detection


Also found in: Dictionary, Thesaurus, Medical, Legal, Wikipedia.
Related to detection: Detection limit, Edge detection

detection

Physics the act or process of extracting information, esp at audio or video frequencies, from an electromagnetic wave

Detection

 

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.

Figure 1. (a) Oscillations with constant amplitude at detector input, (b) current pulses l of identical amplitude at detector output. The detector records the constant component of the current.

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,

Figure 2. (a) Oscillations with amplitude modulation at detector input, (b) current pulses at detector output. The detector records low-frequency alternating current (lower broken line).

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.

Figure 3. (a) Amplitude-modulated oscillation at detector input, (b) current-voltage characteristic of detector, (c) current oscillations at detector output

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.

REFERENCES

Strelkov, 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

detection

[di′tek·shən]
(communications)
The recovery of information from an electrical or electromagnetic signal.
References in classic literature ?
I disregarded both his calls and his threats, and made my way to the woods as fast as my feeble state would allow; and thinking I might be over- hauled by him if I kept the road, I walked through the woods, keeping far enough from the road to avoid detection, and near enough to prevent losing my way.
Detection could not be in your power, and suspicion certainly not in your inclination.
The detection of this blunder in the two veterans, who prided themselves on the sureness and quickness of their sight, produced a hearty laugh at their expense, and put an end to their vauntings.
She did not think that Julia would be able to penetrate his disguise, as he had declared his intentions so to conceal himself, by paint and artifice, as to be able to escape detection.
May not the unworthiness or incapacity of those who assume apostolic functions upon the remote islands of the sea more easily escape detection by the world at large than if it were displayed in the heart of a city?
said Edwards; “this drunken fellow will cause our detection, and we have not a moment to spare.
It was over, and she had escaped without reproaches and without detection.
For although I am safe from detection, although every proof against me is destroyed, I suppose, Gertrude .
To cut another's roding is crime unspeakable on the Banks; yet it was done, and done without detection, three or four times that day.
The detection of types is one of the most elementary branches of knowledge to the special expert in crime, though I confess that once when I was very young I confused the Leeds Mercury with the Western Morning News.
He could not get help from outside, even if he had some one whom he could trust, without the unbarring of doors and considerable risk of detection.
To involve her in the danger of a second detection, to court her into an apartment which must wring her heart, could not be the office of a friend.

Full browser ?