Radar System Engineering

SEC. 1311] ANGLE INDICES 515
bright narrow line is produced at a given azimuth. The electrical
impulse necessary to do this can be provided in any of several ways.
1. The simplest method involves the use of mechanical contractors
on the scanner or on any mechanical repeater of the scanner
motion. The contact is usually made by arnicroswitch operated
byacamon a rotating shaft. Since it isimpossible to open and
close the switchin a short enough angular interval, unless the scan
is very slow, some device for producing a short pulse must be
introduced. This can consist of a flip-flop triggered by the microswitch
although, for medium or slow scam, it is possible to make
use of the transit time, of a mechanical relay.1 In order to produce
a set of markers, an equal numbeqof cams ona one-speed shaft or
6CP
Light bulb
Rotatingdisk
+ 25Gv
6,3v
FIG. 13.33 .—Photocel1 index circuit.
fewer cams on a higher-speed shaft can be used. The latter
method is preferable because of the cleaner action,
2. Figure 1333 illustrates a method for generating angle marks by
using a photocell mounted behind a slotted rotating disk. Since
the slots can be made extremely narrow and no inertia is involved,
this method produces extremely clean-cut markers. The disk
canturn at the scanner speed and have as many slots as there are
markers, or it can be geared up and have fewer slots.
3. Amethodof using acarrier modulated byasynchro is illustrated
in Fig. 13.34. (The figure actually illustrates a movable index.
In the case of fixed indices, the signals can be taken from the first
synchro and the second one omitted. ) The modulation introduced
by the rotating synchro issinusoidal ~vithscanner rotation; if the
latter is uniform the modulation envelope obtained from the
detector hasthiswaveforrn. Thesharp cusp occurring atthe nulls
lSee, for example, Fig. 4.2 of Vol. 22.
*