Radar System Engineering

538 THE RECEIVING SYSTEM—INDICATORS [SEC. 13.17
sweeps. The switched clamps shown can be replaced by simple d-c
restorers if the scan covers less than 90°. The final sweep amplifiers are
of the cathode-feedback type usedin the B-scope of Fig. 13.43.
The centering of the PPI can be adjusted by setting the potentiometers
controlling the clamping points. If extreme off-centering moves
theamplifier grids out of their normal operating range, adelay occursat
the beginning of the sweep which gives the effect of a virtual center at the
desired point. Byusing anextrernel yhigh-voltag esawtooth, off-centering
of as much as six to eight tube radii has been provided. For such
extreme amounts of off-centering, it is best to insert cathode followers
between the potentiometers and the clamps.
The dependable accuracy obtainable with the resolved–time-base
method is not so great as can be obtained with a rotating-coil PPI,
although with care it can be made nearly as good except on very fast
sweeps. However, the present method has much less severe limitations
with respect to scanning speed, and provides a much greater degree of
usable off-cent ering than does the rotating-coil scheme.
Its w-eight is somewhat less and its power requirements somewhat
greater than those typical of the rotating-coil variety without offcentering.
When off-centering must be provided, the present method is
far lighter and less costly in power.
The technique just described can be applied to electrostatic tubes
by substituting voltage amplifiers in the final driving stages, or in some
cases by eliminating amplifiers entirely and connecting the clamped
points directly to one member of each pair of deflecting plates. Unless
weight and power consumption are extremely critical, the saving effected
does not justify the sacrifice in quality entailed.
13.17. Resolved-current PPI.—When the deflection coils are driven
directly by the synchro, the amplifiers of Fig. 13.46 are eliminated, with
the result that the indicator is considerably simplified and errors due to
imperfections in the amplifier response are avoided. These advantages
are accompanied by certain restrictions which, will appear below.
In order to reduce the effects of distributed capacity in the synchro
and the deflecting coils, and of the shunt capacity of the cables, lowinductance
windings of few turns are used throughout. The synchro
rotor is matched to the driving tube by a stepdown transformer. Since
the secondary circuit is almost purely inductive (ideally it would be
precisely so), its current waveform will resemble that in the primary of
the transformer, and a current amplifier similar to that driving the deflection
coil of Fig. 13.44 is appropriate. In many cases imperfections in the
transformer are overcome by taking the feedback from its secondary
circuit.
The low deflection sensitivity of the sweep coils renders it impractical