
• The capacitor discharges negligibly when the diode is not conducting.
• The waveform across the capacitor results in a close approximation to the original modulating signal.
• Because the diode detector recovers the envelope of the AM (modulating) signal, it is sometimes called an envelope detector.
• If the RC time constant is too long, the capacitor discharge will be too slow to follow the faster changes in the modulating signal referred to as diagonal distortion.

Amplitude Demodulators: Synchronous Detection

• Synchronous detectors use an internal clock signal at the carrier frequency in the receiver to switch the AM signal off and on, producing rectification similar to a standard diode detector.
• Synchronous or coherent detectors have less distortion and therefore a better signal-to-noise ratio than standard diode detectors.
• The key to synchronous detection is synchronizing the switching action to match the phase of the received AM carrier.
• An internally generated carrier signal from an oscillator is typically unsuitable.

Balanced Modulator

• A balanced modulator produces a DSB signal by suppressing the carrier, leaving only the sum and difference frequencies.
• Balanced modulator output can be further processed by filters or phase-shifting circuitry to eliminate one of the sidebands, resulting in an SSB signal.
• Types of Balanced Modulators are Lattice, 1496/1596 IC, and the analog multiplier.

Balanced Modulator: Lattice Modulator

• A popular balanced modulator is the diode ring or lattice modulator.
• The lattice modulator consists of an input transformer, an output transformer, and four diodes connected in a bridge circuit.
• The carrier is applied to the center taps of the input and output transformers, while the Modulating Signal is applied to the Input Transformer.
• The output appears across the output transformer.
• The carrier sine wave's frequency and amplitude is higher than the modulating signal.
• The carrier sine wave is used as a source of forward and reverse bias for the diodes, turning them off and on rapidly.
• The diodes act as switches, connecting the modulating signal at the secondary of T₁ to the primary of T₂.

Balanced Modulator: IC Balanced Modulators

• The 1496/1596 IC is a versatile circuit for communication applications, able to work at carrier frequencies up to 100 MHz.
• It achieves carrier suppression levels of 50 - 65dB.
• The 1496/1596 IC can act as a balanced modulator or can be configured for amplitude modulation, product detection, or synchronous detection.

Balanced Modulator: IC Balanced Modulators: Analog Multiplier

• An analog multiplier is an integrated circuit that can serve as a balanced modulator, often used to generate DSB signals.
• It is not a switching circuit like the balanced modulator.
• Instead, the device uses differential amplifiers operating in the linear mode.
• The carrier must be a sine wave and the multiplier produces the true product of two analog inputs.

SSB Circuits

• The filter method, simplest, is used to generate SSB signals.
• The modulating signal is applied to the audio amplifier.
• The amplifier's output is fed to one input of a balanced modulator.
• A crystal oscillator provides the carrier signal which is applied to the balanced modulator.
• The output of the balanced modulator is a double sideband (DSB) signal.
• An SSB signal is produced by passing the DSB signal through a highly selective bandpass filter.
• In the filter method, only the upper or lower sideband can be selected.

SSB Circuits: Phasing Method

• It uses a phase-shift that causes one of the sidebands to be completely canceled out, using two balanced modulators.
• This process eliminates the carrier signal.
• The carrier oscillator is applied to the upper balanced modulator.
• The carrier and modulating signals are both shifted in phase by 90 degrees and applied to another balanced modulator.
• Phase-shifting causes one sideband to be canceled out when the two modulator outputs are added together.

SSB Circuits: DSB and SSB Demodulation

• Reinsertion of the suppressed carrier at the receiver is necessary to recover the intelligence from a DSB or SSB signal..
• A product detector, a balanced modulator, recovers the modulating signal.
• A balanced modulator can be configured as a product detector to demodulate SSB signals.