Page 208 - Radar Technology Encyclopedia

frequency agility frequency, critical (ionospheric) 198

to spread the available energy across a wider bandwidth, well as other parameters commonly used to describe tuning
thereby decreasing the jamming energy density (W/Hz). PCH systems.
Ref.: Skolnik (1990), p. 9.17. Automatic frequency control using frequency and phase
uses two feedback loops, providing both frequency and phase
Angular frequency is frequency expressed in radians per
tuning. An automatic frequency-phase control system con-
second, equal to the frequency in hertz multiplied by 2p. PCH
tains both a frequency detector and a phase detector. In com-
Ref.: Van Nostrand (1983), p. 1,276.
parison with a phase-locked loop, a frequency-phase loop is
Automatic frequency control (AFC) is the automatic main- able to provide a wide capture band, the width of which is
tenance of signal frequencies within the various stages of a determined by the system’s frequency channel. The width of
receiver. Absolute AFC maintains a constant absolute fre- the lock-on band is determined by the phase channel.
quency, while difference AFC maintains a constant difference Automatic frequency control with a PLL is a form of
between the signal frequency and a local oscillator. An AFC automatic frequency control in which a phase detector is used
system comprises a discriminator which determines the mag- to determine the magnitude and sign of the frequency differ-
nitude and sign of required tuning adjustment, a filter that ence between an input signal and a reference. The reference
limits the bandwidth of the adjustment, and a controller to signal generated by a local reference oscillator is presented to
vary the frequency of the local oscillator. the controlling input of the phase detector. There is no resid-
AFC systems may be based on frequency or phase loops. ual mistuning in a phase-locked loop, as there is in a fre-
A frequency feedback system uses a frequency detector and quency-feedback AFC system. This makes phase-locked
has a steady-state residual mistuning. A phase-locked loop loops suitable for use in coherent radar receivers.
(PLL) uses a phase detector and provides exact tuning. The use of digital circuitry increases the accuracy, reli-
AFC systems in pulsed radars may also be categorized as ability, and stability and permits automatic transitions
either fast or slow, with the former providing tuning adjust- between the search and lock modes. IAM
ments in a time shorter than the pulse duration, while the lat- Ref.: Sokolov (1984), pp. 170–174; Skolnik (1980), p. 157; Neri (1991),
ter adjusts the tuning over several pulse repetition intervals. p. 142; Morris (1988), p. 257.
Another differentiating feature is the type of tuning con-
The beat frequency is formed when two ac signals of differ-
trol. An electronic control system uses a voltage-controlled
ent frequencies are combined to form a third signal whose
oscillator, klystron, traveling-wave tube, IMPATT diode
frequency is equal to the difference between the frequencies
oscillator, or Gunn-diode oscillator, and provides fast
of the two original signals, the “beat frequency.” Beat fre-
response but a narrow tuning band. The tuning loop or cavity
quencies can be produced by passing the sum of the two sig-
may also be adjusted mechanically, in which case the AFC
nals through a nonlinear circuit, such as a rectifier, or by
system provides a wide tuning band at the expense of slow
mixing the two signals, a process called heterodyning.
response. The oscillator may also be adjusted thermally, or
The doppler beat frequency is the frequency generated by
with a combination of electronic, mechanical, and thermal
mixing a doppler-shifted echo with the transmitted carrier fre-
means.
quency or other reference signal. In a coherent MTI or dop-
An AFC system may operate in two modes: search mode,
pler system, it is equal to the doppler frequency shift. In a
in which the local oscillator frequency is controlled by the
noncoherent MTI, the reference signal may originate in mov-
search program, and track (locked) mode. Not all AFC sys-
ing clutter, and the beat frequency will then be the difference
tems provide the search mode.
in doppler shifts of the target and the clutter, removing the
The performance of an AFC system is characterized by
first-order effect of radar platform velocity. PCH
the width of its lock-on band (the band within which the sys-
Ref.: Stimson (1983), p. 595.
tem responds to slow changes in the input frequency differ-
ence), the capture band (the band within which the system can Combination frequencies are the frequencies of mixer prod-
acquire the signal frequency, with rapidly-changing mistun- ucts defined by the expression
ing), and the duration of the tuning process. f = nf + mf 2
1
k
Automatic frequency control systems are implemented in where n, m are integers; f and f are the frequencies of inter-
2
1
analog or digital circuitry, or a combination, depending upon acting oscillations. For example, in a superheterodyne
the requirements for accuracy, rapid response and reliability. receiver, the difference combination frequency f - f , where
lo
s
Automatic frequency control with frequency feedback f is the signal frequency and f is the local oscillator fre-
s
lo
uses a frequency detector to determine the magnitude and quency, is typically used as an intermediate frequency.
sign of the frequency difference between an input signal and a Ref.: Skolnik (1970), p. 5.7; Popov (1980), p. 182.
reference. When operating in steady state, there is a residual The critical (ionospheric) frequency is the highest fre-
mistuning, which results in AFC being used mainly in non- quency that will be reflected, at vertical incidence, by the ion-
coherent pulsed radars. The performance of an AFC system is osphere. For electron density N per m 3
e
characterized by the autotuning coefficient, which is the ratio
of the initial frequency difference to the steady state value, as f = 9 N e
c