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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: 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: 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: 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: 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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