Page 281 - Radar Technology Encyclopedia

271 modulation, frequency modulator, active-switch

initial phases are omitted, the signal can be written in the Ref.: Terman (1955), p. 592; Popov (1980), p. 451.
form Polarization modulation is intrapulse modulation whereby
ut () A [ ( sin w t ) m sin(+ Wt ) ]
=
0 0 r the waveform polarization changes in time in accordance
with the preset law. Such a technique is more complex than
where A is the carrier amplitude, w is the carrier frequency;
0 0
m is the index of modulation, and W is the modulation fre- frequency or phase modulation, but it is claimed that it can
improve by a significant amount the radar jamming immu-
quency.
nity. SAL
The width of the frequency span, Dw = 2pD, is termed
f
frequency deviation, is proportional to the amplitude of the Ref.: Leonov (1988), p. 156.
modulating signal, and does not depend on W. Since it is (Modulation) sidebands are the components of the spectrum
impossible to implement continuous variation of the transmit- of a modulated signal, typically located symmetrically rela-
ter frequency, in practice the frequency can change periodi- tive to the carrier frequency, and differing from the carrier
cally relative to a specified level, w = 2pf . One of the
0 0 frequency by the value equal to the modulation frequency, or
simplest is the triangular frequency variation with a modula- a multiple thereof. For example, if the amplitude of the signal
tion period t (Fig. M13b). The spectrum of the FM signal
r with the carrier frequency w is modulated by the sinusoidal
0
consists of an infinite number of sidebands, which differ from oscillation with frequency W, (W w ), then the output

<<
0
carrier frequency by nW, where n is an integer. The amplitude spectrum, besides the component of carrier frequency w will
0
of the nth sideband is equal to A = J (m)·A , where J (m) is
n n 0 n have sidebands components of w ± W. AIL
0
the Bessel function of the first kind of nth order with argu- Ref.: Terman (1955), p. 523; Popov (1980), p. 54.
ment m (Fig. M13c). When m >> 1, the width of the spectrum
MODULATOR. A modulator is the device that imposes
is approximately equal to w = 2Dw. AIL
e
modulation on the carrier. In radar applications it is used to
Ref.: Terman (1955), Ch. 17; Vinitskiy (1961), p. 184.
modulate the transmitted waveform (see pulse modulator,
Frequency-modulation-by-noise is frequency modulation
frequency modulator) and in receiver circuits to transform
with random noise, as used in ECM to jam victim radars,
the electrical signal into acoustical or optical form for signal
especially those using AM and fixed-tuned FM receivers.
processing. (See acousto-optical modulator, space-time
SAL
light modulator.) SAL
Ref.: Johnston (1979), p. 60.
An acousto-optical modulator is based on the effect of dif-
The modulation index is the frequency modulation parame- fraction of light on acoustic waves. It consists of a light con-
ter equal to the ratio of the frequency deviation Df to modula- ductor made from lithium niobate or gallium phosphide in the
tion frequency f . The usual notation is m . AIL microwave band, a piezo-converter of an electrical signal to
m
r
Ref.: Terman (1955), p. 588; Popov (1980), an acoustic wave, an acoustic absorber, as well as an excita-
Intensity modulation is “a process used in certain displays tion circuit of the piezo-converter to ensure stability.
whereby the luminance of the signal indication is the function Acousto-optical modulators use various diffraction
of the received signal strength.” SAL modes: Bragg diffraction (see BRAGG effect), Raman-Nath,
and intermediate. Based on the type of acoustic and optical
Ref.: IEEE (1990), p. 19.
waves, we distinguish among modulators of three-dimen-
Intrapulse modulation is modulation of any pulse parameter
sional acoustic, surface acoustic (for reflection and transmis-
(frequency, phase, or polarization) in accordance with a preset
sion) optical waves of infrared, visible-light, and ultraviolet
law. The most widely used types are frequency modulation
bands.
and biphase modulation (shift keying) that widen the signal
In the microwave band, the use of acousto-optical modu-
spectrum and provide for pulse compression. SAL
lators is limited to a frequency on the order of 3 GHz. The fre-
Phase modulation causes the phase and carrier frequency to quency band is 20 to 30%, and the value of the time aperture
vary in accordance with the modulating signal. If the carrier is equal to the ratio of the aperture of the modulator to the
)
t
phase as the function of time is f = f(en the instanta- velocity of the acoustic wave, to units of microseconds. The
,
th
neous value of the frequency in the moment t is w(t) = df/t, basic advantage of acousto-optical modulators is the capabil-
d
and the complete variation of the phase during period t is ity of inputting a radio signal into an optical system in real
r
time, which establishes its basic use in acousto-optical
t r
devices for processing complex radar signals. The shortcom-
f = ò w t () td ings of these modulators includes the short memory time,
0 which limits the duration of the processed signals to around
There is a linkage between phase and frequency modula- 100 ms, and the comparatively low number of channels
tion, since time-dependent frequency variation w(t) is equiva- (around 100). IAM
lent to phase variation as the integral of w(t), and time- Ref.: Kulikov (1989), p. 5; Lukoshkin (1983), p. 261; Zmuda (1994), p. 215.
dependent variation f(t) is equivalent to frequency variation
An active-switch modulator is one that uses active switches
as the derivative of f(t). AIL
for controlling both the leading and trailing edges of a pulse.