Amplitude Modulation PDF

Amplitude Modulation AM Modulating Circuits Amplitude Modulation Simplified Block Diagram of an AM Transmitter In modulator, the audio modulates the RF amplitude RF Power Low-pass Modulator oscillator amplifier...

Amplitude Modulation AM Modulating Circuits Amplitude Modulation Simplified Block Diagram of an AM Transmitter In modulator, the audio modulates the RF amplitude RF Power Low-pass Modulator oscillator amplifier filter AF amplifier Microphone Amplitude Modulation The modulator converts the frequency of the input signal from the audio range to the carrier frequency of the station 5kHz frequency 610kHz frequency Frequency domain Frequency domain representation of input representation of output Amplitude Modulation Receiver Block Diagram Amplitude Modulation Low Level Modulation modulation takes place prior to the output element of the final stage of the transmitter less modulating signal power is required to achieve a high percentage modulation in high power applications, all the amplifiers that follow the modulator stage must be linear amplifiers High Level Modulation the modulation takes place in the final element of the final stage where the carrier is at its maximum amplitude Amplitude Modulation Low-Leve Modulator a small-signal, class A amplifier two inputs ( carrier and modulating signal) emitter modulation Amplitude Modulation Operation the modulating signal varies the gain of the amplifier at a sinusoidal rate equal to the frequency of the modulating signal the depth of modulation achieved is proportional to the amplitude of the modulating signal Amplitude Modulation Output Waveforms the amplitude of the output signal depends on the amplitude of the carrier and the voltage gain of the amplifier the coefficient of modulation depends entirely on the amplitude of the modulating signal Amplitude Modulation For a low-level AM Modulator similar to the one shown, with a modulation coefficient m = 0.8, a quiescent voltage gain Aq = 100, an input carrier frequency fc=500 kHz with an amplitude Vc= 5mV and 1000 Hz modulating signal, determine: Max and min voltage gains Max and min amplitudes for Vout Sketch the output AM envelope Amplitude Modulation Simplified Medium-Power Transistor AM DSBFC Modulator the modulation takes place in the collector operates in Class C collector modulator RFC – acts as a short to dc and an open to high frequencies Amplitude Modulation Operation: Amplitude Modulation Medium Power Transistor AM DBSBFC Modulator Amplitude Modulation Operation the positive half-cycle of the output waveform is produced in the tank circuit by the flywheel effect the resonant frequency of the tank circuit is equal to the carrier frequency and the bandwidth extends from fc-fm to fc + fm the modulating signal, the harmonics, and all the higher- order cross-products are removed from the waveform, leaving a symmetrical AM DSBFC wave one hundred percent modulation occurs when the peak amplitude of the modulating signal equals VCC Amplitude Modulation Circuit Components R1 is the bias resistor for Q1 R1 and C2 – form a clamper circuit C3 – bypass capacitor that looks like a short to the modulating signal frequencies Cbc – base to collector junction capacitance of Q1 CN – a neutralizing capacitor to provide a feedback path for a signal that is equal in amplitude and frequency but is 180 out of phase with the signal fed back through Cbc C4 – RF bypass capacitor Amplitude Modulation Simultaneous Base and Collector Modulation to achieve symmetrical modulation, operate at maximum efficiency, develop a high output power, and require as little modulating signal drive power as possible, base and collector modulations are sometimes used simultaneously Amplitude Modulation Linear Integrated-Circuit AM Modulators use a unique arrangement of transistors and FETs to perform signal multiplication precisely match current flow, amplifier voltage gain, and temperature variations offer excellent frequency stability, symmetrical modulation characteristics, circuit miniaturization, fewer components, temperature immunity, and simplicity of design ang troubleshooting disadvantages include low output power, a relatively low usable frequency range and susceptibility to fluctuations in the dc power supply Amplitude Modulation XR-2206 Monolithic Function Generator VCO, an analog multiplier and sine-shaper, a unity gain buffer and a set of current switches the VCO frequency of Oscillation fc is determined by the external timing capacitor (C1) between pins 5 and 6 and by timing resistor (R1) connected either pin 7 or 8 and ground Amplitude Modulation Linear Integrated-Circuit AM Modulator Amplitude Modulation AM Transmitters Low Level Transmitters for low-power, low-capacity systems, such as wireless intercoms, remote-control units, pagers, and short-range walkie-talkies Amplitude Modulation Low-Level Transmitter For voice or music transmission – source of modulating signal is generally an acoustical transducer Preamplifier – typically a sensitive, class A linear voltage amplifier with a high input impedance Modulating Signal Driver – linear amplifier that simply amplifies the information signal to an adequate level to sufficiently drive the modulator RF Carrier Oscillator – crystal –controlled oscillators are the most common circuits used Buffer Amplifier – a low-gain, high-input impedance linear amplifier Modulator – either emitter or collector modulation Intermediate and Final Power Amplifiers – either linear class A or class B push-pull modulators Antenna Coupling Network – matches the output impedance of the final power amplifier to the transmission line and antenna Amplitude Modulation High-Level Transmitters modulator : provides the circuitry necessary for modulation to occur , it is the final power amplifier, and it s a frequency up-converter Amplitude Modulation Trapezoidal Patterns used for observing the modulation characteristics of AM transmitters Amplitude Modulation Carrier Shift sometimes called upward or downward modulation a form of amplitude distortion introduced when the positive and negative alternations in the AM modulated signal are not equal an indication of the average voltage of an AM modulated signal Amplitude Modulation AM Envelopes Produced by Complex Non-sinusoidal Signals the shape of the envelope resembles the shape of the original modulating waveform