Radar System Engineering

SEC.1316] RESOLVED TIME BASE PPI 535
or without amplification, to drive an orthogonal deflecting system (Fig.
13.45). When amplifiers are used, or if an electrostatic tube without
amplifiers is involved, the resolver must deliver sawtooth voltages into a
high-impedance load; when a magnetic tube is used without amplifiers,
sawtooth currents must be delivered to an inductive load.
In principle the resolver may be a synchro, a potentiometer, or a
condenser; a synchro is far the most satisfactory and, in spite of certain
shortcomings, it is universally used.
As in all cases of transmission of a nonsymmetncal signal through an
a-c coupling, the d-c component of the signal is lost. This presents one
,
i–”--~
A Primary signal
(unbalanced)
M------------
B %con&ry s,gnal
(no clamping)
_
B, Secondary wgnal
(when clamped)
4 Salanced
waveform
(EXP:cdee time
—
Automat,c
trdgger
generation
Sweep
waveform —
{ Modulator‘-~-
L tr,aer 1 I I I
FIG. 1345.-Resolved-sweep PPI methods.
of the most serious difficulties involved in this technique. With a passive
and linear load, the long-time average of both the current through the
secondary of the synchro and the voltage across it must be zero (Waveform
1?, Fig. 13.45). Thus the absolute current or voltage at the beginning
of the modulated sawtooth will not be constant, but will change
with the modulation amplitude and will reverse sign when the sawtooth
does. As a result, if no corrective measures were taken, each individual
sweep on the display tube would be displaced in the negative direction
by a fixed amount and the 10CUSof the sweep origins would be a “ negative”
circle. This difficult y has been overcome by two general methods,
each of which has variations most suitable to particular circumstances.