FCC Commercial Element 9 pool, section 4

Subelement A

VHF-DSC Equipment & Operation:

Section 4

Receiver Faults-1:

4A1

What condition may prevent a VHF radio receiver from staying tuned to the desired channel?

A.
The local oscillator frequency is beating against the incoming signal to produce an Intermediate Frequency (IF).
Correct Answer
B.
The output signal of the Phase Locked Loop (PLL) circuit in the frequency synthesizer has become unlocked.
C.
The discriminator circuit is alternately developing positive and negative voltages at the modulation frequency.
D.
The ratio detector circuit is producing a variable output that deviates across the receiver's dynamic range.

4A1

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4A2

What fault may exist if a VHF radio receiver fails to produce audible output from the speaker and the squelch control has no effect?

A.
A component in the RF amplifier stage has failed or the signal from a preceding stage is grounded.
B.
The Local Oscillator stage is no longer supplying a signal to the mixer stage at the correct frequency.
Correct Answer
C.
The AF amplifier may be defective or the transmitter Push-To-Talk circuit may be activated.
D.
The mixer stage is no longer producing an output signal on the correct Intermediate Frequency (IF).

4A2

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4A3

What is indicated if a VHF radio fails to receive nearby stations and a voltmeter connected between the positive side of a radio's DC line and ground reads zero line voltage? The fuse in the VHF radio also reads zero when checked with an ohmmeter.

A.
The antenna may be defective or water intrusion has affected antenna system performance.
B.
The coaxial transmission line may be open, shorted, or a connector has become intermittent.
C.
A fault in the discriminator circuit is producing zero output voltage during peak modulation.
Correct Answer
D.
A "crowbar" circuit has prevented an over current condition that might damage the radio.

4A3

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4A4

What fault may exist if a VHF radio receiver fails to produce audible output from the speaker and the squelch control has no effect?

Correct Answer
A.
The AF amplifier may be defective or the transmitter Push-To-Talk circuit may be activated.
B.
A component in the RF amplifier stage has failed or the signal from a preceding stage is grounded.
C.
The Local Oscillator stage is no longer supplying a signal to the mixer stage at the correct frequency.
D.
The mixer stage is no longer producing an output signal on the correct Intermediate Frequency (IF).

4A4

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4A5

What condition may prevent a VHF radio receiver from receiving a weak signal?

A.
The local oscillator frequency is beating against the incoming signal to produce an Intermediate Frequency (IF).
B.
The output signal of the Phase Locked Loop (PLL) circuit in the frequency synthesizer has become unlocked.
C.
The output signal of an AF amplifier stage is greater than the input signal.
Correct Answer
D.
The weak RF signal has been suppressed at the limiter stage of the received by capture effect.

4A5

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4A6

Which of the following is the best test procedure to isolate the source of distortion in the audio output of a receiver?

Correct Answer
A.
Use an oscilloscope to visually display AC signals on the output of each AF amplifier stage.
B.
Use a spectrum analyzer to visually display the amplitude and frequency of each AF amplifier stage input.
C.
Use a signal generator to inject a 1 KHz signal into the first AF amplifier stage and evaluate audio output.
D.
Use a digital voltmeter to measure transistor bias voltages in each AF amplifier stage.

4A6

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