FCC Commercial Element 8 pool, section 24

Subelement C

Receiving Systems

Section 24

Automatic Frequency Control - AFC

824C1

The AFC system is used to:

A.
Control the frequency of the magnetron.
Correct Answer
B.
Control the frequency of the klystron.
C.
Control the receiver gain.
D.
Control the frequency of the incoming pulses.

824C1

AFC stands for Automatic Frequency Control.

The circuit that discriminates ("sees") that the receiver's Klystron is receiving at a frequency different than the frequency the transmitter''s Magnetron is transmitting on is called the Dicriminator.

The Discriminator generates an AFC voltage proportional to the frequency difference to move the receiver's Klystron to match the transmitter's Magnetron frequency.

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824C2

A circuit used to develop AFC voltage in a RADAR receiver is called the:

A.
Peak detector.
B.
Crystal mixer.
C.
Second detector.
Correct Answer
D.
Discriminator.

824C2

AFC stands for Automatic Frequency Control.

The circuit that discriminates ("sees") that the receiver is receiving at a frequency different than the frequency the transmitter is transmitting on is called the Dicriminator.

The Discriminator generates an AFC voltage proportional to the frequency difference.

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824C3

In the AFC system, the discriminator compares the frequencies of the:

Correct Answer
A.
Magnetron and klystron.
B.
PRR generator and magnetron.
C.
Magnetron and crystal detector.
D.
Magnetron and video amplifier.

824C3

AFC stands for Automatic Frequency Control.

The circuit that discriminates ("sees") that the receiver is receiving at a frequency different than the frequency the transmitter is transmitting on is called the Dicriminator.

The transmitter's frequency is determined by the Magnetron, whereas the receiver's frequency is determined by the Klystron.

The Discriminator therefore compares the Magnetron and Klystron frequencies.

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824C4

An AFC system keeps the receiver tuned to the transmitted signal by varying the frequency of the:

A.
Magnetron.
B.
IF amplifier stage.
Correct Answer
C.
Local oscillator.
D.
Cavity duplexer.

824C4

The LO - Local Oscillator, determines the transmitter frequency, thus Answer C. is the right answer.

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824C5

A RADAR transmitter is operating on 3.0 GHz and the reflex klystron local oscillator, operating at 3.060 GHz, develops a 60 MHz IF. If the magnetron drifts higher in frequency, the AFC system must cause the klystron repeller plate to become:

A.
More positive.
Correct Answer
B.
More negative.
C.
Less positive.
D.
Less negative.

824C5

The first sentence is an irrelevant distractor.

If the transmitter's Magentron drifts higher in frequency, the AFC - Automatic Frewuency Control must command the Klystron to increase the receiving frequency to match the transmitter.

To a large extent the Klystron's frequency is determined by the size of it's resonant cavity.

Small changes in the Klystron's frequency can be made by verying the repeller voltage.

The Repeller is one side of the cavity that can be negatively charged.

When the repeller is negatively charged it repels the also negatively-charged electrons oscillating in the cavity (hence the name), thus making the apparent size of the cavity smaller and the resonant frequency of the cavity, and the Klystron, somewhat higher.

Thus the Klystron's repeller voltage must be made increasingly negative.

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824C6

What component is block 50 in Fig. 8A1?

A.
IF amplifier.
B.
AFC amplifier.
Correct Answer
C.
Discriminator.
D.
Crystal detector.

824C6

In a RADAR receiver, the discriminator is a crucial component responsible for detecting and processing the differences in frequency between the transmitted and received signals. It plays a critical role in extracting information about the target's range and velocity.

In Figure 8A1, block 50 represents the discriminator, which processes the intermediate frequency (IF) signal to extract modulation information, such as target range and Doppler frequency shift caused by the relative motion of the target.

Mnemonic: "DISC - Detecting, Intermediate frequency, Signal, Comparator"

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