Page 251 - Radar Technology Encyclopedia

241 lobe switching local oscillator, frequency-multiplier STALO

simultaneous lobing (see MONOPULSE; TRACKING). tor. The former is marked by a pulse mode of operation of the
master oscillator, while the latter is a high-stability master
Lobe switching is an angle-tracking technique in which
oscillator in the continuous mode.
antenna lobes are switched sequentially between two posi-
A COHO is used usually as a second (and sometimes
tions offset from the tracking axis. The switched lobes may be
subsequent) radar local oscillator. IAM
used for both transmitting and receiving, or for receiving only
(see lobe-on-receive-only). DKB Ref.: Ridenour (1947), p. 632; Skolnik (1962), p. 117; Pereverzentsev
(1981), p. 271.
Ref.: Barton (1988), p. 420.
A crystal local oscillator uses crystal stabilization of fre-
time sidelobes (see SIDELOBE, range).
quency, implemented through the use of a crystal resonator.
LOCAL OSCILLATOR (LO). A local oscillator is one used The frequency of the generated oscillations coincides with the
in a superheterodyne receiver to convert the frequency of a frequency of the first (up to 30 MHz) or higher (up to 200
received signal to a given intermediate frequency, and some- MHz) mechanical harmonics of the crystal. To produce oscil-
times to generate oscillations necessary for matched filtering lations of the necessary range, several frequency multiplica-
of the received signals. The latter requires high phase stability tion stages are used with a common coefficient of
in the transmitting and receiving circuits of the radar. In a multiplication reaching 100 and more. (See frequency-multi-
superheterodyne radar receiver, one or more local oscillators plier STALO.) Frequency retuning of a crystal local oscilla-
is used. The first local oscillator makes it possible to tune the tion is usually done by changing the crystals. The frequency
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receiver without touching the intermediate frequency unit. In stability of a crystal local oscillator reaches 10 to 10 . IAM
a coherent system, the parameters of the first local oscillator Ref.: Pereverzentsev (1981), p. 242; Fink (1982), p. 7.32; Rakov (1970),
(stable local oscillator, or STALO) affect the result of signal vol. 2, p. 30.
processing more than the parameters of the transmitter. The A frequency-multiplier STALO is one in which all frequen-
last local oscillator (coherent local oscillator, or COHO) is cies, including the carrier, are obtained from a high-stable fre-
often used to introduce phase adjustments to compensate for quency generator (see OSCILLATOR, crystal) through
the change of phase of the transmitter, the motion of the radar multiplication. A local oscillator of this type is used to form
platform, and so forth. A radar with an amplifier-type trans- frequencies of the first and second local oscillators of the
mitter uses a single local oscillator, which also generates the radar (Fig. L11) and is called a frequency synthesizer. The fre-
carrier frequency of the radar with the required shift relative quency of the first local oscillator is obtained from that of the
to the frequency of the first local oscillator. In pulse transmit- second by a multiplication circuit that uses, for example, a
ters with self-excitation, automatic frequency control (AFC) charge-accumulation diode, which provides a multiplication
is used to maintain the required frequency shift of the carrier factor of 100 or more. The first multiplier creates a series of
frequency and the frequency of the first local oscillator. frequencies that are separated from one another by the fre-
The requirements on stability of the first local oscillator quency of the reference oscillator and are symmetrical with
are usually defined in the form of acceptable spectrum of respect to the frequency of the second local oscillator.
phase modulation. Klystron resonator-stabilized local oscilla-
tors with electronic stabilization are widely used as local
oscillators. The use of a frequency-multiplier circuit, espe-
cially one with low levels of frequency multiplication, pro-
vides the lowest level of near-in sidebands and is
recommended for stabilization of klystron oscillators of non-
coherent and pseudocoherent radars. A local oscillator is a
receiving device, but in truly coherent systems it is also part
of the transmitter. IAM
Ref.: Skolnik (1970), pp. 5.12–5.19.
A coherent local oscillator (COHO) is one whose frequency
and phase of oscillations are strictly associated with the RF
oscillations of the transmitter. Phasing of a coherent local
oscillator is done at an intermediate frequency at the start of
each repetition period of the transmitted pulses during the Figure L11 Local oscillator in frequency multiplication circuit
time of their emission. After the conclusion of the synchroni- (from Skolnik, 1970, Fig. 11, p. 5-15, reprinted by permission
zation pulses, the local oscillator must operate in the mode of of McGraw-Hill).
free continuous oscillations at least for the time correspond-
Control signals arriving from the tracking system con-
ing to the maximum range of the radar. A COHO includes a
nect the signal of the frequency that most closely adjusts the
master oscillator, which has short-term stability, and a phase-
carrier, with allowance for the doppler shift. The pulse repeti-
tuning circuit. Examples of practical COHO include the
tion frequency may also be obtained from a general reference
phase-locked local oscillator and the phase-shift local oscilla-
oscillator using a frequency divider. This makes it possible to