Radar System Engineering

SEC. 17.9] COMPARISON OF SYNCHRONIZATION METHODS 711
The tracking circuits are similar to the circuit shown in Fig. 17.13c
except for changes made to provide for a scan converter and for the fact
that the switching voltages forthesawtooth delay circuits are provided
by the same flip-flops that control the sine- and cosine-pulse switches.
A-c signals for the scan converter are obtained by modulating the 60-cps
line voltage with the sine and cosine voltages, as in Fig. 17.10a. The
regenerative tracking assistsgreatly in overcoming errors in the modulators
by rectifying the modulated alternating current with a phasesensitive
detector and using this result to control the tracking delay
circuit. Thus the amplitude of the modulated alternating current (which
is the quantity of primary interest) is forced to vary in the same manner
as the delay in the signal pulse, regardless of errors in the comparison
circuits, the modulator, any amplifiers, etc.
Equipment essentially like that just described has had a great deal
of use and in its final form has proved satisfactory. An entire system
using this method is described in Sec. 17.16.
17.9. Comparison of Synchronization Methods.-Not all the synchronization
methods described in the previous sections have been used
on actual radar relay systems although nearly all of them have been setup
and tested in the laboratory. Because of the inadequacy of testing in
many of the cases, evaluations are difficult, but the following general
statements can be made.
Pulse US. C-UI Methods. —Pulse methods have been much more highly
developed than those using c-w transmission, partly because of the pulsecircuit
experience available at the Radiation Laboratory, and partly
because the r-f equipment available during most of the work was of the
amplitude-modulation rather than the frequency-modulation type.
However, much can be said in favor of the pulse methods from a fundamental
standpoint, regardless of the type of carrier modulation.
1. Since one pulse, the trigger, is involved in any method, adequate
protection against pulse interference must be provided in any case.
2. When time sharing is used within the radar cycle, the pulse methods
require less complex r-f equipment than do the c-w methods with
their requirement for at least one subcarrier. Were time sharing
with the video signals abandoned and the pulses sent on a subcarrier,
the additional complications would be compensated by
considerable simplification and increased effectiveness of the terminal
equipment. The synchronization signals would automatically
be separated from the video signals. The use of 100 per cent of
the time and the freedom from restrictions imposed by the radar
PRF would lessen the sensitivity to interference and permit much
more flexibility than is available with the time-sharing technique.