Radar System Engineering

536 THE RECEIVING SYSTEM—INDICATORS [SEC.13.16
The first method involves the use of clamps m switches in the secondary
circuit to force the potential or the current to zero in the interval
between sweeps. So long as the sweep component has a given polarity,
this can be very simply accomplished in the potential case by a d-c
rest orer (See. 13“9), and in the current case by a selenium rectifier in series
with the secondary circuit. In cases where the scanning is confined to
90° or less, these variations of the first method are the most satisfactory
of all. If the scan covers more than 90°, one of the diodes must be
switched in polarity from time to timefor example, by a cam arrangement
on the scanner. If the scan covers more than 180° both of them
must be so switched. Although it is difficult to switch quickly and
smoothly enough to avoid aest heti call y displeasing display irregularities,
this method is quite satisfactory and is probably the simplest for the case
of resolved currents, providing the scanning rate is not too high. A
better and only slightly more expensive method of voltage restoration
involves the use of a double clamp (Fig. 13.25) which connects the
secondary to a point of proper potential between sweeps but releases it
when the sweep begins. This is the most widely used method in the
voltage case when polarity reversal is involved. No satisfactory series
switches exist for analogous use with resolved currents.
In the second method of correcting the sweep-origin difficulty, the
positive and negative waveforms are “balanced” about the sweep origin
either by introducing a precisely controlled negative waveform in the
primary circuit between sweeps, or by arranging to trigger the transmitter
automatically at the precise time when the secondary current or
voltage passes through zero. Both variations involve a good deal of
precision and are more costly than the switched clamps mentioned in the
last paragraph as usable when voltage is involved. However, the
present methods have been widely used in connection with resolved
currents, since they require no switching elements in the low-impedance
secondary circuit.
Applications of these various methods will be described in connection
with the following specific examples.
Magnetic PPI Using A mplijiers.-Figure 13.46 shows the secondary
circuit of a magnetic PPI of the resolved–time-base type using the
method of clamping between sweeps. It is supposed that the synchro
is preceded by the sweep generator and amplifier of Fig. 13.44, except
that the circuit of V6 is changed as indicated so that the feedback voltage
is that across the synchro itself.
The clamps on each of the amplifier grids of Fig. 13.46 are of the
double-triode variety illustrated in Fig. 13.25a. They are switched by
the same flip-flop (rectangular-wave generator) that controls the sawtooth
generator and are thus closed during the entire interval between