Radar System Engineering

490 THE RECEIVING SYSTEhf—INDICATORS [SEC. 135
to permit the use of higher carrier frequencies than can readily be passed
through a synchro.
Generators. —In some instances of conical or spiral scanning, part of
che scanner data is provided by means of a d-c excited 2-phase generator
geared directly to the scanner axis. In the case of conical scanning, the
output voltages of this generator are direct measures of the sine and
cosine of the phase of the scan. In the spiral scan, the voltages are
modulated in terms of the nod angle, either by varying the generator
field current by a potentiometer on the nod shaft, or by passing each
sinusoidal voltage through such a potentiometer after generation. The
signals thus produced can be used directly in the synthesis of type B and
type C displays.
13.5. Electromechanical Repeaters.—Two types of electromechanical
devices are used to repeat the motion of a rotating shaft at a remote
point.
}, ; 1
1
e I I
I
0 ~Repeater shaff
A.c line
FIG. 13.8.—Synchro-driven repeater.
The Synchrodriuen Repeater.—If the stators and rotors are attached
respectively in parallel and the rotors are connected to an a-c power
source (Fig. 13.8), the two rotors will tend to align themselves in the
same direction. Any departure from this condition will result in circulating
currents in the stators which will cause a motor action tending
to produce alignment. If one rotor is driven, the other will follow, with
only enough lag to furnish the necessary power. If the second rotor has
little or no mechanical load, this lag will be very small at rotational speeds
up to a few revolutions per second; if the load is appreciable, the lag may
amount to a few degrees. In order to reduce the effect of lag, the system
is often “geared up.” For example, the transmitter synchro can be
attached to a shaft rotating 10 times as fast as the scanner, and the receiving
synchro can then drive a rotating PPI deflection coil through a
10-to-l gear reduction. Under these conditions, the lag error can be
kept to a small fraction of a degree. There is, however, a 10-fold uncertainty
in the position of the deflection coil in our example, since any of
10 antenna positions looks the same to the receiver. The necessity for
phasing the system manually each time it is turned on can be avoided