Page 284 - Radar Technology Encyclopedia

modulator, pulse MONOPINCH 274

which is partially or fully discharged through it. Based on the with a time signal. Various physical effects are used for this,
type of storage element, we distinguish among modulators and determine the basic types of modulators.
with partial discharge, modulators with complete discharge, Based on method of writing information, space-time
linear modulators with pulse formation in the storage line cir- light modulators are divided into three classes: electron-beam
cuit, and magnetic modulators that use a storage device with addressing, optical beam addressing, or electric signal
nonlinear elements. At the end of the pulse, the discharge cir- addressing (see acousto-optical modulator). The first class
cuit is opened and recharging of the storage element begins. includes modulators based on electro-optical crystals (lithium
The shape and width of the pulse are determined both by the niobate, etc.), thermoplastic media, and dielectric oil film.
circuit of the storage device and the linear modulator, and by Their action is based on deformation of the surface of the
the controlled switch in active-switch modulators. modulator through electrical interaction of the charges.
Pulse modulator switches are electronic devices for start- The second class includes modulators based on liquid
ing and stopping discharge of an energy storage device. In crystals and photoconducting electro-optical media.
linear modulators, the switches do not control the shape and For description of the properties of modulators, the fol-
duration of the pulse, but only initiate and maintain the dis- lowing characteristics are used; sensitivity, resolution, con-
charge. Active modulators control timing of both the leading trast, dynamic range, linearity, operating wavelength,
and trailing edges of the pulse. Hydrogen thyratrons, mercury presence of memory, speed (cycling type), and a number of
ignitrons, thyristors as well as preignition dischargers in older others.
types of modulators are used as switches for linear modula- For processing of pulse-compression waveforms in real
tors. Special types of vacuum tubes and thyristors are used as time, the acousto-optical modulator is the only one possible.
active switches. Modulators of other classes are used in opto-electronic
Hydrogen thyratrons have peak powers up to 100 MW, devices as references for processing of radar signals. IAM
average powers of 200 kW, and operating lives of 500 to Ref.: Voskresenskiy (1986), pp. 17, 24; Lukoshkin (1983), p. 259; Zmuda
5,000 hours. The tubes are kept supplied with hydrogen by a (1994), Ch. 6.
reservoir with an independent heater. Their power limitation A thyristor-magnetic modulator is a synchronized mag-
is the heating of the anode. The ignition delay after delivery netic modulator supplied by a dc voltage source with thyris-
of the trigger pulse is in the order of tens of nanoseconds, and tors at the input circuit and first compression stage. The use of
depends on various factors. For better timing stabilization the thyristors in the input circuit makes it possible to obtain out-
thyratron may contain a priming grid, which initiates a low- put power stability with variations of input voltage, choice of
power discharge before the pulsing of the main control grid. pulse repetition frequency independent of the frequency of
Ignitrons are marked by high peak and average powers the supply line, and control of the repetition frequency with
but have limited use due to the complexity of the triggering an external trigger. However the output pulse is delayed sig-
and controlling devices, and the restrictions on the pulse repe- nificantly from the trigger pulse, and this lag changes by sev-
tition frequency to a value of 500 Hz due to the slow deion- eral percent in the process of warm-up and with fluctuations
ization of the mercury vapors. in temperature. These changes are much less than in a purely
Thyristors are similar in parameters to hydrogen thyra- magnetic modulator, but too great for use of these modulators
trons. They are characterized by their peak voltage for moving-target indicator radars.
4
(»1,000V), speed of current buildup (up to 10 A/s). Their Owing to the control of the pulse repetition frequency,
operating life with current operation is practically unlimited. and the capability of obtaining high (hundreds of microsec-
To obtain high power, connection of several thyristors is onds) and low (units of microseconds) values of pulse dura-
widely used. The basic drawback of thyristors is their sensi- tion, thyristor-magnetic modulators are more widely used
tivity to overloads, even instantaneous ones. than magnetic ones. IAM
Vacuum-tubes of active switches are created for nominal
Ref.: Skolnik (1970), p. 7.70; Rakov (1970), vol. 2, p. 104.
voltages up to 260 kV and peak powers up to 30 MW. The
drawbacks include high-voltage breakdowns and a high level MONOPINCH is “a single axis monopulse used in search
of x-rays requiring protective measures for operating radars to provide effective beam narrowing by displacing the
personnel. displayed azimuth by the target angle off axis.” It uses
A comparison of the main types of pulse modulators is monopulse difference-channel information to narrow the
given in Table M4. IAM angular width of the target echo on a display, giving the
appearance of better angular resolution. A voltage taken from
Ref.: Skolnik (1970), pp. 7.72–7.76; (1990), p. 4.32; Davidov (1984), p. 40.
each target pulse in the difference channel is added to the dis-
A space-time light modulator performs space-time modula-
play deflection voltage in such a way as to move the intensi-
tion of the phase (amplitude) of a coherent light wave in
fied blip toward the actual target angle, superimposing all
accordance with the change in parameters of the controlling
target echoes onto a narrow angle cell. It has also been called
signal. The operating principle is based on the change in the
an electrical correction system (ECS) and electronic beam
spatial distribution of the transmission factor in accordance
sharpening (EBS). DKB
Ref.: IEEE (1990), p. 21; Rhodes (1959), p. 60.