Correct answer: stay tuned to the desired signal

Frequency stability refers to a receiver’s ability to maintain its tuning without drifting over time.

A stable receiver:

• remains on the selected frequency

• avoids unwanted changes due to temperature or component variation

• Tracking a drifting signal is a different function.

• Providing a frequency standard or digital readout is unrelated.

Therefore, frequency stability is the ability to stay tuned to the desired signal.

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Correct answer: C — its ability to receive weak signals

Receiver sensitivity is a measure of how well a receiver can detect and usefully demodulate very weak incoming signals. A highly sensitive receiver requires only a tiny amount of RF power at its input to produce an acceptable output signal-to-noise ratio. Sensitivity is typically quoted as the minimum discernible signal (MDS) or as the signal level required to achieve a specified SINAD or signal-to-noise ratio.

• A — the bandwidth of the RF preamplifier: Bandwidth determines which range of frequencies are passed, not how weak a signal can be detected. A wide or narrow preamplifier bandwidth affects selectivity and noise floor indirectly, but is not itself a definition of sensitivity.
• B — the stability of the oscillator: Oscillator stability relates to frequency drift and phase noise, which affect tuning accuracy and sideband noise. This is a separate receiver specification from sensitivity.
• D — its ability to reject strong signals: This describes selectivity or dynamic range — the receiver's ability to handle strong adjacent or interfering signals without overloading. That is a distinct specification from sensitivity.

Therefore, receiver sensitivity specifically describes the ability of a receiver to detect and recover weak signals, making option C correct.

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Correct answer: B — the least internally-generated noise

A receiver's ability to detect weak signals is ultimately limited by the noise generated within its own circuits — particularly in the front-end RF amplifier and mixer stages. This internally-generated noise sets a "noise floor" below which real signals cannot be distinguished. The receiver that produces the least internal noise has the lowest noise floor, and therefore can detect the weakest incoming signals.

• A — an RF gain control: An RF gain control allows the operator to reduce gain when strong signals are present, but it has no bearing on how weak a signal the receiver can detect. It does not reduce internally-generated noise.
• C — the loudest audio output: Audio output level is a function of amplification, not sensitivity. Turning up the volume amplifies noise and signal equally and does not improve the ability to pull out a weak signal.
• D — the greatest tuning range: Covering a wide range of frequencies says nothing about how sensitive the receiver is on any given frequency. A wide tuning range and low noise floor are independent characteristics.

Therefore, the receiver capable of detecting the weakest signals is the one with the least internally-generated noise, giving it the lowest noise floor and the best sensitivity.

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Correct answer: signal plus noise to noise ratio

Receiver sensitivity is commonly specified by the minimum signal required to achieve a usable output.

This is often expressed as a signal-plus-noise to noise ratio (S+N/N), indicating how much stronger the received signal is compared to the noise.

• IF bandwidth relates to selectivity.
• Audio output power is unrelated to sensitivity.
• Number of RF amplifiers does not directly define sensitivity.

Therefore, the relevant specification is signal plus noise to noise ratio.

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Correct answer: C — more signal and less noise

Receiver sensitivity describes how well a receiver can detect weak signals. A more sensitive receiver has a better signal-to-noise ratio (SNR) — it amplifies the desired signal more effectively while introducing less internal noise. This means the recovered audio or data is cleaner and stronger relative to the noise floor, allowing weaker transmissions to be heard clearly.

• A. more than one signal — Sensitivity relates to detecting weak signals, not to how many signals are received. Receiving multiple signals is a selectivity or filtering matter, not a sensitivity one.
• B. less signal and more noise — This describes a less sensitive receiver. Poor sensitivity means more internal noise drowns out weak signals, degrading the output.
• D. a steady oscillator drift — Oscillator stability is a separate characteristic of a receiver. Drift causes frequency instability and is unrelated to sensitivity.

Therefore, the more sensitive receiver produces more signal and less noise, resulting in a superior signal-to-noise ratio and the ability to receive weaker transmissions.

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Correct answer: D — selectivity

Selectivity describes how well a receiver can distinguish a wanted signal from other signals at nearby frequencies. A highly selective receiver rejects adjacent-channel interference effectively, allowing it to tune to one signal without being swamped by a neighbour just a few kilohertz away. Selectivity is primarily determined by the receiver's IF (intermediate frequency) filter characteristics.

• A. Noise figure — this describes how much noise the receiver itself adds to the signal, which affects the weakest signal detectable, not the ability to separate closely spaced signals.
• B. Sensitivity — this is the ability of a receiver to detect weak signals, not to separate signals close in frequency.
• C. Bandwidth — bandwidth describes the range of frequencies passed by a circuit or system; while related to selectivity, it is not the term for the ability to separate nearby signals.

Therefore, the correct term for a receiver's ability to separate signals close in frequency is selectivity.

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Correct answer: narrow bandwidth

Selectivity is the ability of a receiver to distinguish between closely spaced signals.

High selectivity requires:

• rejection of unwanted adjacent signals
• acceptance of only the desired signal

This is achieved by using a narrow bandwidth in the receiver’s filter stages.

• A wide bandwidth would allow more unwanted signals through.
• Tuning range refers to the range of frequencies the receiver can tune.

Therefore, a receiver with high selectivity has a narrow bandwidth.

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Correct answer: IF amplifier

The beat frequency oscillator (BFO) in a superheterodyne receiver generates a signal that is mixed with the received signal in the product detector so that audio can be recovered from signals that have no transmitted carrier, such as SSB and CW.

For this to work correctly, the BFO frequency must be very close to the receiver’s intermediate frequency (IF). The small difference between the BFO and IF produces the audible beat note or recovered audio.

Mathematically, the audio output frequency is:

\[ f_{audio} = | f_{IF} - f_{BFO} | \]

This requires \(f_{BFO}\) to be near \(f_{IF}\).

• RF amplifier operates at the incoming signal frequency, not the IF.
• audio amplifier operates at audio frequencies, far below RF or IF.
• local oscillator operates at a frequency offset from the RF to generate the IF, not at the IF itself.

Therefore, the BFO operates on a frequency nearest to that of the IF amplifier.

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Correct answer: beat with the IF signal to produce an audio tone

Morse code (CW) signals are transmitted as an interrupted carrier with no audio modulation.

In a superheterodyne receiver, the BFO (Beat Frequency Oscillator) injects a signal at or near the IF frequency so that it mixes with the received IF signal.

This mixing produces a difference frequency in the audio range, which can then be heard as a tone corresponding to the Morse code keying.

• The BFO does not produce IF signals.
• It does not beat with the local oscillator.
• It does not produce sidebands.

Therefore, the BFO is used to beat with the IF signal to produce an audio tone.

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Correct answer: single sideband suppressed carrier modulation

A product detector is used to demodulate signals that do not contain a transmitted carrier. It works by multiplying the received signal with a locally generated carrier (from a BFO or insertion oscillator) so the original audio can be recovered.

Single sideband suppressed carrier (SSB-SC) signals have the carrier removed at the transmitter, so the receiver must reinsert a carrier locally and use a product detector for proper demodulation.

• pulse modulation uses different detection methods depending on the pulse type and is not normally demodulated with a product detector.
• double sideband full carrier modulation (conventional AM) contains its own carrier and can be demodulated with a simple envelope detector.
• frequency modulation requires a frequency discriminator or phase detector, not a product detector.

Therefore, the transmission mode usually demodulated by a product detector is single sideband suppressed carrier modulation.

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Correct answer: replace the suppressed carrier for detection

In SSB transmission the carrier is intentionally suppressed to save power and bandwidth. A superheterodyne receiver therefore uses an insertion oscillator (also called a beat frequency oscillator, BFO) to locally recreate the missing carrier so the audio information can be recovered during detection.

The inserted carrier mixes with the received sideband and produces the original audio frequencies.

• phase out the unwanted sideband signal is done by filtering in the transmitter or receiver IF stages, not by the insertion oscillator.
• reduce the passband of the IF stages is the job of IF filters, not the oscillator.
• beat with the received carrier to produce the other sideband does not apply to SSB because there is no transmitted carrier to beat against.

Therefore, the purpose of the insertion oscillator is to replace the suppressed carrier so the signal can be detected.

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Correct answer: RF amplifier

In a receiver, the RF amplifier stage has both its input and output circuits tuned to the received radio frequency. This tuning improves selectivity and sensitivity by amplifying the desired signal while helping reject out-of-band signals before they reach the mixer.

• local oscillator generates a stable frequency for mixing and is not tuned to the received signal frequency.
• audio frequency amplifier operates only on recovered audio frequencies, not RF.
• detector demodulates the RF or IF signal and does not have tuned RF input and output circuits.

Therefore, the stage with input and output circuits tuned to the received frequency is the RF amplifier.

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Correct answer: D — increases the sensitivity of the receiver

In a superheterodyne (superhet) receiver, an RF amplifier placed before the mixer boosts weak incoming signals before they reach the mixer stage. Because the mixer itself introduces noise, amplifying the signal first improves the overall signal-to-noise ratio of the receiver. This means the receiver can detect weaker signals — that is, it has greater sensitivity.

• A. enables the receiver to tune a greater frequency range — Incorrect. Tuning range is determined by the local oscillator and front-end filter design, not by the presence of an RF amplifier.
• B. means no BFO stage is needed — Incorrect. A Beat Frequency Oscillator (BFO) is used for demodulating CW and SSB signals; it is a separate function entirely unrelated to the RF amplifier stage.
• C. makes it possible to receive SSB signals — Incorrect. SSB reception requires a BFO or product detector, not an RF amplifier. The RF amplifier does not enable any particular modulation mode.

Therefore, adding an RF amplifier ahead of the mixer primarily improves receiver sensitivity by amplifying weak signals before the noise-contributing mixer stage processes them.

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Correct answer: improve the reception of different types of signal

Different communication modes require different bandwidths for effective reception.

For example:

• CW requires a narrow bandwidth
• SSB requires a medium bandwidth
• AM or FM may require a wider bandwidth

Selecting an appropriate IF filter bandwidth allows the receiver to pass the desired signal while rejecting adjacent interference.

• The S-meter measures signal strength, not filter selection.
• Sensitivity relates to the receiver’s ability to detect weak signals.
• Increasing noise is not desirable.

Therefore, different IF filters are selected to improve the reception of different types of signal.

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Correct answer: B — mixer stage

In a superheterodyne (superhet) receiver, the mixer stage accepts two inputs: the incoming RF signal (selected by a tunable front-end circuit that tracks across the band) and the output of the local oscillator. The mixer combines these two signals to produce a difference frequency — the Intermediate Frequency (IF) — which is always the same fixed value regardless of which station is tuned in. Because the input tracks the desired station (tunable) but the output is always the same IF (fixed), the mixer is the stage described.

• A. RF amplifier — The RF amplifier has a tunable input but its output is also tunable (it passes the amplified RF signal at the same frequency), not a fixed IF frequency.
• C. IF amplifier — The IF amplifier operates entirely at the fixed intermediate frequency; both its input and output are fixed, not tunable.
• D. Local oscillator — The local oscillator generates a single tuneable frequency used to drive the mixer, but it does not itself have a "fixed tuned output" in the IF sense; it is an oscillator, not a frequency-converting stage.

Therefore, the stage with a tunable input and a fixed-frequency output is the mixer stage, which is the defining characteristic of the superheterodyne principle.

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Correct answer: produces an intermediate frequency signal

In a superheterodyne receiver, the mixer combines the incoming RF signal with the local oscillator signal.

This produces new frequencies equal to the sum and difference of the two:

\[ f_{\text{IF}} = |f_{\text{RF}} - f_{\text{LO}}| \]

One of these is selected as the intermediate frequency (IF) for further amplification and filtering.

• Spurious signals may occur but are not the intended function.
• A buffer stage isolates circuits, not mixes signals.
• Demodulation is performed later in the receiver.

Therefore, the mixer stage produces an intermediate frequency signal.

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Correct answer: 9 and 23 MHz

A mixer produces output frequencies equal to the sum and difference of its input frequencies.

Given:

• Signal frequency = \(7\ \mathrm{MHz}\)
• Oscillator frequency = \(16\ \mathrm{MHz}\)

The mixer output will include:

\[ f_{\text{sum}} = 7 + 16 = 23\ \mathrm{MHz} \]

\[ f_{\text{difference}} = |16 - 7| = 9\ \mathrm{MHz} \]

• 8 MHz does not result from sum or difference.
• 7 MHz is already one of the inputs.
• 3.5 MHz is unrelated to the mixing process.

Therefore, the output will contain 9 MHz and 23 MHz.

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Correct answer: IF amplifier

In a superheterodyne receiver, selectivity is mainly provided by the intermediate frequency (IF) amplifier. The IF stages operate at a fixed frequency, allowing high-quality filters with narrow bandwidth and steep skirts to separate the desired signal from adjacent signals.

Because the frequency is constant, the filters can be optimized for stable and precise selectivity, which is much harder to achieve at the constantly changing RF tuning frequency.

• RF amplifier provides some front-end filtering and image rejection but does not provide the primary selectivity.
• mixer performs frequency conversion and does not contribute meaningful selectivity.
• audio stage affects audio quality and volume, not RF channel selectivity.

Therefore, selectivity in a superhet receiver is achieved primarily in the IF amplifier.

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Correct answer: B — automatic gain control

AGC (Automatic Gain Control) is a feedback circuit used in receivers to automatically adjust the gain of one or more amplifier stages. As the received signal strength varies, the AGC circuit increases or decreases amplifier gain to maintain a relatively constant output level. This prevents strong signals from overloading the receiver and weak signals from becoming inaudible.

• A — attenuating gain capacitor: Not a real circuit term; "attenuating gain" is self-contradictory and capacitors are not described this way.
• C — anode-grid capacitor: This describes an inter-electrode capacitance in a vacuum tube, not an abbreviation in common use as AGC.
• D — amplified grid conductance: Not a standard term; "grid conductance" relates to valve (tube) parameters but has no recognised AGC abbreviation.

Therefore, AGC stands for automatic gain control, a fundamental receiver circuit that stabilises output level across a wide range of incoming signal strengths.

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Correct answer: D — RF and IF stages

Automatic Gain Control (AGC) is a feedback system that keeps the receiver's output level stable when the incoming signal strength varies. It does this by sampling the signal level (usually after the detector) and feeding a control voltage back to the RF amplifier and IF amplifier stages, reducing their gain when a strong signal arrives and allowing more gain when the signal is weak.

Controlling gain at the RF and IF stages is effective because these are the high-gain amplification stages that handle the signal before detection — adjusting them early in the chain prevents overloading the detector and maintains a consistent audio output level.

• A. Audio stage — AGC does not act on the audio stage; volume control handles audio level, and by the time the signal reaches audio, it is too late to prevent detector overload.
• B. Mixer stage — the mixer converts frequency but is not typically a variable-gain stage under AGC control; gain adjustment there would also introduce unwanted intermodulation products.
• C. Power supply — varying the power supply voltage is not how AGC works; the power supply provides a stable rail voltage for all stages.

Therefore, AGC maintains a steady output by applying its control voltage to the RF and IF amplifier stages, which are the primary gain elements in the receive chain.

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Correct answer: local oscillator

In a superheterodyne receiver, the received signal is converted to a fixed intermediate frequency (IF) by mixing it with a local oscillator (LO). The IF remains constant so that filtering and gain can be optimized at one frequency.

When the tuning control is adjusted, it changes the frequency of the local oscillator. This shifts the mixing product so that different incoming RF signals are converted to the same IF.

Mathematically:

\[ f_{IF} = | f_{RF} - f_{LO} | \]

To keep \(f_{IF}\) constant while tuning to different \(f_{RF}\) values, the receiver varies \(f_{LO}\).

• audio amplifier operates only on recovered audio and is not frequency tuned.
• IF amplifier is fixed-tuned to the intermediate frequency and normally does not change with tuning.
• post-detector amplifier processes audio and is unrelated to RF tuning.

Therefore, the tuning control changes the frequency of the local oscillator.

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Correct answer: 14.5 MHz

In a superheterodyne receiver:

\[ f_{\text{IF}} = |f_{\text{RF}} - f_{\text{LO}}| \]

Given:

• \(f_{\text{RF}} = 14\ \mathrm{MHz}\)
• \(f_{\text{IF}} = 0.5\ \mathrm{MHz}\)

So the local oscillator frequency is:

\[ f_{\text{LO}} = f_{\text{RF}} + f_{\text{IF}} = 14 + 0.5 = 14.5\ \mathrm{MHz} \]

• 19 MHz and 28 MHz do not produce a 500 kHz IF.
• 500 kHz is the IF, not the oscillator frequency.

Therefore, the local oscillator frequency is 14.5 MHz.

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Correct answer: signals leaving the detector are weak

In an AM receiver, the detector (demodulator) recovers the audio signal from the modulated RF carrier.

However, the recovered audio signal is typically of very low amplitude and not strong enough to drive a loudspeaker directly.

An audio amplifier is therefore required to increase the signal level to a usable output.

• The carrier is removed, not replaced.
• Demodulation is performed by the detector.
• RF signals are converted to audio before reaching the amplifier.

Therefore, an audio amplifier is necessary because signals leaving the detector are weak.

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Correct answer: match the output impedance of the audio amplifier to the speaker

An audio output transformer is used to match the high output impedance of the audio amplifier to the low impedance of the loudspeaker (typically 4–8 \(\Omega\)).

Impedance matching allows:

• maximum power transfer to the speaker

• efficient operation of the amplifier stage

• It does not increase gain directly.

• It is not primarily for speaker protection.

• Tone improvement is not its main function.

Therefore, the audio output transformer is required to match the output impedance of the audio amplifier to the speaker.

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Correct answer: two oscillators

A single-conversion superheterodyne receiver uses:

• a local oscillator (LO) to mix with the incoming RF signal and produce the intermediate frequency (IF)
• a carrier insertion oscillator (BFO) to reinsert a carrier for demodulating signals such as CW or SSB

So, counting the carrier insertion oscillator, the receiver contains:

• one local oscillator

• one beat frequency oscillator (BFO)

• More than two would be required only in multi-conversion receivers.

• One oscillator alone would not allow for IF generation and carrier reinsertion.

Therefore, a single-conversion superhet receiver has two oscillators.

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Correct answer: 22 MHz is the image frequency

In a superheterodyne receiver:

\[ f_{\text{IF}} = |f_{\text{signal}} - f_{\text{LO}}| \]

Given:

• \(f_{\text{signal}} = 21.0\ \mathrm{MHz}\)
• \(f_{\text{IF}} = 0.5\ \mathrm{MHz}\)

The local oscillator frequency may be:

\[ f_{\text{LO}} = 21.0 + 0.5 = 21.5\ \mathrm{MHz} \]

An unwanted signal that also produces the same IF is the image frequency:

\[ f_{\text{image}} = f_{\text{LO}} + f_{\text{IF}} = 21.5 + 0.5 = 22.0\ \mathrm{MHz} \]

So a signal at 22 MHz will mix with the LO to produce the same IF and pass through the receiver.

• Insufficient IF selectivity cannot remove an image signal once converted to IF.
• The signal is within the receiver’s RF range.
• RF gain does not determine image rejection.

Therefore, the interference is caused because 22 MHz is the image frequency.

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Correct answer: A — 455 kHz

In a superheterodyne (superhet) receiver, the mixer stage combines the incoming RF signal with the local oscillator (LO) signal to produce an intermediate frequency (IF). The IF is the difference between the two input frequencies, and the IF amplifier is tuned to this fixed difference frequency.

\[ f_{IF} = f_{LO} - f_{RF} \]

Given:

• RF signal = 3540 kHz
• LO signal = 3995 kHz

\[ f_{IF} = 3995\ \mathrm{kHz} - 3540\ \mathrm{kHz} = 455\ \mathrm{kHz} \]

455 kHz is a very common IF frequency used in AM and HF receivers.

• B. 3540 kHz — This is simply the incoming RF signal frequency, not the IF.
• C. 3995 kHz — This is the local oscillator frequency, not the IF.
• D. 7435 kHz — This is the sum of the two frequencies (3540 + 3995). The mixer produces both a sum and a difference, but the IF amplifier is tuned to the difference, and the sum product is filtered out.

Therefore, the IF amplifier is tuned to 455 kHz, the difference between the local oscillator and the incoming signal frequencies.

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Correct answer: two IF stages and two local oscillators

A double conversion receiver converts the incoming RF signal to an intermediate frequency twice. Each frequency conversion requires its own local oscillator, and each conversion produces its own IF stage.

The signal path is:

1. RF signal is mixed with the first local oscillator to produce the first IF.
2. The first IF is mixed with the second local oscillator to produce the second IF.
3. The final IF is then demodulated.

For SSB reception, a carrier insertion oscillator (BFO) is also required to reinsert the suppressed carrier for audio recovery. The BFO is separate from the two local oscillators used for frequency conversion.

So the receiver contains:

• two IF stages (first IF and second IF)

• two local oscillators (one for each conversion)

• plus a carrier insertion oscillator for detection

• one IF stage and one local oscillator describes a single-conversion receiver.

• two IF stages and one local oscillator cannot perform two frequency conversions.

• two IF stages and three local oscillators includes one extra oscillator that is not required.

Therefore, a double conversion SSB receiver has two IF stages and two local oscillators.

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Correct answer: has improved image rejection characteristics

A double conversion receiver uses two frequency conversion stages, converting the received signal first to a high intermediate frequency (IF), then to a lower IF for filtering and amplification.

Using a high first IF moves the image frequency much farther away from the wanted signal. This makes it much easier for the RF front-end filtering to reject the image, significantly improving image rejection performance compared with a single-conversion receiver.

The second, lower IF then allows sharp filtering and stable gain.

• does not drift off frequency is incorrect, frequency stability depends on oscillator design and temperature control, not on the number of conversions.
• produces a louder audio signal depends on audio amplifier gain, not receiver architecture.
• is a more sensitive receiver is not guaranteed, sensitivity depends mainly on noise figure and front-end design.

Therefore, a key advantage of a double conversion receiver is improved image rejection characteristics.

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Correct answer: B — silences the receiver speaker during periods of no received signal

A squelch circuit monitors the received signal level (or noise level) and mutes the audio output when no carrier or signal is present. Without squelch, an FM receiver produces a loud, continuous hiss between transmissions because the discriminator outputs noise when no signal is being received. The squelch circuit sets a threshold: when the signal falls below that threshold, the audio path is cut, keeping the speaker silent until a transmission arrives.

• A is wrong — automatic level control (ALC) or AGC manages audio/gain levels; squelch does not boost or maintain audio at maximum.
• C is wrong — squelch does the opposite: it removes noise from the listening environment rather than creating one.
• D is wrong — squelch circuits are compact and are standard in virtually all pocket-sized FM transceivers (handheld radios, HTs), where silent standby operation is especially desirable.

Therefore, a squelch circuit improves the listening experience by automatically silencing the speaker whenever no usable signal is present on the channel.

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