FCC Commercial Element 8 pool, section 17

Subelement B

Transmitting Systems

Section 17

Timer - Trigger - Synchronizer Circuits

817B1

What RADAR circuit determines the pulse repetition rate (PRR)?

A.
Discriminator.
Correct Answer
B.
Timer (synchronizer circuit).
C.
Artificial transmission line.
D.
Pulse-rate-indicator circuit.

817B1

All time-dependent electrical circuits in a RADAR System are controlled by the Synchronizer, thus the PRR - Pulse Repetition Rate is also controlled by the Synchronizer and Answer B. is the right answer.

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817B2

The triggering section is also known as the:

A.
PFN.
B.
Timer circuit.
C.
Blocking oscillator.
Correct Answer
D.
Synchronizer.

817B2

"Triggering Section" is short for Pulse Triggering Section.

All time-dependent electrical circuits in a RADAR System are controlled by the Synchronizer, thus the Triggering Section is also controlled by the Synchronizer and Answer D. is the right answer.

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817B3

Operation of any RADAR system begins in the:

Correct Answer
A.
Triggering section.
B.
Magnetron.
C.
AFC.
D.
PFN.

817B3

The Triggering Section triggers the RADAR pulse, so the RADAR action pretty much starts with the Triggering Section and Answer A. is the right answer.

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817B4

The timer circuit:

A.
Determines the pulse repetition rate (PRR).
B.
Determines range markers.
C.
Provides blanking and unblanking signals for the CRT.
Correct Answer
D.
All of the above

817B4

Timing is everything in a RADAR System, so it follows that Answer D., "all of the above", is the right answer.

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817B5

Pulse RADARs require precise timing for their operation. Which type circuit below might best be used to provide these accurate timing pulses?

Correct Answer
A.
Single-swing blocking oscillator.
B.
AFC controlled sinewave oscillator.
C.
Non-symmetrical astable multivibrator.
D.
Triggered flip-flop type multivibrator.

817B5

The AFC is the Automatic Frequency Control signal and AFC is unrelated to pulse generation, so answer B. is wrong.

An Astable Multivibrator is an unstable oscillator and you wouldn't want to drive a complex timing-dependent machine such as a RADAR System with an unstable oscillator, so Answer C. is wrong.

A Triggered Multi-Vibrator depends on being triggered by an outside signal and you wouldn't want an outside signal to trigger the pulses of a RADAR System, so answer D. is wrong.

A Blocking Oscillatorm, a.k.a. a Pulse Oscillator, seems ideal. Notinh could be better than a Pulse Oscillator to generate the pulses a RADAR Sytem needs to operatoe, so Answer A. is the right answer.

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817B6

Unblanking pulses are produced by the timer circuit. Where are they sent?

A.
IF amplifiers.
B.
Mixer.
Correct Answer
C.
CRT.
D.
Discriminator.

817B6

hint: Timer Circuit - CRT

Blanking pulses inhibit the raster in a CRT - Cathode-Ray Tube from rendering anything in the screen while the raster moves from the lower right corner of the screen to the top left corner to start rendering a new frame.

In practice rather than a a blanking pulse CRTs can implement an un-blanking pulse taht allows the raster to render an image on-screen.

The un-blanking pulse applies a positive voltage to the CRT's grid thus allowing electrons from the cathode to reach the creen.

Un-Blanking pulses thus refer to a CRT so Answer C. is the right one.

Answers A., B. and D. do not involve an un-blanking pulse in any way and are therefore wrong.

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