Radar System Engineering

SEC.13.10] SAWTOOTH GE,VBliATOh?S 513
This method is used in the precision delay circuit of Fig. 13.36,
described in the next section.
3. The most common method of using linear amplifier gain to keep
the necessary signal voltage small is illustrated in the diagrams of
Fig. 13.32. The amplified signal is applied to the “bottom” of the
condenser, so that the potential of X changes only by the difference
between the charge across the condenser and that fed back. Thus
the change in potential across R is kept very small and the charging
current is very constant. The most commonly used of such
methods is the Miller “run-down” circuit of Fig. 13.32b and c.
The two circuits shown differ only in the methods of switching.
In both cases the amplifier consists of a single tube on whose grid
the original signal is formed. The entire plate swing is applied
to the condenser, so that the feedback ratio is 1. In the case of
suppressor switching, all of the current goes to the screen between
sweeps. The control grid is held against the cathode by virtue
of its positive bias. When the suppressor is switched on, most of
the current transfers to the plate, which experiences a negative
surge that is passed on to the control grid. The sawtooth generation
then begins as indicated in the waveform diagrams. The step
at the beginning of the sawtooth is in some cases detrimental and
in others
useful.
INDICES
It is always desirable and usually mandatory to provide some form
of quantitative indices or markers for the radar indicator. These may
consist simply of a gridwork of lines or, when high accuracy is required,
of one or more movable indices. In addition, it is often desirable to
superpose some form of map or chart on a radar display, in order to provide
accurate correlation with fixed echoes for navigational purposes or
to show at a glance the geographical position of a ship or aircraft target.
Markers can be provided either by placing a surface containing the marks
as nearly as possible in optical superposition with the display, or by
modulating the electron beam in such a way that the marks appear as
part of the display
itself.
Indices or charts ruled on a transparency over the tube face are the
simplest of all to provide, but their use results in errors due to display
inaccuracies, to parallax, and to faulty interpolation. Furthermore, if
the origin of the display is to be moved, it is necessary to provide a
corresponding motion of the reference system, which is usually cumbersome,
or if only a few positions are involved, to furnish multiple sets of
marks in such a way that no confusion results. The methods of optical
superposition described in Sec. 7.3 largely eliminate parallax and are