Radar System Engineering

438 THE RECEIVING SYSTEM—RADAR RECEIVERS [SEC.122
Since time is always measured with respect to the instant at which the
modulator fires, synchronization between the modulator and the timer is
of basic importance. In some cases, thetimer exerts precise control of
exact firing time of thernodulat or bysendingitatriggerpulse;in others,
it responds to a trigger from the modulator. In the latter case, the system
is said to be ‘i self-synchronous. ” 1 Although certain advantages
can often be derived by giving control to the timer, this cannot always
be done,z and it is sometimes inconvenient in a multiple-display system.
Figure 12.1 illustrates a self-synchronous system. Some of the departures
possible when the timer does the synchronizing will be pointed out
in the appropriate places.
The cathode-ray tube must be turned on only during the fraction of
the pulse cycle in which it is used. One portion of the timer generates a
square wave (Waveform b, Fig. 12”1) which performs this function and
usually finds other applications within the timer.
If the display includes a range sweep, as is nearly always the case, the
timer generates the waveform that ultimately produces it. This waveform
usually consists of a linearly increasing voltage wave which begins
at the initiation of the sweep and returns to the initial condition at its end
(Waveform c, Fig. 12.1). This involves a switching action which, as
indicated in Fig. 12.1, is usually provided by the same square wave that
turns on the cathode-ray tube.
The timer generates a set of discrete range markers: sharp video pulses
occurring at regular, precisely known intervals. In a self-synchronous
system, the markers must be recycled on each transmitter pulse; this
requires a transient oscillator. In the simple example illustrated, the
square wave b provides the necessary switching voltage. Often the
discrete range markers are supplemented by a continuously variable
range marker which may be generated in any of several ways. As indicated
in Fig. 12.1, the markers are usually mixed with the radar video
signals and the combination applied to the cathode-ray tube. In some
cases, however, the signals and the markers are applied separately to
different electrodes of the cathode-ray tube.
The equipment illustrated does not provide for the use of a delayed
sweep. If one is to be used, separate square-wave generators are necessary
to switch the range-marker circuit and to perform the other functio&.
The former square-wave generator must be triggered directly as
1A trigger generator for the modulator is sometimes housed with the indicator
equipment for convenience, but the actual synchroni~atiOnis accomplished bY transmitting
the modulator pulse to the timer proper. This case is functionally identical
with that in which the trigger sourceis physically part of the modulator.
2Certainmodulators,such as the rotary gap, cannot be triggeredat Ill. Other
types can be triggeredbut have so variablea responsetime that the modulatorpulse
itselfmustbe usedfor synchronizationin orderto providethe necessarvprecision.