Page 252 - Radar Technology Encyclopedia

local oscillator, frequency-multiplier STALO local oscillator, quartz 242

get rid of parasitic combination components of the signal at
the input of the mixer. IAM
Ref.: Skolnik (1970), pp. 5.15–5.16.
A phase-locked COHO is a second local oscillator with
phase locking implemented through synchronization with a
downconverted sample of the transmitter pulse in the reso-
nant circuit of the oscillator. It is used as the second (coher-
ent) local oscillator to compensate for phase fluctuation of a
pulsed-oscillator type of radar transmitter (e.g., magnetron).
Just before each transmission, the COHO oscillations are
quenched, destroying the phase information from the previ-
ous transmission. The reference phase for the new transmis-
sion is obtained at the intermediate frequency by mixing part Figure L13 Phase-shift COHO (from Skolnik, 1970, Fig. 14,
of the transmitter RF with the first local oscillator in a mixer p. 5-17, reprinted by permission of McGraw-Hill).
controlled by a lock pulse. The pulse voltage of the difference
oscillator in the figure creates a linear spectrum in the micro-
(intermediate) frequency is amplified, and then is sent to the
wave band with a frequency interval of Df using a snap diode.
resonant circuit of the second local oscillator (Fig. L11). The
(See DIODE, charge storage.)
phase of oscillations of the second local oscillator is then
Some of the output power of the klystron is mixed with
matched to the phase difference between the first local oscil-
the linear spectrum and an oscillation with a beat frequency
lator and radar transmitter, and the continuing COHO output
of f/2 is applied, after amplification, to the f/2 frequency
preserves this random phase for the remainder of the pulse
phase detector. An operational amplifier provides the required
repetition interval. IAM
transient characteristics of the circuit for phase autotuning of
Ref.: Skolnik (1970), p. 5.16
the frequency. The klystron is synchronized in phase at the
discrete frequencies f (N ± 1/2). The stability of the klystron is
determined by that of the reference frequencies. IAM
Ref.: Skolnik (1970), p. 5.14.

Figure L12 Keyed COHO (from Skolnik, 1970, Fig. 12,
p. 5.16, reprinted by permission of McGraw-Hill).

A phase shift COHO is a second local oscillator with auto-
matic phase tuning of oscillations of crystal local oscillator
with phase of the synchronization pulse. The tuning is done
using a phase-shifting device with electronic control
(Fig. L13). It is used as a second loop of a radar.
The circuit for automatic phase tuning is turned on with
an electronic switch only for the time of operation of the syn-
chronization pulse. In the interval between the successive Figure L14 Phase-locked STALO (from Skolnik, 1970, Fig. 10,
pulses, the voltage of the phase correction is retained in the p. 5-14, reprinted by permission of McGraw-Hill).
integrating capacitor C. The quadratic phase detector and the
high-speed phase-shifting device with an interval of 180° are
A quartz local oscillator uses a quartz resonator for fre-
required to maintain the phase delay between 0 and 180° .
quency stabilization. The oscillator frequency coincides with
The process of phase strobing and storage does not influ-
the frequency of the first (up to 30 MHz) or higher (up to 200
ence the frequency of the second local oscillator and the
MHz) mechanical harmonics of the quartz. Multiple fre-
requirements on stability are minimal. The use of a crystal
quency multiplications can be employed to get oscillations
resonator assures higher stability of a local oscillator with
with the required frequency. Frequency agility typically is
phase shift than of a phase-locked COHO. IAM
obtained by changing the quartz slab. The frequency stability
Ref.: Skolnik (1970), pp. 5.16–5.17. - 7 - 8
is about 10 to 10 . IAM
A phase-locked STALO is a local oscillator with phase auto- Ref.: Pereverzentsev (1981), p. 242; Fink (1982), p. 7.32.
tuning of frequency, which has the capability of retuning. It is
used as a first local oscillator of a radar (Fig. L14). The local