Amplitude modulator
A device for moving the frequency of an information signal, which is generally at baseband (such as an audio or instrumentation signal), to a higher frequency, by varying the amplitude of a mediating (carrier) signal. The motivation to modulate may be to shift the signal of interest from a frequency band (for example, the baseband, corresponding to zero frequency or dc) where electrical disturbances exist to another frequency band where the information signal will be subject to less electrical interference; to isolate the signal from shifts in the dc value, due to bias shifts with temperature or time of the characteristics of amplifiers, or other electronic circuits; or to prepare the information signal for transmission.
Familiar applications of amplitude modulators are standard-broadcast or amplitude-modulation (AM) radio; data modems (modem = modulator + demodulator); and remote sensing, where the information signal detected by a remote sensor is modulated by the remote signal-conditioning circuitry for transmission to an electrically quiet data-processing location. See Modem
The primary divisions among amplitude modulators are double sideband (DSB); single sideband (SSB); vestigial sideband (VSB); and quadrature amplitude (QAM), where two DSB signals share the same frequency and time slots simultaneously. Each of these schemes can be additionally tagged as suppressed carrier (SC) or transmitted carrier (TC).
To amplitude-modulate an information signal is (in its simplest form) to multiply it by a second signal, known as the carrier signal (because it then carries the information). A real (as opposed to complex) information signal at baseband, or one whose spectrum is centered about zero frequency, has an amplitude spectrum which is symmetric (an even function; illus. a) and a phase spectrum which is asymmetric (an odd function; illus. b) in frequency about zero. Because of this symmetry, the information in the upper or positive sideband (positive frequencies) replicates the information in the lower or negative sideband (negative frequencies). Balanced modulation or linear multiplication (that is, four-quadrant multiplication where each of the two inputs is free to take on positive or negative values without disturbing the validity of the product) of this information signal by a sinusoidal carrier of single frequency, fc, moves the information signal from baseband and replicates it about the carrier frequencies fc and -fc (illus. c).
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Linear multiplication (described above) produces double-sideband suppressed-carrier (DSSC or DSBSC) modulation of the carrier by the information. One of the redundant sidebands of information may be eliminated by filtering (which can be difficult and costly to do adequately) or by phase cancellation (which is usually a much more reasonable process) to produce the more efficient single-sideband suppressed-carrier (SSBSC or SSB) modulation (illus. d, e). The process of only partially removing the redundant sideband (usually by deliberately imperfect filtering) and leaving only a vestige of it is called vestigial-sideband (VSB) modulation (illus. f).
Receivers demand a carrier-signal reference to properly demodulate (detect) the transmitted signal. This reference may be provided in one of three ways: it may be transmitted with the modulated signal, for example, in double-sideband transmitted-carrier (DSTC or DSBTC) modulation, which is the method used for standard-broadcast AM radio; transmitted on a separate channel (often the case for instrumentation systems); or generated at the receiver.
DSSC modulation, no matter how it is disguised, is just ordinary (linear) multiplication, or a reasonable approximation to that multiplication. Furthermore, a DSTC signal can be modeled as a DSSC signal with the carrier added. Any amplitude modulator is therefore simply some sort of embodiment of a linear multiplier with or without a means to add the carrier.
High-level DSTC modulation is usually done by varying the power-supply voltage to the final high-power amplifier in the radio-frequency transmitter by summing the information signal with the dc output voltage of the power supply. Low-level DSTC modulation is carried out by integrated circuits that approximate the multiplications, followed by a linear amplifier. See Amplifier
Because of the falling costs and improving performance of digital devices, signals are now generally represented by digital data, that is, signal values which are sampled in time and encoded as numbers to represent their sampled values. Digital signal-processing (DSP) functions are carried out by some sequence of addition (or subtraction), multiplication, and delays. Efficient mechanization of each of these functions has been the subject of considerable effort. Digitally, waveform generation and linear multiplication are highly optimized processes. Advances in integrated-circuit fabrication techniques have so lowered the cost of digital circuits for modulation that the overwhelming majority of amplitude modulators in use are now digital. Custom application-specific integrated circuits (ASICs), general-purpose programmable DSP devices, and customizable arrays such as programmable logic arrays (PLAs) and field-programmable arrays (FPAs) are all in widespread use as amplitude modulators and demodulators. Digital modems as data transmission equipment dominate the production of amplitude modulators. See Integrated circuits, Modulator
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