B-003
Subelement B-003
Section B-003-013
Section B-003-013
What causes the loud noise heard from an FM receiver in the absence of a signal?
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The additional gain following the discriminator
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The nature of atmospheric noise in the VHF range
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The higher intermediate frequency used in FM receivers
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Correct AnswerThe very large gain of stages ahead of the discriminator
In an FM receiver, a very large gain is required ahead of the limiter so it can work effectively. The consequence of that gain is a very loud noise in the absence of a signal at the operating frequency. To counter that annoyance, a "squelch" circuit is included in the audio stage to silence the noise in the absence of a signal.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
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You are using an FM repeater configured for 5 kHz deviation, but your transmitter is set to 2.5 kHz deviation. What is the consequence?
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Your audio will be distorted
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Correct AnswerYour audio will be low
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The repeater will not respond
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Your range will be shorter
In frequency modulation (FM), a loud sound produces a large frequency deviation and a high-pitched sound produces rapid frequency deviation. If the deviation setting on your transmitter is lower than expected, the effect will be the same as speaking too low.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
What term defines the change in frequency caused by modulation in an FM transmitter?
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Spectrum spread
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Shift
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Modulation index
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Correct AnswerDeviation
In frequency modulation (FM), a loud sound produces a large frequency deviation and a high-pitched sound produces rapid frequency deviation.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
What kind of emission would your FM transmitter produce if its microphone failed to work?
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An amplitude-modulated carrier
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A frequency-modulated carrier
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Correct AnswerAn unmodulated carrier
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A phase-modulated carrier
The concept here is that IF NO AUDIO is fed into an FM transmitter, the carrier put out at the Power Amplifier has full amplitude anyway. A carrier which conveys no message is an 'unmodulated carrier'.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
Why is FM voice best for local VHF/UHF radio communications?
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Its RF carrier stays on frequency better than the AM modes
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The carrier is not detectable
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It is more resistant to distortion caused by reflected signals
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Correct AnswerIt provides a good signal-to-noise ratio at low RF signal levels
FM -- Frequency Modulation. As the process removes much of the electrical noise picked up along the way, weak signals can be rendered with a better 'signal to noise' ratio. However, this comes at a price of more occupied bandwidth, 10 to 20 kilohertz in usual amateur radio communications.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
What is the approximate bandwidth of a frequency modulated signal using 5 kHz deviation?
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Less than 5 kHz
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Between 5 kHz and 10 kHz
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Greater than 20 kHz
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Correct AnswerBetween 10 kHz and 20 kHz
How is a higher level of the modulating signal represented in an FM signal?
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By a larger pulse width in the transmitted wave train
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Correct AnswerBy a larger excursion of the carrier frequency
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By a larger peak envelope power
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By a larger amplitude of the carrier
In frequency modulation (FM), a loud sound produces a large frequency deviation and a high-pitched sound produces rapid frequency deviation.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
What modulation method is most closely related to frequency modulation?
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Correct AnswerPhase modulation
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Amplitude modulation
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Multiplex modulation
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Pulse modulation
Direct FM: Use a variable reactance element as one of the elements of an oscillator to cause frequency deviation. Indirect FM: apply the modulating voltage to a variable reactance element connected to a tuned circuit later in the transmit chain, where it will produce phase modulation rather than frequency modulation.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
Why isn't FM used as an amateur radio emission mode below 28 MHz?
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Harmonics could not be attenuated to practical levels
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The frequency stability would not be adequate
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The transmitter efficiency for this mode is low
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Correct AnswerThe bandwidth would exceed limits in the regulations
The usual bandwidth of FM with 5 kHz deviation on amateur bands is between 10 to 20 kilohertz. On the 10-metre band (28.0 to 29.7 MHz), maximum permitted bandwidth is 20 kHz. "Radiotelephone signals in a frequency band below 29.50 MHz cannot be automatically retransmitted unless these signals are received from a station operated by a person qualified to transmit on frequencies below 29.50 MHz (RBR-4)."
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
Several stations report that your FM transmission is loud and distorted, but on frequency. Which of the following is the most probable cause of the distortion?
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Excessive transmit power
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Correct AnswerSpeaking too loudly into the microphone
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Cross-polarized antenna
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Setting the wrong CTCSS tone
Key word: DISTORTION. 'Overdeviation (FM)' or 'Overmodulation (AM, SSB)' results in distorted speech plus using excessive bandwidth on the air (splatter) and interfering with stations using adjacent frequencies ('out-of-channel emissions').
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
When more than one signal is present, the FM receiver is likely to demodulate only the strongest signal. What is this behaviour called?
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Surrender effect
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Overpower effect
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Interference effect
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Correct AnswerCapture effect
The 'Capture Effect' is specific to FM receivers: only the stronger of two signals at or near the same frequency will be demodulated. The complete suppression of the weaker signal occurs at the receiver limiter. When both signals are nearly equal in strength, or are fading independently, the receiver may switch from one to the other. https://en.wikipedia.org/wiki/Capture_effect
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
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