The Radio Transmitter
In the transmitter block diagram shown, the "oscillator"
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is variable in frequency
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generates an audio frequency tone during tests
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Correct Answeruses a crystal for good frequency stability
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may have a calibrated dial
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In the transmitter block diagram shown, the "balanced modulator"
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balances the high and low frequencies in the audio signal
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Correct Answerperforms double sideband suppressed carrier modulation
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acts as a tone control
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balances the standing wave ratio
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In the transmitter block diagram shown, the "filter"
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removes mains hum from the audio signal
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suppresses unwanted harmonics of the RF signal
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Correct Answerremoves one sideband from the modulated signal
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removes the carrier component from the modulated signal
Correct answer: removes one sideband from the modulated signal
In an SSB transmitter, the balanced modulator produces a double sideband suppressed carrier (DSB-SC) signal. This signal contains both the upper and lower sidebands, but only one sideband is desired for transmission.
The filter following the balanced modulator is a narrow bandpass filter that selects the wanted sideband and rejects the unwanted sideband. This process creates a true single sideband (SSB) signal.
- removes mains hum from the audio signal is the function of audio filtering in the microphone or speech amplifier stages, not the RF filter shown here.
- suppresses unwanted harmonics of the RF signal is typically done by output filtering after the power amplifier, not at this stage of modulation.
- removes the carrier component from the modulated signal is performed by the balanced modulator itself, not by the filter.
Therefore, the filter’s purpose in this block diagram is to remove one sideband from the modulated signal.
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In the transmitter block diagram shown, the "mixer"
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adds the correct proportion of carrier to the SSB signal
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mixes the audio and RF signals in the correct proportions
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Correct Answertranslates the SSB signal to the required frequency
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mixes the two sidebands in the correct proportions
Correct answer: translates the SSB signal to the required frequency
In this SSB transmitter, the balanced modulator and filter first generate a clean single sideband signal at an intermediate frequency. The mixer then combines this SSB signal with the signal from the VFO.
Mixing produces sum and difference frequencies, effectively shifting the SSB signal to the desired transmit frequency band. This process is called frequency translation or upconversion.
- adds the correct proportion of carrier to the SSB signal is incorrect, the carrier remains suppressed in an SSB transmitter unless deliberately reinserted elsewhere.
- mixes the audio and RF signals in the correct proportions describes the function of the balanced modulator earlier in the chain, not the mixer.
- mixes the two sidebands in the correct proportions is incorrect because only one sideband exists after the filter stage.
Therefore, the mixer’s role is to translate the SSB signal to the required frequency.
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In the transmitter block diagram shown, the "linear amplifier"
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has all components arranged in-line
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Correct Answeramplifies the modulated signal with no distortion
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aligns the two sidebands correctly
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removes any unwanted amplitude modulation from the signal
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In the transmitter block diagram shown, the "VFO" is
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a voice frequency oscillator
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a varactor fixed oscillator
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a virtual faze oscillator
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Correct Answera variable frequency oscillator
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In the transmitter block diagram shown, the "master oscillator" produces
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Correct Answera steady signal at the required carrier frequency
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a pulsating signal at the required carrier frequency
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a 800 Hz signal to modulate the carrier
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a modulated CW signal
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In the transmitter block diagram shown, the "driver buffer"
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filters any sharp edges from the input signal
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drives the power amplifier into saturation
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Correct Answerprovides isolation between the oscillator and power amplifier
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changes the frequency of the master oscillator signal
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In the transmitter block diagram shown, the "Morse key"
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turns the DC power to the transmitter on and off
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Correct Answerallows the oscillator signal to pass only when the key is depressed
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changes the frequency of the transmitted signal when the key is depressed
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adds an 800 Hz audio tone to the signal when the key is depressed
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In the transmitter block diagram shown, the "power amplifier"
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Correct Answerneed not have linear characteristics
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amplifies the bandwidth of its input signal
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must be adjusted during key-up conditions
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should be water-cooled
Correct answer: need not have linear characteristics
In a CW (continuous wave) transmitter, the signal is a single RF carrier that is simply switched on and off by the Morse key. There is no amplitude or phase modulation that must be preserved.
Because the waveform contains no modulation information, the power amplifier does not need to operate linearly. It can be run in a more efficient non-linear class (such as Class C) without distorting the transmitted signal.
- amplifies the bandwidth of its input signal is incorrect, amplifiers increase power or voltage, not bandwidth.
- must be adjusted during key-up conditions is incorrect, tuning and adjustment are normally done with the transmitter keyed and producing RF.
- should be water-cooled depends on power level and design, not on the basic function of a CW power amplifier.
Therefore, in a CW transmitter the power amplifier need not have linear characteristics.
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In the transmitter block diagram shown, the "speech amplifier"
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Correct Answeramplifies the audio signal from the microphone
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is a spectral equalization entropy changer
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amplifies only speech, while discriminating against background noises
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shifts the frequency spectrum of the audio signal into the RF region
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In the transmitter block diagram shown, the "modulator"
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is an amplitude modulator with feedback
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is an SSB modulator with feedback
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causes the speech waveform to gate the oscillator on and off
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Correct Answercauses the speech waveform to shift the frequency of the oscillator
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In the transmitter block diagram shown, the "oscillator" is
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an audio frequency oscillator
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Correct Answera variable frequency RF oscillator
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a beat frequency oscillator
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a variable frequency audio oscillator
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In the transmitter block diagram shown, the "frequency multiplier"
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translates the frequency of the modulated signal into the RF spectrum
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changes the frequency of the speech signal
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Correct Answerproduces a harmonic of the oscillator signal
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multiplies the oscillator signal by the speech signal
The Frequency Multiplier stage is an RF amplifier with a tuned output - the output tuned to a harmonic of the input signal.
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In the transmitter block diagram shown, the "power amplifier"
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increases the voltage of the mains to drive the antenna
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amplifies the audio frequency component of the signal
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amplifies the selected sideband to a suitable level
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Correct Answeramplifies the RF signal to a suitable level
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