Page 139 - Radar Technology Encyclopedia

detector, phase device, charge-coupled (CCD) 129

ate frequency). When digital doppler filters are used for An acoustic-wave device uses the propagation of acoustic
clutter rejection prior to extracting amplitude and phase of the waves in special materials to produce the effect of signal
target return, the fully digital phase detector extracts the amplification, processing, or delay. The primary advantage of
phase information from the in-phase and quadrature compo- acoustic waves is a relatively low velocity, typically 10 - 5
nents of input digital codes, and the traditional (analog) phase times that of the electromagnetic waves, which makes possi-
detectors discussed above are inapplicable. In this case, digi- ble relatively long signal-delay times in a physically small
tal I and Q data are converted into logarithmic format, the space. Both bulk-mode propagation and surface waves have
polarities of the I and Q signals define the quadrant, and the been employed in bulk acoustic-wave devices and surface-
2
2
phase within the quadrant is a function of log I - log Q . acoustic-wave (SAW) devices, respectively. The main range
2
2
SAL of applications for radar purposes is in signal processors and
Ref.: Skolnik (1990), pp. 3.32–3.38; Chistyakov (1986), p. 155. delay lines. SAL
square-law detector (see detector model). Ref.: Fink (1975), p. 9.72.
A bulk acoustic-wave device is an acoustic-wave device
A synchronous detector is an amplitude-phase detector in
employing propagation of bulk waves in the solid crystals,
which output signal polarity and amplitude depend upon the
primarily to produce amplification in acoustic amplifiers. SAL
phase difference between input and reference signals. A bal-
Ref.: Fink (1975), p. 13.77.
anced phase diode detector may be used as a synchronous
detector. The reference voltage (a local oscillator) must be A charge-coupled device (CCD) is based on the transfer of a
synchronized in phase with the input signal carrier. Voltage at charge in a semiconductor structure in which, owing to the
the detector output is maximum when the phase difference controlling potentials in the electrodes, a potential relief is
f= 0. When f = 90° the output voltage is zero, and when created in the form of a series of potential holes. The operat-
,
,
f=180° the output voltage polarity reverses. Thanks to the ing principle of the devices is based on the significant time
linear dependence of phase detector output voltage on input (from hundreds of milliseconds to tens of seconds) necessary
voltage, the detector may be used for detection of ampli- to fill the holes with secondary carriers, with the result that it
tude-modulated signals (amplitude-phase detection). Syn- can be viewed as a memory cell. Transfer of charge from one
chronous detectors were used in older types of radars to memory cell to an adjacent one is effected with delivery of a
implement moving target indication. They are also called more negative voltage to the corresponding electrode. Ini-
phase-sensitive [discriminating] detectors. AIL tially devices with charge transfers were produced in two
Ref.: Chistyakov (1986), p. 157; Skolnik (1990), p. 3.32. types: “bucket brigades” and CCDs. In bucket brigades, diffu-
sion p-regions are created in the semiconductor between elec-
A video detector is an amplitude detector designed for detec-
trodes located at a greater distance than in charge-coupled
tion of pulse waveforms. Such detectors are divided into two
devices. Under the action of control voltages in the elec-
types: the pulse detector and the peak detector. The first is for
trodes, packets of charges are successively moved along a
conversion of RF pulses into direct current pulses (i.e., for
chain from one region to another. Because of the difficulty
reproduction of the envelope of each pulse). A peak detector
of forming diffusion regions, bucket brigades are practically
is for extraction of the envelope of the entire pulse train.
never used now.
Since, during envelope extraction, the output voltage at each
A basic element of charge-coupled devices is the metal-
moment in time must be proportional to the amplitude (peak
oxide-semiconductor (MOS) capacitor, which consists of a
value) of the pulses, this is referred to as a peak detector. Peak
metal electrode and semiconductor substrate of the n- or p-
detection may occur by means of single or double detection.
type, separated from each other by a thin layer of this semi-
In single detection, RF pulses are supplied directly to a peak
conductor. The role of such a capacitor is frequently per-
detector, the most common type being the diode detector (see
formed by more sophisticated surface-charge transistors. The
amplitude detector). In double peak detection, RF pulses
capability of organizing movement of the charge package
initially are converted into video pulses by a pulse detector,
along the surface in any direction is a specific feature of oper-
are amplified, and reach the peak detector, which extracts the
ation of the CCD. If necessary, the charge can be read from
pulse train envelope. AIL
any intermediate electrode through a capacitor.
Ref.:Fink (1982), p. 20.81; Chistyakov (1986), p. 150.
CCDs are capable of operating at lower frequencies (to
The zero-crossing detector is a circuit that produces an out- 1 GHz) than other functional devices. They are usually used
put pulse when the input voltage passes through zero. It is as devices for discrete-analog processing of analog signals.
widely used in time (range) discriminators and other receiver They can also be used for digital processing of signals, com-
circuits. SAL bining the high precision of digital systems with the low
Ref.: Skolnik (1990), p. 10.18. power consumption and great functional capabilities of ana-
log devices. Analog CCD delay lines and CCD filters of all
DEVICE, microwave
types are widely used: band-pass, nonrecursive, transversal,
acoustic-surface-wave device (see surface-acoustic-wave recursive, matched, and so forth. IAM
device). Ref.: Gassanov (1988), p. 227; Galati (1993), p. 516.