Radar System Engineering

SEC. 16.16] THE COHERENT 08CILLA TOR 663
to pass to the mixer with so little loss that their amplitude is comparable
to or greater than that of the fundamental frequency.
A further source of locking trouble is statistical noise. If a simple
cable-coupling circuit is used in the absence of a locking-pulse preamplifier,
care must be taken to maintain adequate pulse amplitude.
Circuit Design.-Any conventional type of freely running oscillator
can be phase-locked by injecting into its tuned circuit a sufficient y large
carrier pulse. There is a certain degree of incompatibility between the
requirements that the oscillator be extremely stable and yet precisely
phasable. A simple method of reconciling these requirements is to stop
the oscillator completely before each radar transmission so that the locking
pulse is also a starting pulse. By this process, all previous phase
information is destroyed and the locking pulse is required to overwhelm
merely the statistical noise fluctuations present in the oscillator tuned
circuit. The stopping of the coherent oscillator can be accomplished by
applying a bias “gate” perhaps 100 psec before each transmitted pulse.
This gate is released just before, during, or just after the locking pulse,
while the resonant circuit of the oscillator is still ringing. The amplifying
circuits that precede the oscillator can be made reasonably narrow since
pulse shape is unimportant. These circuits must be broad enough to
allow for local-oscillator detuning and to maintain a large locking-pulseto-noise
ratio. It is advantageous to use a bias slightly beyond cutoff
in a late amplifying stage in order to suppress any spurious low-level
oscillations which, by their persistence, might introduce an appreciable
phase error either before or after locking. The actual injection of the
locking pulse into the oscillator might be accomplished in any of several
ways, including the use of an extra oscillator control grid. The most
flexible method and the one most widely used is the connection of the plate
of a pent ode injection-tube across all, or part of, the LC-circuit of the
oscillator.
The oscillator itself must be designed with considerable care. The
resonant circuit must have a high Q. Input and output loading must be
held to a minimum. Heater hum modulation must be minimized by
keeping the d-c and r-f impedance between cathode and ground as smali
as possible. The oscillator tube must be of a nonmicrophonic type. The
circuit as a whole must be rigidly constructed and shock-mounted,
Power-supply ripple must not exceed a few millivolts. When these
precautions are observed, free-running stabilities of around 1 kc/sec2 can
be obtained at 30 Me/see. A typical oscillator with its preceding and
following stages is shown in Fig. 16.27.
Phase-shift Unit.—Figure 16.28 shows, in more detail than the simplified
diagram of Fig. 16.25b, the arrangement of the phase-shift unit
required for MTI on a moving system. For station motion up to 400 mph,