or
Subelement T7
PRACTICAL CIRCUITS
Section T7A
Station equipment: receivers, transceivers, transmitter amplifiers, receive amplifiers, transverters; Basic radio circuit concepts and terminology: sensitivity, selectivity, mixers, oscillators, PTT, modulation
Which term describes the ability of a receiver to detect the presence of a signal?
• Linearity
Sensitivity
• Selectivity
• Total Harmonic Distortion

Sensitivity is one of the most important characteristics of a receiver - the ability to detect the presence (or absence) of a signal.

Selectivity is the ability to detect a signal in the presence of other signals that are not on the frequency you want to receive. Indeed, if a strong signal adjacent to you is getting through, you wouldn't hear the one you're trying to receive, but let's ignore that, since it's the wrong answer.

Total Harmonic Distortion (THD) is a popular term that describes how well an amplifier reproduces its input. It's usually applied to high-end audio amplifiers. In this case, however, they're asking about detecting a signal, not how well an amplifier reproduces a signal.

And linearity refers to getting a proportional increase or decrease when you vary something, like the gain of an amplifier - this is a topic covered on the General exam, but not on the Technician exam. Besides, linearity isn't about detecting, but about how well the output of a circuit matches the shape of the input to the circuit.

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What is a transceiver?
A device that combines a receiver and transmitter
• A device for matching feed line impedance to 50 ohms
• A device for automatically sending and decoding Morse code
• A device for converting receiver and transmitter frequencies to another band

A transmitter is used to convert information to radio waves, and a receiver is used to convert radio waves back into information. Your radio in your car used to listen to music is an example of a receiver, where the station broadcasting the music has a transmitter that is tuned to the same frequency your car radio is.
A transceiver is ideal for 2-way communication because it facilitates both the talking and listening aspects of communication. Transceivers are the more common in ham radio than dedicated transmitters, although many hams have receive-only sets as well.

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Which of the following is used to convert a signal from one frequency to another?
• Phase splitter
Mixer
• Inverter
• Amplifier

A mixer allows two signals to be combined such that there will be an output signal at the sum and difference of the two signals.

For instance, if you mix a 14.250 MHz signal with a 2.4 kHz signal, there will be an output at 14.2524 MHz and 14.2476 MHz.

Sum of 14.250 MHz + 2.4 kHz

• = 14250000 Hz + 2400 Hz
• = 14252400 Hz
• = 14.2524 MHz

Difference of 14.250 MHz - 2.4 kHz

• = 14250000 Hz - 2400 Hz
• = 14247600 Hz
• = 14.2476 MHz

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Which term describes the ability of a receiver to discriminate between multiple signals?
• Discrimination ratio
• Sensitivity
Selectivity
• Harmonic distortion

Selectivity is the ability of a receiver to discriminate a desired signal from other adjacent signals. If the IF (Intermediate Frequency) circuits do not filter out adjacent signals then they will also be detected and interfere with the desired signal. To help learn this remember: We need very good Selectivity in order to Discriminate the signal properly.

The distractor answers may need some explanation so you know why they are not the correct answer:

• Tuning rate - Rate means speed, how fast the tuner can change frequencies.
• Sensitivity - this is a measure of how well weak signals can be amplified and picked up out of noise.
• Noise floor - this is the noise energy that hides or obscures our signal; if the signal is less than the noise it cannot be seen just like a floor hides what is below it. Or it is the lowest level that can be detected.

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What is the name of a circuit that generates a signal at a specific frequency?
• Reactance modulator
• Phase modulator
• Low-pass filter
Oscillator

Oscillators generate a signal of a desired frequency.

Reactance modulators are circuits that are used to add voice information to radio signals - they don't generate the signal.

A product detector is a circuit in a receiver - it doesn't generate a radio signal.

And a low pass filter keeps radio frequency energy above a certain cut-off level from passing through it, but it doesn't generate RF energy, so it doesn't generate any sort of signal.

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What device converts the RF input and output of a transceiver to another band?
• High-pass filter
• Low-pass filter
Transverter
• Phase converter

This question wants to know what electrical circuit converts a lower frequency Single-Side-Band (SSB) frequency to a higher frequency. This is done by a Transverter. The other choices will not convert one frequency to another.

• A high-pass filter will reduce a signal's amplitude if it is not higher than the corner frequency of the filter.
• A low-pass filter will reduce a signal's amplitude if it is not lower than the corner frequency of the filter.
• A phase converter will alter the phase of a signal but the frequency will stay the same.

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What is the function of a transceiver’s PTT input?
• Input for a key used to send CW
Switches transceiver from receive to transmit when grounded
• Provides a transmit tuning tone when grounded
• Input for a preamplifier tuning tone

PTT stands for “Push To Talk.” Its function is as it says: it is used to talk to other amateur radio users on the set frequency. Your transceiver will be receiving signals on the frequency until the PTT input is pushed, which will switch to transmitting mode.

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Which of the following describes combining speech with an RF carrier signal?
• Impedance matching
• Oscillation
Modulation
• Low-pass filtering

A Modulator is the circuit that "modulates" or encodes the RF carrier frequency with the speech signal in a transmitter. The keyword "combines" indicates that the voice and carrier are put together, which should help you envision a circuit that modulates the signal.

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What is the function of the SSB/CW-FM switch on a VHF power amplifier?
• Change the mode of the transmitted signal
Set the amplifier for proper operation in the selected mode
• Change the frequency range of the amplifier to operate in the proper segment of the band
• Reduce the received signal noise

A RF power amplifier must operate differently when amplifying a FM vs an AM (such as SSB or CW) signal. VHF power amplifiers are more likely to be used with both modes by different people who may need both functions.

Therefore the function of the SSB/CW-FM (Single Side Band/Continuous Wave <-> Frequency Modulation) switch on a VHF power amplifier is to set the amplifier for proper operation in the selected mode.

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What device increases the transmitted output power from a transceiver?
• A voltage divider
An RF power amplifier
• An impedance network
• All these choices are correct

This one is probably pretty obvious if you think about it; an RF power amplifier amplifies (or increases) an RF signal from another device.

Note that a voltage regulator (the most common incorrect response to this question) only regulates the voltage, and does not directly affect power. A voltage regulator might be used as part of a power supply to keep the output voltage constant.

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Where is an RF preamplifier installed?