Radar System Engineering

SEC. 13.17] RESOLVED-CURRENT PPI 539
to use them for off-centering; therefore, when this is desired, separate
coils are provided on the same form. Since they are inductively coupled
to the sweep coils, the number of turns in the off-centering coils must also
be limited. Off-centering is therefore expensive in current, and its
extreme amount is usually limited to not more than two tube-radii.
Since vacuum tubes are not appropriate as switches in such lowimpedance
circuits, it is not possible to fix the sweep origin by methods
analogous to that of Fig. 13.46. It has been necessary to use either the
method of a series dry-disk rectifier switched (if necessary) by the scanner,
or to apply some method of waveform balance. Applications of
these methods appear in the following descriptions.
The Use of Series Rectifiers.-Figure 13.47 illustrates those parts that
must be added to the sweep generator and amplifier of Fig. 13.44 to
produce a complete PPI involving the use of disk rectifiers. By provid-
b/
4‘~~’~
Selenlum
rectifiers Switch reversed
at 0“ and 180”
Alternative
f\ ---
--- feedback
points
/
Switch reversed
V6 of at 90”and 27o”
F]g.1344
oCRT
~‘ ~
FXG, 1347,-Resolved-current PPI usingswitchedrectifiers,
ing two rectifiers for each secondary as indicated, only one point need be
switched. Optimum smoothness at the instant of switching is accomplished
if contact is made through the second rectifier before the first
one is opened, so that the circuit is never broken.
This method has the virtue of extreme simplicity and in many circumstances
it gives excellent performance. However, it has two principal
limitations in addition to the restriction on off-centering mentioned
above: the mechanical switching limits the scanning rates at which it can
be used, and some distortion is introduced on “long” sweeps (more than
50 miles) by the low but appreciable resistance of the rectifiers. If this
resistance were constant, the only effect would be a droop caused by
inadequate low-frequency response. This droop could easily be overcome
by shaping the primary waveform, but unfortunately the rectifier
resistance is nonlinear, being much larger at very low than at intermediate
or high currents. As a result, the effect of the rectifier resistance changes
with scan angle. “ Circles” of equal range are slightly “ squared, ” the
“ corners” coming at the 90° positions. 1
1This effect should not be confused with the “ square circles” found on certain
early PPI’s of the type shown in Fig, 13.46. These errors, which resulted from