FCC Commercial Element 8 pool, section 23

Subelement C

Receiving Systems

Section 23

Detectors - Video Amplifiers

823C1

Which of the following statements is correct?

A.
The video amplifier is located between the mixer and the I.F. amplifier.
B.
The video amplifier operates between 60 MHz and 120 Mhz.
Correct Answer
C.
The video amplifier is located between the I.F. amplifier and the display system.
D.
The video amplifier is located between the local oscillator and the mixer.

823C1

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823C2

Video amplifiers in pulse RADAR receivers must have a broad bandwidth because:

Correct Answer
A.
Weak pulses must be amplified.
B.
High frequency sine waves must be amplified.
C.
The RADARs operate at PRFs above 100.
D.
The pulses produced are normally too wide for video amplification.

823C2

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823C3

In video amplifiers, compensation for the input and output stage capacitances must be accomplished to prevent distorting the video pulses. This compensation is normally accomplished by connecting:

A.
Inductors in parallel with both the input and output capacitances.
B.
Resistances in parallel with both the input and output capacitances.
C.
An inductor in parallel with the input capacitance and an inductor in series with the output capacitance.
Correct Answer
D.
An inductor in series with the input capacitance and an inductor in parallel with the output capacitance.

823C3

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823C4

Which of the following signals is not usually an input to the video amplifier?

Correct Answer
A.
Resolver.
B.
Range.
C.
Brilliance.
D.
Contrast.

823C4

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823C5

Which of the following signals are usually an input to the video amplifier?

A.
Range.
B.
Brilliance.
C.
Contrast.
Correct Answer
D.
All of the above.

823C5

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823C6

The video (second) detector in a pulse modulated RADAR system would most likely use a/an:

A.
Discriminator detector.
Correct Answer
B.
Diode detector.
C.
Ratio detector.
D.
Infinite impedance detector.

823C6

The video (second) detector in a pulse modulated RADAR system would most likely use a/an:

Diode detector.

A pulse-modulated RADAR system transmits short pulses of radiofrequency (RF) energy and then listens for the echoes reflected from targets. These echoes contain information about the distance and characteristics of the targets. The received echoes are typically mixed with the local oscillator signal to down-convert them to a lower frequency, typically an intermediate frequency (IF).

After the down-conversion, the next step is to extract the modulating information or video signal from the IF. This process is carried out by the video detector. The video detector's primary task is to rectify the IF signal, effectively converting it from alternating current (AC) to direct current (DC). The DC output represents the amplitude variations of the IF signal, which carries the target information.

A diode detector is a simple and common type of video detector used in pulse-modulated RADAR systems. It employs a semiconductor diode as the rectifying element. When the IF signal is applied to the diode, it allows current flow only in one direction, effectively rectifying the signal. The rectified output is then smoothed using a capacitor, resulting in a DC voltage proportional to the amplitude of the IF signal.

Diode detectors are preferred for video detection in pulse-modulated RADAR systems due to their simplicity, low cost, and ability to handle the high pulse repetition frequencies (PRFs) commonly used in RADAR applications.

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