Radar System Engineering

702 RADAR RELAY [SEC.17.7
be obtained most simply by exciting a synchro on the radar scanner and
relaying each of the resulting voltage “components” together with
a constant reference voltage (Fig. 17”9). This method, which would
require the use of multiple subcarriers, and would thus involve excessive
bandwidth, complex multiple modulation and filtering, or both, has never
been used. Two component voltages and one reference voltage can be
provided at different a-c frequencies, but this arrangement is not suitable
for use with a servomechanism. It can, however, be used to produce
slowly varying sine and cosine voltages. These can then be used to modulate
alternating current of proper frequency to provide the required voltage
components for a synchro-controlled servomechanism. The method
Coae
c1
Modulator
A%line
?5
voltage
Modulator
~’
RAmplifier
-- Motor
(a)
Data
transmitter
0or PPI coil
Cos e
Double
clamp
L
x Square ~
wave
Sm.9
Amplifier
CRT
mo
Double
clamp
: ‘cFAmplifier
F:~. 17. 10.—Use of S1OW1 y varying sine and cosine voltages: (a) production of mechanical
motion; (b) fixed-coil PPI synthesis.
is illustrated in Fig. 17.10a. It has been successfully used in connection
with various types of data transmission resulting in slowly varying sine
and cosine voltages.
In contrast to the reproduction of mechanical motion, the fixed-coil
PPI can best use slowly varying polarized direct current. 1 Since alternating
current can easily be rectified to give such voltages, any of the
methods is applicable to this case. As shown in Fig. 17. 10b, the sweep
components are produced by sawtooth generators using the signal voltages
as their source potentials so that the ‘‘ sawteeth” are modulated in
the proper manner as the scanner rotates (Sec. 13.16). Through the
provision of low-impedance or multiple-output circuits for the sine and
cosine voltages, as many indicators as desired can be used. The sweep
length for each can be chosen independently of those of the others.
1 It is poseible, of course, to relay the sweep components themselves. However, a
separate sub carrier would be required for each. Furthermore, each “sawtooth”
must endure for the time of the longest desired sweep, and fast sweeps must be
achieved by amplification of a short segment. Errors would be greatly magnified in
such a process.
(b)
T