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the conversion of electrical oscillations, as a result of which lower-frequency oscillations or a direct current is produced. The most widespread case of detection —demodulation—consists in the isolation of a low-frequency modulating signal from modulated high-frequency oscillations. Detection is used in radio receivers to isolate audiofrequency oscillations and in television to isolate image signals.
In the simplest case, an amplitude-modulated oscillation is the aggregate of three high frequencies ω, ω + Ω, and ω - Ω, where ω is a high carrier frequency and Ω is a low modulation frequency. Since there is no signal of frequency Ω in a modulated oscillation, detection must entail frequency conversion. Electrical oscillations are fed to a device (a detector) that passes current only in one direction. In the process the oscillations are converted into a number of current pulses of the same sign. If the amplitude of the detected oscillations is constant, then the current pulses at the detector output have constant amplitude (Figure 1). If the amplitude of the oscillations at the detector input changes, the amplitude of the cur-rent pulses becomes different. Here the pulse envelope con-forms to the law governing the change in the amplitude of modulated oscillations fed to the detector (Figure 2). If the oscillations are only partially rectified—that is, if the current flows through the detector in both directions but the electric conductivity of the detector is different—detection also takes place. Thus, any device having different electric conductivity in different directions, such as a diode, may be used for detection. The frequency spectrum of the current passing through the diode is much richer than the spectrum of the initial modulated oscillation. It contains a constant component, an oscillation of frequency Ω, as well as components having frequencies ω, 2ω, 3ω, and so on. To isolate a signal of frequency Ω the diode current passes through a linear filter that has high resistance at frequency Ω and low resistance at frequencies ω, 2ω, and so on. The simplest filter consists of a resistor R and a capacitor C whose values are defined by the conditions ωRC > > 1 and ΩRC < < 1. The voltage at the output of the filter has frequency Ω and an amplitude proportional to the depth of modulation of the input high-frequency oscillation.
The detector examined above, which has a piecewise linear relationship between the current and the voltage (Figure 3,b), is called a linear detector and reproduces virtually without distortion the low-frequency oscillation Ω used to modulate the input signal (Figure 3,c). Much greater distortions are produced in a square-law detector,
will induce a current through a detector whose spectrum contains the frequencies Ω, 2Ω, ω - Ω, ω, ω + Ω, 2ω - Ω, 2ω + Ω, and so on, A linear filter easily screens all frequencies beginning with the third, but an oscillation of frequency 2O is weakly attenuated by the filter and is “noise” that distorts the signal Ω. It can be eliminated only when the modulation depth is small, since the amplitude of a current of frequency 2Ω is proportional to the square of the modulation depth of the input signal.
A given diode may function either as a square-law or linear detector, depending on the magnitude of the signal fed to it. For a weak signal the diode curve is square, but for a strong signal the curve may be considered “piecewise linear.” Thus, for detectors having low distortion it is desirable to feed a rather strong signal to the detector.
The nonlinearity of the dependence of current on voltage in vacuum and semiconductor diodes (diode detection), the nonlinearity of the curve of the grid-cathode section of a vacuum triode (grid detection), and the nonlinearity of the dependence of the anode current of a triode on the voltage at the grid (plate detection) are used for detection. In all cases the very process of detection reduces to diode detection; it is accompanied by signal amplification in the triode only in grid and plate detectors. Detection is also possible in the optical range, where it is accomplished by means of photocells (photoelectric cells, photoamplifiers, photodiodes, and so on) or nonlinear crystals.
REFERENCESStrelkov, S. P. Vvedenie v teoriiu kolebanii, 2nd ed. Moscow, 1964.
Siforov, V. I. Radiopriemnye ustroistva, 5th ed. Moscow, 1954. Chapter 6.
Gutkin, L. S. Preobrazovanie sverkhvysokikh chastot i detektirovanie. Moscow-Leningrad, 1953.
V. N. PARYGIN