Radar System Engineering

SEC. 17.5] THE METHOD OF INCREMENTAL ANGLE 689
are large enough to allow Vla to rest below cutoff even when on, Vlb is
unnecessary. Alternative types of video switches are described in
Sec. 139. The sensitive time must be long enough to allow for all
uncertainties in the periodicity of the incoming signal and for changes in
the flip-flop circuit timing.
This method can also be used to distinguish between different signals
which have been transmitted on a time-sharing basis: for example, to
separate pulses from video signals. If the coincidence circuit is to be
used to decode more than one set of pulses, the flip-flop can be triggered
by the last set or by a pulse from a delay circuit which spans the signal
interval. More often, a train of switches is necessary to separate the
various pulses from one another after decoding so that no switch is
necessary at the input circuit. Somewhat more elegant methods which
allow narrower open intervals can be used in certain special cases in which
the opening of the switch can be controlled from a sequence of events
within the cycle. Various arrangements will appear in later sections in
connection with specific methods of data transmission.
17.6. The Method of Incremental Angle.-This section and the next
three will describe various specific methods of relaying the scanner
information, the basis of classification being the geometrical quantities
used.
As stated in Sec. 17.2, the method of angular increments usually
involves, as a final stage, a synchronous motor driven by a sinusoid.
Three methods of relaying the necessary data have been used.
In the first method, the sinusoid itself may be transmitted directly
on a subcarrier. The extra expenditure of power involved is usually not
justifiable for this rather inflexible method, especially since the pulse
methods are extremely simple.
As a second method, the modulator pulses themselves can be used to
represent the increments of scanner angle 1 if the scanner motion can be
made sufficiently constant to control the modulator triggering satisfactorily.
Such a system is represented in Fig. 17.4.2 Some form of
signal generator—usually electromagnetic-geared to the scanner produces
periodic signals of frequency proportional to the scanner velocity
and suitable for the pulse recurrence frequency. These signals control a
blocking oscillator or some other device to produce sharp pulses which are
then coded for transmission over the relay link. The third pulse of the
code (from point E of Fig. 17.2) is used for the modulator trigger.
Some method must be provided for separating pulses from video
1This ia,of course,an exampleof time sharingin whichtwo of the signalscoincide.
2In all the diagramsof this chapterthe individual blocks are functional and are
intendedto include proper input and output circuits including amplifiers,cathode
followers, blocking o.willators, etc.