Receivers.
Subelement L08
Section L08
What are the advantages of the frequency conversion process in a superheterodyne receiver?
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Automatic soft-limiting and automatic squelching
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Automatic squelching and increased sensitivity
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Correct AnswerIncreased selectivity and optimal tuned circuit design
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Automatic detection in the RF amplifier and increased sensitivity
Down-converting the operating frequency to a lower Intermediate Frequency facilitates selectivity: for example, 0.6% of 455 kilohertz is 2.7 kHz, 0.6% of 3.7 megahertz is 22 kHz.
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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What factors should be considered when selecting an intermediate frequency?
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Interference to other services
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Cross-modulation distortion and interference
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Correct AnswerImage rejection and responses to unwanted signals
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Noise figure and distortion
Two frequencies, one above and one below the Local Oscillator frequency, can produce a mixing result at the Intermediate Frequency. The one resulting in the unwanted product is the Image Frequency. With a low Intermediate Frequency, selectivity and gain are easier to achieve but image rejection suffers. With a high Intermediate Frequency, image rejection is facilitated but selectivity is more difficult to achieve. A Double-conversion receiver deals with image rejection with an initial conversion and restores selectivity with a subsequent down-conversion. Two conversions, however, expose the designer to twice the risk of spurious responses due to spurious oscillations in local oscillators.
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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One of the greatest advantages of the double-conversion over the single-conversion receiver is that it:
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is much more stable
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is much more sensitive
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produces a louder signal at the output
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Correct Answergreater reduction of image interference for a given front end selectivity
Two frequencies, one above and one below the Local Oscillator frequency, can produce a mixing result at the Intermediate Frequency. The one resulting in the unwanted product is the Image Frequency. With a low Intermediate Frequency, selectivity and gain are easier to achieve but image rejection suffers. With a high Intermediate Frequency, image rejection is facilitated but selectivity is more difficult to achieve. A Double-conversion receiver deals with image rejection with an initial conversion and restores selectivity with a subsequent down-conversion. Two conversions, however, expose the designer to twice the risk of spurious responses due to spurious oscillations in local oscillators.
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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In a communications receiver, a crystal filter would be located in the:
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Correct AnswerIF circuits
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local oscillator
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audio output stage
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detector
The Intermediate Frequency chain is responsible for most of the selectivity. Crystal filters or mechanical filters can be used at the Intermediate Frequency. Digital Signal Processing (DSP) is used in modern receivers.
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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A multiple conversion superheterodyne receiver is more susceptible to spurious responses than a single-conversion receiver because of the:
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Correct Answeradditional oscillators and mixing frequencies involved in the design
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poorer selectivity in the IF caused by the multitude of frequency changes
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greater sensitivity introducing higher levels of RF to the receiver
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AGC being forced to work harder causing the stages concerned to overload
Two frequencies, one above and one below the Local Oscillator frequency, can produce a mixing result at the Intermediate Frequency. The one resulting in the unwanted product is the Image Frequency. With a low Intermediate Frequency, selectivity and gain are easier to achieve but image rejection suffers. With a high Intermediate Frequency, image rejection is facilitated but selectivity is more difficult to achieve. A Double-conversion receiver deals with image rejection with an initial conversion and restores selectivity with a subsequent down-conversion. Two conversions, however, expose the designer to twice the risk of spurious responses due to spurious oscillations in local oscillators.
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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In a dual-conversion superheterodyne receiver what are the respective aims of the first and second conversion:
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selectivity and dynamic range
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image rejection and noise figure
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Correct Answerimage rejection and selectivity
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selectivity and image rejection
The first conversion to a high IF places the image frequency far away from the operating frequency so it can be optimally rejected by the front-end filtering. The second conversion to a low IF performs the traditional function of ensuring selectivity to protect the receiver from adjacent channels.
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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Which stage of a receiver has its input and output circuits tuned to the received frequency?
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The detector
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Correct AnswerThe RF amplifier
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The local oscillator
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The audio frequency amplifier
Which stage of a superheterodyne receiver lies between a tuneable stage and a fixed tuned stage?
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Local oscillator
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Correct AnswerMixer
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Radio frequency amplifier
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Intermediate frequency amplifier
The superheterodyne concept is based on converting the operating frequency to a fixed Intermediate Frequency: the Mixer performs that function by combining the output of the tuneable RF amplifier with the Local Oscillator signal to feed the fixed-tuned Intermediate Frequency chain.
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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A single conversion receiver with a 9 MHz IF has a local oscillator operating at 16 MHz. The frequency it is tuned to is:
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Correct Answer7 MHz
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16 MHz
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21 MHz
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9 MHz
In a superheterodyne receiver, injection from the Local Oscillator can be above or below the operating frequency. There could be two answers: Local Oscillator minus Intermediate Frequency or Local Oscillator plus Intermediate Frequency.
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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A double conversion receiver designed for SSB reception has a beat frequency oscillator and:
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one IF stage and one local oscillator
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two IF stages and three local oscillators
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two IF stages and one local oscillator
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Correct Answertwo IF stages and two local oscillators
The advantage of a double conversion receiver over a single conversion receiver is that it:
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produces a louder audio signal
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Correct Answersuffers less from image interference for a given front end sensitivity
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does not drift off frequency
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is a more sensitive receiver
Two frequencies, one above and one below the Local Oscillator frequency, can produce a mixing result at the Intermediate Frequency. The one resulting in the unwanted product is the Image Frequency. With a low Intermediate Frequency, selectivity and gain are easier to achieve but image rejection suffers. With a high Intermediate Frequency, image rejection is facilitated but selectivity is more difficult to achieve. A Double-conversion receiver deals with image rejection with an initial conversion and restores selectivity with a subsequent down-conversion. Two conversions, however, expose the designer to twice the risk of spurious responses due to spurious oscillations in local oscillators.
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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The mixer stage of a superheterodyne receiver is used to:
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Correct Answerchange the frequency of the incoming signal to that of the IF
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allow a number of IF frequencies to be used
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remove image signals from the receiver
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produce an audio frequency for the speaker
The superheterodyne concept is based on converting the operating frequency to a fixed Intermediate Frequency: the Mixer performs that function by combining the output of the tuneable RF amplifier with the Local Oscillator signal to feed the fixed-tuned Intermediate Frequency chain.
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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A superheterodyne receiver designed for SSB reception must have a beat-frequency oscillator (BFO) because:
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Correct Answerthe suppressed carrier must be replaced for detection
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it phases out the unwanted sideband signal
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it reduces the pass-band of the IF stages
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it beats with the receiver carrier to produce the missing sideband
The Beat Frequency Oscillator feeds the Product Detector for CW and SSB detection. Mixing the Intermediate Frequency with the BFO signal in the Product Detector produces an audio output. In Single Sideband, it is said to "reinsert the carrier" as it recreates a reference at the exact frequency at which the carrier, suppressed at the transmitter, would have appeared out of the Intermediate Frequency chain.
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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The first mixer in the receiver mixes the incoming signal with the local oscillator to produce:
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an audio frequency
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a radio frequency
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a high frequency oscillator (HFO) frequency
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Correct Answeran intermediate frequency
The superheterodyne concept is based on converting the operating frequency to a fixed Intermediate Frequency: the Mixer performs that function by combining the output of the tuneable RF amplifier with the Local Oscillator signal to feed the fixed-tuned Intermediate Frequency chain.
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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If the incoming signal to the mixer is 3 600 kHz and the first IF is 9 MHz, at which one of the following frequencies would the local oscillator (LO) operate?
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10 600 kHz
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21 600 kHz
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Correct Answer5 400 kHz
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3 400 kHz
In a superheterodyne receiver, injection from the Local Oscillator can be above or below the operating frequency. There could be two answers: Intermediate Frequency plus operating frequency or Intermediate Frequency minus operating frequency.
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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The BFO is off-set slightly (500 - 1 500 Hz) from the incoming signal to the detector. This is required:
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to protect the incoming signal from interference
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Correct Answerto beat with the incoming signal
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to pass the signal without interruption
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to provide additional amplification
The Beat Frequency Oscillator feeds the Product Detector for CW and SSB detection. Mixing the Intermediate Frequency with the BFO signal in the Product Detector produces an audio output. In Single Sideband, it is said to "reinsert the carrier" as it recreates a reference at the exact frequency at which the carrier, suppressed at the transmitter, would have appeared out of the Intermediate Frequency chain.
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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It is very important that the oscillators contained in a superheterodyne receiver are:
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sensitive and selective
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stable and sensitive
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selective and spectrally pure
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Correct Answerstable and spectrally pure
Oscillators need to be free of drift regardless of voltage and temperature variations or mechanical vibrations. Spectral purity is the absence of harmonics, other spurious oscillations or noise; purity limits spurious responses in the subsequent mixing processes.
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In a superheterodyne receiver, a stage before the IF amplifier has a variable capacitor in parallel with a trimmer capacitor and an inductance. The variable capacitor is for:
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tuning of the beat-frequency oscillator (BFO)
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tuning both the antenna and the LO
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Correct Answertuning of the local oscillator (LO)
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tuning both the antenna and the BFO
Two stages may require tuning ahead of the Intermediate Frequency amplifier: the preselector and the high-frequency oscillator, commonly known as the Local Oscillator. As the question alludes to one trimmer and one inductance, only one circuit can be tuned.
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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In a superheterodyne receiver without an RF amplifier, the input to the mixer stage has a variable capacitor in parallel with an inductance. The variable capacitor is for:
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tuning the beat-frequency oscillator
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tuning both the antenna and the local oscillator
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Correct Answertuning the receiver preselector to the reception frequency
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tuning both the antenna and the beat-frequency oscillator
What receiver stage combines a 14.25-MHz input signal with a 13.795-MHz oscillator signal to produce a 455-kHz intermediate frequency (IF) signal?
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Correct AnswerMixer
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BFO
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VFO
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Multiplier
The superheterodyne concept is based on converting the operating frequency to a fixed Intermediate Frequency: the Mixer performs that function by combining the output of the tuneable RF amplifier with the Local Oscillator signal to feed the fixed-tuned Intermediate Frequency chain.
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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Which two stages in a superheterodyne receiver have input tuned circuits tuned to the same frequency?
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IF and local oscillator
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RF and IF
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RF and local oscillator
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Correct AnswerRF and first mixer
Through an elimination process, only one answer makes sense. The input and the output of the radio-frequency amplifier run at the operating frequency. The output of the RF amplifier constitutes the input to the Mixer.
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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The mixer stage of a superheterodyne receiver:
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acts as a buffer stage
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demodulates SSB signals
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Correct Answerproduces an intermediate frequency
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produces spurious signals
The superheterodyne concept is based on converting the operating frequency to a fixed Intermediate Frequency: the Mixer performs that function by combining the output of the tuneable RF amplifier with the Local Oscillator signal to feed the fixed-tuned Intermediate Frequency chain.
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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What is meant by the noise floor of a receiver?
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Correct AnswerThe weakest signal that can be detected above the receiver internal noise
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The weakest signal that can be detected under noisy atmospheric conditions
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The minimum level of noise that will overload the receiver RF amplifier stage
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The amount of noise generated by the receiver local oscillator
A receiver's "Noise Floor" is the power level at which an incoming signal exhibits a Signal-To-Noise ratio of zero decibel: that is, the signal power equals the internal noise power level. Noise Floor is evaluated while measuring "Minimum Discernible Signal (MDS)". The "Noise Figure" of a receiver is a comparison of Signal-to-Noise ratio at the input and Signal-to-Noise ratio at the output; it assesses the degradation in Signal-to-Noise ratio caused by added noise. A low Noise Figure suggests that little noise was added internally and is a hallmark of sensitivity. The front-end of the receiver, where signals are weakest, is responsible for the noise performance of a receiver.
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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Which of the following is a purpose of the first IF amplifier stage in a receiver?
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Correct AnswerTo improve selectivity and gain
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To tune out cross-modulation distortion
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To increase dynamic response
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To improve noise figure performance
The Intermediate Frequency chain is responsible for the selectivity and a large part of the gain. The front-end of the receiver, where signals are weakest, is responsible for the noise performance of a receiver.
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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How much gain should be used in the RF amplifier stage of a receiver?
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Correct AnswerSufficient gain to allow weak signals to overcome noise generated in the first mixer stage
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As much gain as possible, short of self-oscillation
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It depends on the amplification factor of the first IF stage
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Sufficient gain to keep weak signals below the noise of the first mixer stage
The Radio-Frequency Amplifier should only introduce enough gain to override the internal noise of the subsequent Mixer. Too much gain will degrade Dynamic Range; Dynamic Range is broadly defined as a ratio between the strongest signals that can be tolerated near the passband and the "Minimum Discernible 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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What is the primary purpose of an RF amplifier in a receiver?
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To provide most of the receiver gain
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Correct AnswerTo improve the receiver noise figure
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To vary the receiver image rejection by using the AGC
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To develop the AGC voltage
The Intermediate Frequency chain is responsible for the selectivity and a large part of the gain. The front-end of the receiver, where signals are weakest, is responsible for the noise performance of a receiver.
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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How is receiver sensitivity often expressed for UHF FM receivers?
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Noise Figure in decibels
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Overall gain in decibels
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Correct AnswerRF level for 12 dB SINAD
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RF level for a given Bit Error Rate (BER)
The SINAD (signal + noise + distortion over noise + distortion) ratio takes the SNR (signal + noise over noise ratio) one step further by including distortion. A 12 dB SINAD ratio ensures that speech remains intelligible. Sensitivity expressed in those terms is the lowest RF level that will produce a usable message. The RF signal generator must be calibrated so the number of microvolts is precisely determined. Total Harmonic Distortion compares unwanted harmonic components added to the desired fundamental frequency, an audio tone in this instance.
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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What is the term used for the decibel difference (or ratio) between the largest tolerable receiver input signal (without causing audible distortion products) and the minimum discernible signal (sensitivity)?
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Stability
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Noise figure
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Correct AnswerDynamic range
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Design parameter
Dynamic Range is broadly defined as a ratio between the strongest signals that can be tolerated near the passband and the "Minimum Discernible Signal". The "Noise Figure" of a receiver is a comparison of Signal-to-Noise ratio at the input and Signal-to-Noise ratio at the output; it assesses the degradation in Signal-to-Noise ratio caused by added noise. A low Noise Figure suggests that little noise was added internally and is a hallmark of sensitivity. The front-end of the receiver, where signals are weakest, is responsible for the noise performance of a receiver.
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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The lower the receiver noise figure becomes, the greater will be the receiver's _________:
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Correct Answersensitivity
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rejection of unwanted signals
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selectivity
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stability
The "Noise Figure" of a receiver is a comparison of Signal-to-Noise ratio at the input and Signal-to-Noise ratio at the output; it assesses the degradation in Signal-to-Noise ratio caused by added noise. A low Noise Figure suggests that little noise was added internally and is a hallmark of sensitivity. The front-end of the receiver, where signals are weakest, is responsible for the noise performance of a receiver.
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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The noise generated in a receiver of good design originates in the:
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detector and AF amplifier
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BFO and detector
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IF amplifier and detector
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Correct AnswerRF amplifier and mixer
The "Noise Figure" of a receiver is a comparison of Signal-to-Noise ratio at the input and Signal-to-Noise ratio at the output; it assesses the degradation in Signal-to-Noise ratio caused by added noise. A low Noise Figure suggests that little noise was added internally and is a hallmark of sensitivity. The front-end of the receiver, where signals are weakest, is responsible for the noise performance of a receiver.
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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Why are very low noise figures relatively unimportant for a high frequency receiver?
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The use of SSB and CW on the HF bands overcomes the noise
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Regardless of the front end, the succeeding stages when used on HF are very noisy
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Correct AnswerExternal HF noise, man-made and natural, are higher than the internal noise generated by the receiver
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Ionospheric distortion of the received signal creates high noise levels
Below 30 megahertz, the antenna picks-up atmospheric noise and man-made noise at levels far more important than internal noise. As the frequency of operation rises, those types of noise become less prevalent. On Ultra High Frequencies (UHF) and above, the internal noise becomes the limiting factor in receiving weak signals. The front-end of the receiver, where signals are weakest, is responsible for the noise performance of a receiver: for weak signal work on 2 metres and up, more attention must be placed on reducing internal noise in the front-end.
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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The term which relates specifically to the amplitude levels of multiple signals that can be accommodated during reception is called:
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noise figure
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Correct Answerdynamic range
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AGC
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cross-modulation index
Dynamic Range is broadly defined as a ratio between the strongest signals that can be tolerated near the passband and the "Minimum Discernible Signal". Other related notions include Blocking Dynamic Range and Intermodulation Dynamic Range. Blocking Dynamic Range measures how much of a single strong off-channel signal can be tolerated while receiving a weak signal, it is a measure of desensitization or immunity to overload. Intermodulation Dynamic Range verifies how strong two off-channel signals can be without spurious responses being generated in the receiver, it is a measure of resistance to intermodulation.
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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Normally, front-end selectivity is provided by the resonant networks both before and after the RF stage in a superheterodyne receiver. This whole section of the receiver is often referred to as the:
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pass-selector
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Correct Answerpreselector
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preamble
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preamplifier
A preselector is a tuned stage which passes a desired range of signals to a receiver: it ensures a certain preliminary selection. It may or may not be amplified; in other words, active or passive.
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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What audio shaping network is added at an FM receiver to restore proportionally attenuated lower audio frequencies?
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A heterodyne suppressor
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Correct AnswerA de-emphasis network
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A pre-emphasis network
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An audio prescaler
With "true" FM, deviation is independent of modulating frequency, actual deviation is determined solely by the modulating amplitude. With Phase Modulation, deviation depends on the amount of phase shift and its rapidity, increasing modulating frequency results in proportionally more deviation even if amplitude is held constant. Because commercial standards were based on Phase Modulation, an FM transmitter requires an artificial boost in high frequency response so that PM and FM sound the same at the receiver. A pre-emphasis network tailors the frequency response in the FM transmitter. De-emphasis is employed in the receiver to restore a flat audio response.
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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What does a product detector do?
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It amplifies and narrows band-pass frequencies
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It detects cross-modulation products
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Correct AnswerIt mixes an incoming signal with a locally generated carrier
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It provides local oscillations for input to a mixer
The Beat Frequency Oscillator feeds the Product Detector for CW and SSB detection. Mixing the Intermediate Frequency with the BFO signal in the Product Detector produces an audio output. In Single Sideband, it is said to "reinsert the carrier" as it recreates a reference at the exact frequency at which the carrier, suppressed at the transmitter, would have appeared out of the Intermediate Frequency chain.
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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Distortion in a receiver that only affects strong signals usually indicates a defect in or mis-adjustment of the:
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RF amplifier
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Correct Answerautomatic gain control (AGC)
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IF amplifier
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AF amplifier
In a superheterodyne receiver with automatic gain control (AGC), as the strength of the signal increases, the AGC:
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distorts the signal
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introduces limiting
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Correct Answerreduces the receiver gain
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increases the receiver gain
The AGC (Automatic Gain Control) circuit reduces receiver gain as signal strength increases. AGC can be "IF-derived", some say "RF-Derived" (a slight misnomer), by sampling the output of the last Intermediate Frequency stage or "AF-derived" by sampling the output of the detector. The resulting control voltage is applied to the Intermediate Frequency amplifiers and, sometimes, Radio-Frequency amplifier.
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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The amplified IF signal is applied to the ____________ stage in a superheterodyne receiver:
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LO
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Correct Answerdetector
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RF amplifier
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audio output
The low-level output of a detector is:
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fed directly to the speaker
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applied to the RF amplifier
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Correct Answerapplied to the AF amplifier
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grounded via the chassis
The overall output of an AM/CW/SSB receiver can be adjusted by means of manual controls on the receiver or by use of a circuit known as:
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automatic load control
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Correct Answerautomatic gain control
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automatic frequency control
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inverse gain control
The AGC (Automatic Gain Control) circuit reduces receiver gain as signal strength increases. AGC can be "IF-derived", some say "RF-Derived" (a slight misnomer), by sampling the output of the last Intermediate Frequency stage or "AF-derived" by sampling the output of the detector. The resulting control voltage is applied to the Intermediate Frequency amplifiers and, sometimes, Radio-Frequency amplifier.
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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AGC voltage is applied to the:
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Correct AnswerRF and IF amplifiers
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AF and IF amplifiers
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RF and AF amplifiers
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detector and AF amplifiers
The AGC (Automatic Gain Control) circuit reduces receiver gain as signal strength increases. AGC can be "IF-derived", some say "RF-Derived" (a slight misnomer), by sampling the output of the last Intermediate Frequency stage or "AF-derived" by sampling the output of the detector. The resulting control voltage is applied to the Intermediate Frequency amplifiers and, sometimes, Radio-Frequency amplifier.
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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AGC is derived in a receiver from one of two circuits. Depending on the method used, it is called:
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IF derived or RF derived
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detector derived or audio derived
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Correct AnswerIF derived or audio derived
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RF derived or audio derived
The AGC (Automatic Gain Control) circuit reduces receiver gain as signal strength increases. AGC can be "IF-derived", some say "RF-Derived" (a slight misnomer), by sampling the output of the last Intermediate Frequency stage or "AF-derived" by sampling the output of the detector. The resulting control voltage is applied to the Intermediate Frequency amplifiers and, sometimes, Radio-Frequency amplifier.
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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Which two variables primarily determine the behaviour of an automatic gain control (AGC) loop?
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Blanking level and slope
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Slope and bandwidth
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Clipping level and hang time
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Correct AnswerThreshold and decay time
The AGC threshold is the level in the monitored circuit at which the AGC circuit begins to reduce gain. The AGC decay time determines how quickly gain is restored once the strong signal disappears.
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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What circuit combines signals from an IF amplifier stage and a beat-frequency oscillator (BFO), to produce an audio signal?
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A VFO circuit
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Correct AnswerA product detector circuit
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An AGC circuit
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A power supply circuit
The Beat Frequency Oscillator feeds the Product Detector for CW and SSB detection. Mixing the Intermediate Frequency with the BFO signal in the Product Detector produces an audio output. In Single Sideband, it is said to "reinsert the carrier" as it recreates a reference at the exact frequency at which the carrier, suppressed at the transmitter, would have appeared out of the Intermediate Frequency chain.
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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What part of a superheterodyne receiver determines the image rejection ratio of the receiver?
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AGC loop
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IF filter
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Correct AnswerRF amplifier pre-selector
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Product detector
The Image is the other frequency that can successfully mix with the Local Oscillator and produce an output out of the Mixer at the Intermediate frequency. Selectivity ahead of the Mixer must be employed to prevent that signal from reaching the Mixer.
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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What is the term for the reduction in receiver sensitivity caused by a strong signal near the received frequency?
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Correct AnswerDesensitization
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Cross-modulation interference
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Squelch gain rollback
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Quieting
Desensitization is a symptom of front-end overload where a strong adjacent off-channel signal provokes a drop in receiver sensitivity. The only cure for desensitization is to keep the offending signal out of the receiver. Other manifestations of front-end overload are intermodulation and cross-modulation where strong signals push the RF amplifier or Mixer into non-linear operation resulting in spurious responses.
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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What causes receiver desensitization?
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Squelch gain adjusted too high
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Squelch gain adjusted too low
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Audio gain adjusted too low
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Correct AnswerStrong near frequency signals
Desensitization is a symptom of front-end overload where a strong adjacent off-channel signal provokes a drop in receiver sensitivity. The only cure for desensitization is to keep the offending signal out of the receiver. Other manifestations of front-end overload are intermodulation and cross-modulation where strong signals push the RF amplifier or Mixer into non-linear operation resulting in spurious responses.
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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What is one way receiver desensitization can be reduced?
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Increase the transmitter audio gain
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Correct AnswerUse a cavity filter
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Decrease the receiver squelch gain
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Increase the receiver bandwidth
Desensitization is a symptom of front-end overload where a strong adjacent off-channel signal provokes a drop in receiver sensitivity. The only cure for desensitization is to keep the offending signal out of the receiver. Other manifestations of front-end overload are intermodulation and cross-modulation where strong signals push the RF amplifier or Mixer into non-linear operation resulting in spurious responses.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
What causes intermodulation in an electronic circuit?
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Correct AnswerNonlinear circuits or devices
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Too little gain
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Positive feedback
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Lack of neutralization
Desensitization is a symptom of front-end overload where a strong adjacent off-channel signal provokes a drop in receiver sensitivity. The only cure for desensitization is to keep the offending signal out of the receiver. Other manifestations of front-end overload are intermodulation and cross-modulation where strong signals push the RF amplifier or Mixer into non-linear operation resulting in spurious responses.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
Which of the following is an important reason for using a VHF intermediate frequency in an HF receiver?
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To provide a greater tuning range
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To tune out cross-modulation distortion
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To prevent the generation of spurious mixer products
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Correct AnswerTo move the image response far away from the filter passband
Whether injection from the Local Oscillator is above or below the operating frequency, the Image Frequency is always separated from the operating frequency by twice the Intermediate Frequency. A very high Intermediate Frequency moves the Image well out of the preselector bandpass.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
Intermodulation interference is produced by:
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the mixing of more than one signal in the first or second intermediate frequency amplifiers of a receiver
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Correct Answerthe mixing of two or more signals in the front-end of a superheterodyne receiver
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the interaction of products from high-powered transmitters in the area
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the high-voltage stages in the final amplifier of an amplitude or frequency-modulated transmitter
Desensitization is a symptom of front-end overload where a strong adjacent off-channel signal provokes a drop in receiver sensitivity. The only cure for desensitization is to keep the offending signal out of the receiver. Other manifestations of front-end overload are intermodulation and cross-modulation where strong signals push the RF amplifier or Mixer into non-linear operation resulting in spurious responses.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
Which of the following is NOT a direct cause of instability in a receiver?
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Correct AnswerDial display accuracy
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Mechanical rigidity
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Feedback components
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Temperature variations
Key words: NOT A DIRECT CAUSE. Temperature variations, voltage variations and movements due to mechanical stresses will cause changes in frequency. The selection of feedback components, notably their temperature coefficient, is paramount for stability. Dial accuracy is not instability per se.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
Poor frequency stability in a receiver usually originates in the:
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mixer
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Correct Answerlocal oscillator and power supply
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detector
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RF amplifier
Stability is the ability to stay on frequency despite other variations. The Local Oscillator indirectly sets the operating frequency. Temperature variations, voltage variations and movements due to mechanical stresses will cause changes in frequency.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
Poor dynamic range of a receiver can cause many problems when a strong signal appears within or near the front-end bandpass. Which of the following is NOT caused as a direct result?
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Cross-modulation
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Correct AnswerFeedback
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Desensitization
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Intermodulation
Desensitization is a symptom of front-end overload where a strong adjacent off-channel signal provokes a drop in receiver sensitivity. The only cure for desensitization is to keep the offending signal out of the receiver. Other manifestations of front-end overload are intermodulation and cross-modulation where strong signals push the RF amplifier or Mixer into non-linear operation resulting in spurious responses.
Original copyright; explanations transcribed with permission from Francois VE2AAY, author of the ExHAMiner exam simulator. Do not copy without his permission.
Tags: none
Which of these measurements is a good indicator of VHF receiver performance in an environment of strong out-of-band signals?
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Intermediate frequency rejection ratio
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Correct AnswerTwo-tone Third-Order IMD Dynamic Range, 10 MHz spacing
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Third-Order Intercept Point
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Blocking Dynamic Range
"The FM Two-tone, third-order dynamic range, 10-MHz offset ... is a wide-band dynamic-range test on VHF equipment, using two strong signals just outside the amateur band (usually the abode of nearby pager transmitters). (...) This test is a good indicator of relative IMD performance". (RFI - Intermodulation, ARRL, Ed Hare, W1RFI)
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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