8-32D: Fixed Range Markers

Question 1

8-32D1

Accurate range markers must be developed using very narrow pulses. A circuit that could be used to provide these high-quality pulses for the CRT is a:

A. Ringing oscillator.
B. Monostable multivibrator.
C. Triggered bi-stable multivibrator.
D. Blocking oscillator.

Answer 1

ANSWER D
A blocking oscillator, like all oscillators, makes use of feedback. That is, some of the output of the circuit is sent back to the input. But, as its name suggests, the blocking oscillator “blocks out” most of the input and feedback, and allows only a very sharp, narrow pulse at its output. Of course this is perfect for generating the thin, unobtrusive range rings that allow you to see the distances to tagets on a RADAR scope without “blocking out” the targets.

Question 2

8-32D2

Range markers are determined by:

A. The CRT.
B. The magnetron.
C. The timer.
D. The video amplifier.

Answer 2

ANSWER C
It’s the timer circuit that determines the range rings.

Question 3

8-32D5

What is the distance between range markers if the controlling
oscillator is operating at 20 kHz?

A. 1 nautical miles.
B. 2 nautical miles.
C. 4 nautical miles.
D. 8 nautical miles.

Answer 3

ANSWER C
First convert oscillator freequency into time, or period. This is easy enough - simply take the reciprocal. 1/20,000 = 0.000050 seconds (50 microseconds). Our old freind “seasix” shows up again; 6 microseconds per nautical mile. Divide our 50 microseconds by 6 microseconds-per-mile, which gives us 8.333 nautical miles. We can round that off to 8 mailes because the “seasix” 6 we use is also rounder off a bit. But wait! We have to cut that distance in half because our RADAR signal has to make a round trip. 8/2 = = 4. Ah, that’s better!