Page 300 - Radar Technology Encyclopedia

NULLING oscillation, parametric 290

NULL AXIS (see AXIS, antenna). with radar display, an operator detects the targets and selects
which of them are important for subsequent processing. There
NULLING
are some specific requirements depending on the tasks per-
Feed-through nulling is a technique used in CW radars to formed by radar, the features of its structure, and conditions
null out the transmitted signal leaking into the receiver. It of work, which require special knowledge and training. (See
uses negative feedback at the receiver input to cancel what- TRAINER, radar.) AIL
ever transmitter signal may appear there. It is arranged to can- Ref.: Skolnik (1962), pp. 439, 566; Romanov (1980), pp. 5-24.
cel only the signal carrier, as signals offset from the carrier
OSCILLATION. Oscillation is “the variation, usually with
include the desired signals with doppler shift. SAL
time, of the magnitude of a quantity with respect to a speci-
Ref.: Skolnik (1990), p. 4.3; Brookner (1988), p. 58.
fied reference when a magnitude is alternately greater and
Nulling within the main lobe (see NOTCHER, mainlobe). smaller than the reference.” In radar applications, this term is
typically applied to the variation of an electromagnetic wave.
Nulling within the sidelobes (see CANCELLATION, side- SAL
lobe).
Ref.: IEEE (1993), p. 896.
NYQUIST Coherent oscillation is when two (or more) oscillations have
a phase difference between them which does not change over
The Nyquist formula [theorem] describes the dependence of
time in interval T. (See COHERENCE.) AIL
the mean-square value of the thermal noise voltage v in an
n
active resistance R, on the value of the absolute temperature T Ref.: Popov (1980), p. 178.
and the frequency bandwidth B, as A damped oscillation is a natural oscillation with amplitude
continuously decreasing based on an exponential law, due to
v = 4kTRB (V)
n
loss of energy in an oscillatory system. AIL
where k is Boltzmann’s constant. The available noise power
Ref.: Popov (1980), p. 131.
output from the resistor is 1/4 this value. IAM
Forced oscillation arises in an isolated system driven exter-
Ref.: Rakov (1970), vol. 2, p. 110; Skolnik (1990), p. 2.26.
nally. During forced oscillations, the oscillatory process
Nyquist rate (see SAMPLING). depends on both the nature of the external force and the prop-
erties of the system. AIL
Nyquist sampling theorem (see SAMPLING).
Ref.: Popov (1980), p. 75.
harmonic oscillation (see sinusoidal oscillation).
O Modulated oscillation results from the interaction of two
oscillations, modulated and modulating. A modulated oscilla-
OPERATOR, radar. A radar operator is the person who per- tion is an oscillation serving as a carrier for a modulating
forms two basic functions in the process of radar service: to oscillation. A sinusoidal oscillation or a periodic square pulse
ensure proper operation of the radar through its maintenance train is used as the modulated oscillation. A modulating oscil-
and to ensure that the radar performs its main task such as lation is one containing transmitted information and used to
detection, tracking, target recognition, and so forth. if the per- change a certain parameter (depending upon modulation
formed task requires operator involvement. The degree of type) of a modulated oscillation. (See MODULATION.) AIL
operator involvement depends on the concept of radar opera- Ref.: Popov (1980), p. 235.
tion. In automatic radar, an operator does not interfere in the
Monochromatic oscillation has amplitude, frequency, and
process of operation, but only controls proper operation
phase that do not depend on time. AIL
through the radar monitoring system and eliminates the fail-
Ref.: Popov (1980), p. 238.
ure if it had occurred. In automated radars, an operator is the
important link in the process of operation (Fig. O1). Working Natural oscillation arises in an isolated system as a result of
an initial external effect delivering energy for the entire pro-
cess. Such an effect might be connection of voltage or open-
ing (closing) a section of a circuit. Natural oscillations arise at
the resonant frequency of a system, the value of which is
determined by system parameters. AIL
Ref.: Popov (1980), p. 402.
Parametric oscillation arises under the effect of an external
periodic force that transmits energy to a system through a
change in a certain system parameter (usually capacitance or
inductance). AIL
Figure O1 Block diagram of automated radar data processing.
Ref.: Popov (1980), p. 276.