Amplitude Modulation and Superheterodynes The balanced modulator, superheterodynes, the autodyne converter, automatic gain control, amplitude and frequency modulation, and function generator chips Modulation We briefly encountered the balanced modulator as a phase detector for phase-locked loops.
Increasing the modulating signal beyond that point, known as overmodulationcauses a standard AM modulator see below to fail, as the negative excursions of the wave envelope cannot become less than zero, resulting in distortion "clipping" of the received modulation.
Such circuits are sometimes referred to as a vogad. In that case, negative excursions beyond zero entail a reversal of the carrier phase, as shown in the third waveform below. This cannot be produced using the efficient high-level output stage modulation techniques see below which are widely used especially in high power broadcast transmitters.
Rather, a special modulator produces such a waveform at a low level followed by a linear amplifier. Rather, synchronous detection is required. Such systems more often attempt a radical reduction of the carrier level compared to the sidebands where the useful information is present to the point of double-sideband suppressed-carrier transmission where the carrier is ideally reduced to zero.
In all such cases the term "modulation index" loses its value as it refers to the ratio of the modulation amplitude to a rather small or zero remaining carrier amplitude. In the diagram, the unmodulated carrier has an amplitude of 1. Modulation methods[ edit ] Anode plate modulation.
The resistor R1 sets the grid bias; both the input and output are tuned circuits with inductive coupling. Modulation circuit designs may be classified as low- or high-level depending on whether they modulate in a low-power domain—followed by amplification for transmission—or in the high-power domain of the transmitted signal.
Calculated digital samples are converted to voltages with a digital-to-analog convertertypically at a frequency less than the desired RF-output frequency. The analog signal must then be shifted in frequency and linearly amplified to the desired frequency and power level linear amplification must be used to prevent modulation distortion.
High-level generation[ edit ] High-power AM transmitters such as those used for AM broadcasting are based on high-efficiency class-D and class-E power amplifier stages, modulated by varying the supply voltage.
The following types are for vacuum tube transmitters but similar options are available with transistors: In plate modulation, the plate voltage of the RF amplifier is modulated with the audio signal.
The audio power requirement is 50 percent of the RF-carrier power. The AM modulation tube plate is fed through the same inductor, so the modulator tube diverts current from the RF amplifier.
The choke acts as a constant current source in the audio range. This system has a low power efficiency. The operating bias and gain of the final RF amplifier can be controlled by varying the voltage of the control grid. This method requires little audio power, but care must be taken to reduce distortion.
Clamp tube screen grid modulation: It is difficult to approach percent modulation while maintaining low distortion with this system. One tube provides the power under carrier conditions and another operates only for positive modulation peaks.
Overall efficiency is good, and distortion is low. Two tubes are operated in parallel, but partially out of phase with each other. As they are differentially phase modulated their combined amplitude is greater or smaller.
Efficiency is good and distortion low when properly adjusted. A highly efficient high voltage power supply is applied to the tube plate. The output voltage of this supply is varied at an audio rate to follow the program.
This system was pioneered by Hilmer Swanson and has a number of variations, all of which achieve high efficiency and sound quality.
Demodulation methods[ edit ] The simplest form of AM demodulator consists of a diode which is configured to act as envelope detector. Another type of demodulator, the product detectorcan provide better-quality demodulation with additional circuit complexity.Definition of Amplitude Modulation.
The amplitude modulation is the process of transmitting the information signal by superimposing it on the high-frequency wave called carrier wave. The information signal can be of any type based on the type of information it is carrying such as voice, data etc. When the Carrier also has a frequency of Hz (a moderately deep bass note), the three frequencies produced by Amplitude Modulation lie at 0Hz, Hz, and Hz.
These are, of course, the Difference, the Carrier, and the Sum signals. AM (or Amplitude Modulation) and FM (or Frequency Modulation) are ways of broadcasting radio signals.
Both transmit the information in the form of electromagnetic waves. Both transmit the information in the form of electromagnetic waves. Now, this is the point at which things get a little weird, because for any given Modulator frequency, it is the Modulation Index (and, therefore, the amplitude of the Modulator) that determines the amplitude of each of the components in .
AM (or Amplitude Modulation) and FM (or Frequency Modulation) are ways of broadcasting radio signals. Both transmit the information in the form of electromagnetic waves. Both transmit the information in the form of electromagnetic waves. The modulation index is about 3, making the peak frequency deviation about 15 Hz.
That means the frequency will vary somewhere between 15 and 45 Hz. How fast the cycle is completed is a function of the modulating frequency.