3-13
the radar is tracking, or the manual positioning of the range-volts potentiometer when the radar is not
tracking (in the search mode). The range-gate triggers the range-strobe multivibrator, from which the
output is amplified and sent to the blocking oscillator (which sharpens the pulses), as shown in figure 3-
11. This range gate is used to select the target to be tracked. When in the track mode, the range gate
brightens the trace or brackets the blip (depending on the system) to indicate what target is being tracked.
Range-gate generators are used most often in weapons-control track radar A-scope presentations, but they
can also be used with ppi presentations. When used with a ppi presentation, the range gate must be
movable in both range and bearing.
The range-gate generator can easily be modified to produce a range strobe instead of a range gate. A
range strobe is simply a single brightened spot that is movable both in range and bearing. In operation, the
range strobe or range gate control also controls a dial or digital readout to provide a range readout to the
operator.
Range-Marker Generator
Several types of range-marker generators are in common use. Figure 3-12 shows a simplified version
of a circuit that produces both range markers and the basic system timing triggers. The master oscillator in
this case is a blocking oscillator that operates at a frequency of 80.86 kilohertz. By dividing 80.86
kilohertz into 1 (t = 1/frequency), we find the time required for one cycle of operation is 12.36
microseconds. Thus the blocking oscillator produces pulses 1 radar mile apart. These are fed to the 5:1
divider circuit. Five of the 1-mile marks are required to produce an output from the divider circuit. These
five-mile marks are sent to the indicator for display and to the 10:1 divider circuit. In the latter case, ten of
the five-mile marks are required to produce an output from the 10:1 divider. Thus the output triggers are
50 miles apart. These basic timing triggers are for a radar with a range of fifty miles. The period between
triggers could be extended through the use of additional dividers for use with longer range systems.
Figure 3-12.—Range-marker generator.
Another version of a range-mark generator is shown in figure 3-13. This circuit provides range
marks at 1,000-, 2,000-, or 3,000-yard intervals. Generation of the marks begins with the ringing
oscillator, which is started by a delayed master trigger from the synchronizer. A ringing oscillator
produces a sinusoidal output of a fixed duration and frequency when triggered. The output is
synchronized to the input trigger. In this circuit, the trigger causes the oscillator to produce a 162-
kilohertz signal that lasts for 4 1/2 cycles. The emitter follower isolates the ringing oscillator from the
countdown multivibrator and clips the oscillator output signals. This action allows only the positive half
of each sine wave to reach the multivibrator. The positive triggers from the ringing oscillator are at 1,000-
yard intervals. This input signal results in an output from the countdown multivibrator of 1,000-, 2,000-,
or 3,000-yard range marks, depending on the position of the RANGE MARK SELECT SWITCH.
