Question 1

Image frequency calculation in a superheterodyne receiver A superheterodyne receiver is tuned such that its mixer sees a desired RF of 555 kHz, while the local oscillator (LO) input is at 1010 kHz and the IF is 455 kHz. What is the image frequency f…

Accepted Answer

Introduction / Context: Image frequency rejection is a crucial aspect of superhet design. Signals at the image will also convert to IF and can cause double spotting unless sufficiently attenuated by the RF front end. Given Data / Assumptions: IF = 455 kHz, LO = 1010 kHz. Desired RF = 555 kHz since 1010 − 555 = 455. High-side injection (LO above RF). Concept / Approach: For high-side injection, the image frequency is located at f_image = f_LO + IF (since both f_LO − f_RF and f_image − f_LO can equal IF depending on side). Equivalently, f_image = f_RF + 2IF. Step-by-Step Solution: Compute f_RF from LO and IF: f_RF = f_LO − IF = 1010 − 455 = 555 kHz.Image for high-side LO: f_image = f_RF + 2IF.Calculate: f_image = 555 + 2455 = 555 + 910 = 1465 kHz.Thus the image is 1465 kHz. Verification / Alternative check: Using the alternate relation f_image = f_LO + IF = 1010 + 455 = 1465 kHz gives the same result. Why Other Options Are Wrong: 555 kHz: The desired signal, not the image. 1010 kHz: The LO frequency. 1920 kHz: Unrelated arithmetic combination. 910 kHz: Equals 2IF; not the image frequency here. Common Pitfalls: Confusing image formula for high-side and low-side injection; always check whether LO is above or below the RF. Final Answer: 1465 kHz

Question 2

Functions of the RF amplifier in a radio receiver front end Why is an RF amplifier stage commonly included ahead of the mixer in superheterodyne receivers?

Accepted Answer

Introduction / Context: The RF amplifier in a receiver front end provides controlled gain and helps with noise figure and image rejection. It prepares the signal for mixing while reducing the impact of mixer noise and improving matching to the antenna system. Given Data / Assumptions: Standard superheterodyne architecture. Reasonable quality RF amplifier with appropriate bandwidth and selectivity. Goal is robust reception across a range of signal strengths. Concept / Approach: Adding low-noise gain early improves the cascade noise figure (Friis formula). A tuned RF amplifier also provides preselection that suppresses images and out-of-band interferers. Proper input matching improves power transfer from the antenna into the receiver chain. Step-by-Step Solution: Sensitivity improves because the RF stage boosts weak signals before the mixer and IF.SNR improves since low-noise first-stage gain dominates system noise.Impedance matching and preliminary selectivity reduce unwanted signals and reflections.Thus, all listed benefits are valid. Verification / Alternative check: Receiver design texts emphasize a low-noise, high-linearity front end to set overall noise performance and dynamic range, confirming these roles of the RF amplifier. Why Other Options Are Wrong: The single-benefit choices ignore other equally important roles. Protection alone is not the main purpose; gain, noise, and selectivity are primary. Common Pitfalls: Assuming the RF stage is optional; while some minimal receivers omit it, most quality receivers include it for the reasons above. Final Answer: All of the above

Question 3

Superheterodyne receiver fundamentals In a radio receiver, which single stage accepts a radio-frequency (RF) signal at its input and delivers an intermediate-frequency (IF) signal at its output?

Accepted Answer

Introduction / Context: This question targets the core concept of the superheterodyne architecture that dominates modern receivers. Understanding which stage translates incoming radio-frequency (RF) signals to a fixed intermediate frequency (IF) is essential for grasping selectivity, image rejection, and overall sensitivity design. Given Data / Assumptions: A conventional superheterodyne receiver is considered. Stages present typically include an RF front end, a frequency changer (mixer plus local oscillator), IF amplifiers and filters, a detector (demodulator), and audio stages. We assume standard continuous-wave or modulated RF reception. Concept / Approach: The superheterodyne method converts the desired RF channel to a constant IF by mixing it with a local oscillator (LO). The mixer output contains sum and difference frequencies. A subsequent IF filter selects the IF component. This translation to a fixed IF allows highly selective, stable filtering and consistent bandwidth across the tuning range. Step-by-Step Solution: Identify the stage that performs frequency conversion → the frequency changer (mixer+LO).Note inputs and outputs: RF in, LO in, IF out (desired mixing product).Confirm that demodulator works at IF or baseband, not at RF-to-IF translation.Audio and loudspeaker stages operate after detection and are not frequency-translating blocks. Verification / Alternative check: Block diagrams of superheterodyne receivers universally label the mixer/frequency changer between the RF stage and the IF strip. The IF is fixed (e.g., 455 kHz for AM broadcast), enabling standardized narrowband filters. Why Other Options Are Wrong: Loudspeaker and audio amplifier: act on audio, not on RF/IF translation. Demodulator: extracts baseband from IF; it does not create IF. RF preselector filter: provides selectivity at RF but does not change frequency. Common Pitfalls: Confusing the detector with the mixer; the detector removes the carrier, whereas the mixer shifts frequency. Assuming IF is produced by the RF amplifier; it is produced by the mixer. Final Answer: Frequency changer (mixer with local oscillator)

Question 4

Receiver performance terminology In broadcast and communication receivers, “fidelity” refers to which characteristic measured across the modulation-frequency range at a resistive load?

Accepted Answer

Introduction / Context: Receiver specifications use distinct terms: sensitivity, selectivity, fidelity, and signal-to-noise ratio. Candidates often confuse these metrics. Fidelity specifically addresses how accurately the receiver reproduces the audio (or baseband) spectrum without amplitude distortion across the intended frequency range. Given Data / Assumptions: Baseband (audio) output is measured across a resistive load. We consider small-signal linear operation without intentional compression. The receiver processes amplitude-modulated or frequency-modulated signals and presents an audio-frequency spectrum. Concept / Approach: Fidelity is the flatness of the amplitude-versus-frequency response at baseband. A receiver with high fidelity delivers an output whose amplitude does not unduly rise or fall with modulation frequency within the specified audio band (for example, 50 Hz–5 kHz in AM broadcast). This is distinct from selectivity (channel discrimination) and sensitivity (minimum discernible signal). Step-by-Step Solution: Define fidelity → faithfulness of the output waveform spectrum to the input modulation.Measurement → sweep modulation frequency at constant modulation index and note output amplitude across a resistive load.Interpretation → smaller variation indicates better fidelity. Verification / Alternative check: Receiver datasheets specify audio frequency response (e.g., ±3 dB from 100 Hz to 3 kHz). This response curve is a direct indicator of fidelity. Why Other Options Are Wrong: Sensitivity: relates to minimum input for specified output-noise ratio, not spectral flatness. Selectivity: measures discrimination of nearby channels and adjacent frequencies, not audio response. AGC action per dB: pertains to dynamic range control, not fidelity. “None of the above” is incorrect because a precise definition is given in option (c). Common Pitfalls: Equating fidelity with distortion alone; although connected, fidelity is commonly quantified via frequency response flatness. Confusing baseband frequency response with IF/RF filter bandwidth specifications. Final Answer: Variation of output amplitude with modulation frequency when the output impedance is resistive

Question 5

Image rejection of a single-tuned RF stage A superheterodyne receiver uses an IF of 450 kHz and is tuned to an incoming signal at 1200 kHz. The RF tuned circuit has Q = 65. Estimate the image-rejection ratio (unitless).

Accepted Answer

Introduction / Context: Image rejection quantifies how well the RF front end discriminates against the image frequency that also converts to IF in a superheterodyne receiver. With a single tuned RF circuit, the image attenuation depends on the circuit Q and the separation between the tuned frequency and the image frequency. Given Data / Assumptions: IF = 450 kHz. Desired RF (tuned) frequency f0 = 1200 kHz. Single-tuned RF stage with Q = 65. High-side local oscillator assumed; image at f_image = f0 + 2IF. Concept / Approach: For a single-tuned circuit, the relative response at an off-tune frequency f is approximately: |H(f)| ≈ 1 / sqrt(1 + Q^2 * ( (f/f0) − (f0/f) )^2 ) Tuning at f0 makes the on-channel response unity. Thus the image-rejection ratio R is: R ≈ 1 / |H(f_image)| = sqrt(1 + Q^2 * ( (f_image/f0) − (f0/f_image) )^2 ) Step-by-Step Solution: Compute image: f_image = 1200 + 2450 = 2100 kHz.Evaluate term T = (f_image/f0) − (f0/f_image) = (2100/1200) − (1200/2100) = 1.75 − 0.5714 ≈ 1.1786.Compute Q^2 * T^2 = 65^2 * 1.1786^2 = 4225 * 1.388 ≈ 5865.Rejection ratio R ≈ sqrt(1 + 5865) ≈ sqrt(5866) ≈ 76.6 in voltage.If the question defines “image rejection” as the power ratio, it is R^2 ≈ 5866, matching the tabulated answer 5870. Verification / Alternative check: Many exam tables quote the power-ratio definition (1 + Q^2 T^2); hence 5870 is consistent with that convention. The corresponding voltage ratio is ~76.6, which equals √5866. Why Other Options Are Wrong: 3655 or 236 or 13.3: inconsistent with the calculated expression. 76.6: this is the voltage ratio; if the key expects the power ratio, it will be ~5870. Common Pitfalls: Using f_image = f0 ± IF instead of f0 ± 2*IF. Confusing voltage and power ratios; know which convention the problem uses. Final Answer: 5870

Question 6

Radio-noise taxonomy Which one of the following noise types arises from terrestrial weather phenomena rather than extraterrestrial sources such as the Sun or the Milky Way?

Accepted Answer

Introduction / Context: Understanding noise sources helps engineers design front ends and set expectations for minimum discernible signal. Not all noise originates in the receiver; significant components arrive from the environment. Given Data / Assumptions: Four named categories: solar, cosmic, galactic, and atmospheric. The question seeks the one with a different (non-space) origin. Concept / Approach: Solar, cosmic, and galactic noises are extraterrestrial. Atmospheric noise, also called static, is produced by lightning discharges and other meteorological electrical activity in Earth’s atmosphere. Its intensity varies with time of day, season, and storm activity, and it is more severe at lower frequencies (e.g., HF). Step-by-Step Solution: Identify source domains: extraterrestrial vs terrestrial.Classify: solar (Sun), cosmic (outside the galaxy), galactic (within Milky Way), atmospheric (Earth’s atmosphere).Conclude: atmospheric noise is the one with terrestrial origin. Verification / Alternative check: Radio propagation texts show atmospheric noise curves dominating the external noise temperature at HF and reducing at VHF/UHF as sky noise becomes more relevant. Why Other Options Are Wrong: Solar, galactic, and cosmic noises are all extraterrestrial emissions. Thermal noise in a resistor is internal to the receiver component, not an external environmental source. Common Pitfalls: Confusing “cosmic” and “galactic” categories. Assuming all external noise is space-borne; thunderstorms are powerful RF emitters. Final Answer: Atmospheric noise (static from lightning and storms)

Question 7

Receiver sensitivity determinants In a superheterodyne receiver, which factors primarily determine the sensitivity (i.e., the minimum input required for a specified output and S/N)?

Accepted Answer

Introduction / Context: Sensitivity indicates how weak a signal a receiver can detect at a specified output and signal-to-noise ratio. It depends on both gain distribution and noise performance across stages, as summarized by Friis’s formula for cascaded noise figure. Given Data / Assumptions: Superheterodyne receiver with RF amplifier, mixer, IF strip, detector. Target is a standard output level and S/N at the demodulator or audio output. Wideband thermal and device noises are present. Concept / Approach: Overall sensitivity improves with adequate front-end gain that suppresses contributions of noisy subsequent stages, and with low noise figure in the first stage. IF gain also matters because the detector needs sufficient input. Therefore, both gains (RF and IF) and noise figure together define sensitivity. Step-by-Step Solution: Apply Friis’s formula: system noise figure is dominated by the first active stage’s NF and its gain.Ensure enough IF gain to reach the detector’s required drive without adding excessive noise.Conclude that sensitivity is determined by RF/IF gains and overall noise figure. Verification / Alternative check: Manufacturer datasheets state sensitivity together with IF gain and front-end NF; improving any of these (to a point) enhances minimum discernible signal. Why Other Options Are Wrong: Single-stage answers (RF-only or IF-only) ignore the cascaded nature. Noise figure alone does not guarantee adequate detector drive without gain. Antenna impedance matters for matching but is not the dominant determinant in isolation. Common Pitfalls: Overemphasizing IF gain while neglecting front-end NF. Forgetting that too much gain can cause overload; sensitivity must be paired with linearity. Final Answer: All of the above acting together (RF/IF gains and noise figure)

Question 8

Why IF selectivity is superior to RF selectivity in superheterodyne receivers In a superheterodyne, the IF strip generally provides better and more consistent selectivity than the RF stage primarily because of what design advantage?

Accepted Answer

Introduction / Context: One of the chief benefits of the superheterodyne architecture is excellent selectivity from well-controlled IF filters. By converting all channels to a single IF, engineers can design sharp, stable filters independent of the tuned RF frequency. Given Data / Assumptions: Receiver uses a fixed IF (e.g., 455 kHz for AM, several MHz for others). IF strip includes narrowband filters (LC or ceramic/quartz/SAW) and gain stages. RF stage must tune across a wide band and cannot maintain a constant passband easily. Concept / Approach: At a fixed IF, filter elements can be optimized and cascaded to produce steep skirts and a controlled bandwidth (constant passband). This is far more difficult at RF, where the center frequency changes continuously and tracking multiple tuned circuits while maintaining shape factor is challenging. Step-by-Step Solution: Translate RF to a constant IF via mixing.Design multi-pole filters at this IF with precise bandwidth (e.g., 6 kHz for AM).Achieve consistent selectivity across the entire tuning range. Verification / Alternative check: Commercial receivers rely on quartz or ceramic IF filters with tight tolerances and repeatable passbands—that consistency is central to channel spacing and adjacent-channel rejection. Why Other Options Are Wrong: “Lower” or “higher” frequency alone does not guarantee selectivity; the fixed frequency and filter technology do. High L/C ratio helps but is not the primary architectural reason. “Random variations” is not a design principle. Common Pitfalls: Assuming selectivity is only about Q; filter topology and fixed IF are key. Ignoring the role of multi-resonator coupling and standardized IF components. Final Answer: The passband can be made constant at a fixed IF, enabling highly selective multi-pole filtering

Question 9

Limitations of the tuned-radio-frequency (TRF) receiver Which of the following is a recognized drawback of the TRF architecture when compared to the superheterodyne?

Accepted Answer

Introduction / Context: Before the superheterodyne, the TRF receiver used multiple cascaded RF tuned stages at the signal frequency. While workable for narrow ranges, TRF is fundamentally limited for wide tuning and stringent selectivity requirements. Given Data / Assumptions: TRF uses several RF amplifiers all tuned to the desired RF channel. Bandwidth and gain must be consistent over a large tuning span. Oscillations and tracking errors are practical concerns. Concept / Approach: Because each RF stage must track the others as the tuning knob moves, small misalignments or component tolerances degrade selectivity. High gain at RF can cause instability (oscillation). Also, the achievable bandwidth (shape factor) varies with frequency, reducing adjacent-channel rejection. Step-by-Step Solution: Assess stability → many high-Q RF stages in cascade increase likelihood of oscillation.Assess selectivity → tracking multiple resonators compromises skirt selectivity across the band.Assess bandwidth → passband changes with tuned frequency because LC reactances change and coupling is not constant. Verification / Alternative check: Historical receiver literature shows the superheterodyne superseded TRF largely due to these issues, especially for broadcast bands and beyond. Why Other Options Are Wrong: Each listed drawback is real; hence “none of the above” is incorrect. Claiming parity with superhet ignores the architectural advantages of a fixed IF. Common Pitfalls: Underestimating the difficulty of multi-circuit tracking at RF. Confusing image problems (a superhet issue) with TRF’s bandwidth inconsistency; both architectures have different challenges. Final Answer: All of the above

Question 10

Noise contributors in a superheterodyne receiver chain Considering typical gain distribution, which statement most accurately reflects the dominant source of receiver-generated noise?

Accepted Answer

Introduction / Context: Receiver noise performance is governed by the noise figures and gains of cascaded stages. Practical superheterodynes can be limited by mixer noise unless sufficient low-noise RF gain precedes it. Recognizing the typical culprit guides front-end design and sensitivity improvements. Given Data / Assumptions: Standard superheterodyne chain: RF amp → mixer → IF amp → detector → audio. RF gain is finite; mixer has significant conversion loss and noise figure. Friis’s formula applies: early stages dominate overall noise figure. Concept / Approach: In many designs, the mixer (frequency changer) has higher noise figure than a well-designed RF amplifier. If RF gain is modest, the mixer’s noise is not sufficiently suppressed and becomes the dominant contributor. With high RF gain, mixer noise impact diminishes, but it still remains a critical stage to optimize. Step-by-Step Solution: Identify stage noise figures and gains.Apply Friis’s formula: NF_total ≈ NF_RF + (NF_mixer − 1)/Gain_RF + …If Gain_RF is low, the second term is large; thus the mixer often dominates. Verification / Alternative check: Receiver datasheets frequently quote mixer conversion loss and noise figure as key limits. Low-noise preamps are added to reduce the system noise temperature. Why Other Options Are Wrong: Equal contribution: contradicted by cascaded-noise theory. RF stage has no effect: false; the first stage is crucial. Only audio stage determines S/N: audio contributes little compared to RF/mixer in properly designed systems. Antenna thermal noise does not always dominate—especially on bands where external noise is low. Common Pitfalls: Ignoring that adequate RF gain is needed to swamp mixer noise. Overlooking the trade-off between high RF gain and intermodulation/overload risks. Final Answer: The mixer stage often contributes the largest share of internally generated noise (especially if RF gain is modest)

Question 11

Sideband generation in communication systems Which stage in a standard radio transmitter–receiver chain is responsible for creating the upper and lower side frequencies around a carrier?

Accepted Answer

Introduction / Context: Amplitude and angle modulations produce spectral components known as sidebands. Knowing which block creates sidebands is essential for understanding bandwidth, channel spacing, and spectral masks. Given Data / Assumptions: Classical transmitter chain: audio source → modulator → RF power amplifier → antenna. Receiver chain: antenna → RF/mixer/IF → detector → audio. We consider conventional AM (double-sideband) or other linear modulations. Concept / Approach: In AM, a carrier at frequency fc is combined with a baseband signal at fm to produce components at fc + fm (upper side) and fc − fm (lower side). This spectral translation occurs in the modulator, which multiplies (or otherwise combines) the carrier and the modulating signal. Step-by-Step Solution: Identify the operation needed: frequency translation by multiplication of signals.Locate the stage performing multiplication: the modulator in the transmitter.Confirm other blocks: microphones only produce audio; demodulators reverse the process; oscillators generate carriers but do not create sidebands alone. Verification / Alternative check: Spectral plots of a modulated carrier show the sidebands appear at the modulator output; removing modulation collapses the spectrum to the unmodulated carrier. Why Other Options Are Wrong: Microphone: no RF sidebands; only audio generation. Demodulator: removes sidebands to recover baseband. Local oscillator: provides a pure carrier; sidebands arise only when it is modulated or mixed with information. Audio power amplifier: operates at audio frequencies, not at RF. Common Pitfalls: Confusing mixing (frequency conversion) in receivers with modulation in transmitters; both are nonlinear but serve different purposes. Assuming any nonlinear stage makes sidebands; controlled modulation creates the intended sidebands. Final Answer: Modulator in the radio transmitter

Question 12

Superheterodyne broadcast receiver (no RF amplifier) Given a loaded Q of the antenna coupling circuit equal to 100 and an intermediate frequency (IF) of 455 kHz, find the image frequency corresponding to a desired station at 1000 kHz for a standard …

Accepted Answer

Introduction / Context: Image frequency is a classic topic in superheterodyne receiver design. When mixing a wanted RF signal with a local oscillator (LO), another unwanted frequency (the image) can also convert to the same IF, potentially causing interference. Understanding how to compute the image frequency and why high front-end Q is desirable is fundamental for receiver selectivity and performance. Given Data / Assumptions: Desired station (signal) frequency fs = 1000 kHz. Intermediate frequency IF = 455 kHz. Typical broadcast receiver with a high-side LO (LO above the signal). No RF amplifier stage; antenna coupling circuit has loaded Q = 100 (useful for image rejection discussion but not needed to compute the image frequency itself). Concept / Approach: For a high-side LO, the oscillator frequency is fLO = fs + IF. The image frequency fim is the frequency that also satisfies |fLO − fim| = IF, which leads to fim = fs + 2IF. This places the image on the higher-frequency side of the desired station in AM broadcast receivers with high-side injection. Step-by-Step Solution: Compute fLO: fLO = 1000 kHz + 455 kHz = 1455 kHz.Image condition: |fLO − fim| = IF.Solve for fim: fim = fLO + IF = 1455 kHz + 455 kHz = 1910 kHz (since image is on the high side). Verification / Alternative check: Alternatively use fim = fs + 2IF = 1000 kHz + 2*455 kHz = 1910 kHz. Both routes agree, confirming the result. Why Other Options Are Wrong: 1010 kHz and 545 kHz are near-image distractors but do not satisfy the mixing condition with IF = 455 kHz. 1455 kHz is the LO, not the image. 1365 kHz does not meet the image equation. Common Pitfalls: Confusing LO frequency with image frequency; using low-side injection without stating it; or assuming Q changes the image frequency (it affects image rejection, not the image location). Final Answer: 1910 kHz

Question 13

Receiver performance linkage A receiver that exhibits poor intermediate-frequency (IF) selectivity will also suffer from which degradation most prominently, assuming other sections are typical of AM broadcast superheterodynes?

Accepted Answer

Introduction / Context: Selectivity determines how well a receiver separates a desired channel from adjacent or spurious responses. In a superheterodyne, IF selectivity strongly influences the shape of the passband and how unwanted signals near the IF are handled. One symptomatic issue is double spotting, wherein the same station is heard at two dial positions because of inadequate rejection of spurious responses near the IF path. Given Data / Assumptions: AM superheterodyne architecture. IF selectivity is poorer than intended (wider or insufficient attenuation of adjacent responses). Typical front end and local oscillator operation. Concept / Approach: Poor IF selectivity broadens the effective passband, allowing unintended responses within or near the IF to leak through. This can present as hearing the same station twice (double spotting), usually due to residual image or second-conversion-like responses being insufficiently attenuated by the IF filters. Step-by-Step Solution: Identify failure mode: inadequate IF filtering → broader passband.Result: spurious/adjacent signals are not sufficiently rejected.Symptom: receiver detects a station at two tuning points → double spotting. Verification / Alternative check: In practice, improving IF selectivity (sharper skirts, better filter alignment) removes secondary responses, reducing double spotting. Sensitivity is more tied to front-end noise figure and gain, and blocking mainly involves strong-signal handling and AGC linearity rather than IF bandwidth alone. Why Other Options Are Wrong: Sensitivity: Primarily governed by noise figure and gain, not IF bandwidth alone. Blocking: A strong adjacent signal desensitizing the receiver is more about dynamic range and AGC. Image rejection: Dominated by RF front-end selectivity and LO choice; IF selectivity has limited effect on true image rejection in single-conversion sets. Common Pitfalls: Assuming “all of the above” whenever performance degrades; conflating IF selectivity issues with RF image rejection; overlooking AGC and dynamic-range origins of blocking. Final Answer: Double spotting

Question 14

Squelch operation in broadcast/communications receivers What does a typical squelch circuit do under no-signal (no carrier) conditions to avoid loudspeaker noise?

Accepted Answer

Introduction / Context: Squelch (muting) improves user experience by silencing the loud hiss or noise when a receiver is not tuned to a station or when the incoming carrier falls below a threshold. It is widely used in FM communications receivers and in AM/FM sets to avoid irritating audio between stations. Given Data / Assumptions: Standard superheterodyne receiver with AGC/AVC. No valid carrier or the carrier is below the squelch threshold. We are interested in the typical action performed by the squelch circuit. Concept / Approach: Most squelch systems monitor signal metrics (noise level, carrier presence, or recovered DC from discriminator) and then control the audio path. When no carrier is detected, they mute the audio stage so that only silence is heard. This avoids blasting noise while tuning or during weak-signal fades. Step-by-Step Solution: Detect absence of carrier via noise gate or carrier detector.Drive a control line that biases the audio amplifier into cutoff or opens an electronic switch.Result: loudspeaker is muted until a valid carrier is present. Verification / Alternative check: Service manuals often show the squelch voltage muting the audio path or disabling AF preamplifiers. IF muting exists in some designs but typical squelch silences the AF chain rather than completely shutting down IF gain. Why Other Options Are Wrong: Cutting off IF stages per AGC extremes is not standard squelch action; “eliminating RF interference” is too vague; boosting LO power is unrelated and undesirable. Common Pitfalls: Confusing AGC (automatic gain control) with squelch; AGC adapts gain to signal strength, while squelch mutes audio to hide noise. Final Answer: Cuts off an audio amplifier when the carrier is absent

Question 15

Where most of the gain comes from in a superheterodyne receiver In a well-designed AM superheterodyne set, which stage typically provides the majority of the amplification of the received signal?

Accepted Answer

Introduction / Context: Superheterodyne receivers translate incoming RF to a fixed intermediate frequency (IF), where stable, high-gain, and selective amplification is easier to implement. Understanding where most gain is applied helps with noise figure, stability, and selectivity trade-offs in practical designs. Given Data / Assumptions: Single-conversion AM superhet with IF around a few hundred kHz (e.g., 455 kHz). Moderate RF preselection, fixed IF filtering. Standard audio chain after detection. Concept / Approach: At a fixed IF, narrowband high-Q filters and multiple cascaded IF amplifier stages can be stabilized and aligned for both gain and selectivity. RF stages usually offer lower gain to avoid oscillation and radiation from the antenna. Audio power stages amplify only after detection, acting on baseband, not on the weak RF/IF signal. Step-by-Step Solution: RF → mixer → IF. The IF strip comprises multiple tuned/filtered stages.Apply most of the receiver gain at IF for stability and selectivity.Detect (envelope), then provide modest AF power gain for the loudspeaker. Verification / Alternative check: Alignment procedures emphasize IF peak gain and bandwidth; data sheets for IF ICs show most overall gain contribution in the IF chain. Why Other Options Are Wrong: RF stage gain is limited to maintain stability and prevent LO radiation; audio power stage does not amplify the RF/IF signal; “none of the above” and LO buffer are distractors. Common Pitfalls: Assuming more RF gain always improves sensitivity; it can worsen strong-signal handling and stability compared to placing gain at IF. Final Answer: IF stage

Question 16

Detector used in household AM broadcast receivers Which detector topology is most commonly used in standard household AM radio receivers for demodulating the envelope of the received signal?

Accepted Answer

Introduction / Context: AM broadcast receivers need a simple, inexpensive, and robust method to recover audio from an amplitude-modulated carrier. The envelope detector (usually a diode detector with an RC load) has been the workhorse solution for consumer radios for decades. Given Data / Assumptions: Standard AM broadcast (medium-wave) receiver. Noncoherent detection scheme acceptable (no carrier phase recovery required). Low cost and simplicity are desired. Concept / Approach: A diode rectifies the RF, and an RC time constant follows the envelope—the audio content—while filtering out the RF carrier. Proper RC selection avoids distortion and diagonal clipping. This method is simple, low power, and adequate for AM fidelity requirements in household radios. Step-by-Step Solution: Rectify the incoming IF (post-AGC) with a diode.Use an RC network to track the signal envelope (audio).Feed recovered audio to the AF amplifier and loudspeaker. Verification / Alternative check: Service schematics of classic and modern AM radios show diode envelope detectors. Synchronous or product detectors are used for improved performance or SSB/CW but add complexity and cost. Why Other Options Are Wrong: Synchronous and product detectors require carrier/oscillator synchronization; “radio detector” and “none of the above” are not specific or are incorrect in this context. Common Pitfalls: Choosing RC values that are too small leads to diagonal clipping; too large causes loss of high audio frequencies. Final Answer: Envelope detector

Question 17

Applicability of the superheterodyne principle For which types of modulation can a superheterodyne receiver architecture be implemented effectively?

Accepted Answer

Introduction / Context: The superheterodyne principle uses frequency conversion to a fixed intermediate frequency, enabling stable filtering and amplification. This approach is versatile and forms the basis of most modern receivers across a wide range of modulation formats. Given Data / Assumptions: Heterodyning (mixing) to a fixed IF is available. Appropriate demodulators for each modulation exist at IF or baseband. Front-end selectivity and image rejection are suitably addressed. Concept / Approach: Whether the incoming signal is AM, FM, or SSB, it can be downconverted to a convenient IF. After IF filtering and gain, a suitable detector is used: envelope or synchronous detectors for AM, discriminators/quadrature detectors for FM, and product detectors/BFO for SSB. Therefore, the superheterodyne framework is modulation-agnostic with the right detector and filtering. Step-by-Step Solution: RF → Mixer with LO → IF.Apply IF selectivity and gain.Use appropriate detector: AM (envelope/product), FM (discriminator/PLL), SSB (product/BFO). Verification / Alternative check: Commercial radios, scanners, and communications receivers use superhet chains for AM broadcast, FM broadcast, and SSB/HF communications alike. Why Other Options Are Wrong: Choosing any single modulation ignores the generality of the architecture; “none of the above” is incorrect given widespread practice. Common Pitfalls: Equating the detector type with the overall receiver architecture; in a superhet, detection is a final stage choice adapted to the modulation. Final Answer: All of the above

Question 18

Effect of low AC load impedance in a diode envelope detector In an AM diode detector, what is the principal distortion observed when the AC load impedance is too low (i.e., the RC time constant is too small)?

Accepted Answer

Introduction / Context: AM envelope detection depends on an RC network that follows the modulation envelope while rejecting the carrier frequency. Incorrect selection of the RC time constant leads to distortion. The classic failure for too small an RC is diagonal clipping, where the recovered envelope cuts across the modulation valleys. Given Data / Assumptions: Simple diode detector with capacitor and load resistor. Modulation envelope varying much slower than the carrier. AC load impedance too low → RC too small. Concept / Approach: When RC is too small, the capacitor discharges too quickly between carrier peaks, so the output cannot faithfully track the envelope; instead, it forms straight-line segments connecting peaks—“diagonal clipping.” If RC is too large, high-frequency audio components are lost (muffled sound). Step-by-Step Solution: Model detector output as charge-and-decay between carrier peaks.Small RC → fast decay → envelope troughs clipped diagonally.Result is distortion and audible harshness. Verification / Alternative check: Textbook plots of diode detector waveforms explicitly label this effect as diagonal clipping for undersized RC time constants. Why Other Options Are Wrong: “Negative peak clipping” refers to overdrive or AGC bias issues; “poor AF response” is broader and not the specific distortion mechanism here; “poor AGC operation” is secondary; “crossover distortion” is a push-pull audio amplifier issue, not an AM detector artifact. Common Pitfalls: Trying to fix diagonal clipping by raising RF gain instead of choosing a correct RC that balances distortion and audio bandwidth. Final Answer: Diagonal clipping

Question 19

Broadcast band used by Vividh Bharati (India) Historically and in widespread practice, the Vividh Bharati programme of All India Radio has been transmitted on which broadcast band?

Accepted Answer

Introduction / Context: Vividh Bharati is a popular entertainment and information service from All India Radio. Knowing its traditional transmission bands helps students connect receiver concepts (antenna, propagation, and selectivity) with real services in the broadcast spectrum. Given Data / Assumptions: Focus on classic and widespread transmission practice. India’s broadcast allocations historically include MW AM for wide-area coverage. FM relays exist in many cities, but the core legacy is MW. Concept / Approach: Medium-wave AM (roughly 530–1700 kHz) has long provided extensive coverage with simple receivers. While Vividh Bharati today may also be available on FM relays and digital platforms, exam-style questions typically associate Vividh Bharati with the MW/AM band. Step-by-Step Solution: Identify traditional band of service → Medium frequency (AM/MW).Note existence of FM relays but not as the original or defining band.Choose the option that best matches the historical and common context. Verification / Alternative check: Receiver guides and historical AIR frequency lists show numerous Vividh Bharati MW transmitters across regions. Why Other Options Are Wrong: Short wave has been used for international/national coverage but is not most associated with Vividh Bharati. VHF/FM is used in many cities but is not the traditional identification for exam purposes. “All of the above” overgeneralizes. Common Pitfalls: Assuming modern FM relays negate the traditional MW association; exam conventions usually emphasize MW for Vividh Bharati. Final Answer: Medium frequency (AM/MW)

Question 20

Common demodulators: AM vs FM Consider the statements:  A diode is the most common device used for demodulation (detection).  A discriminator is a frequency-modulation (FM) detector. Choose the correct option.

Accepted Answer

Introduction / Context: Demodulation recovers baseband information from modulated RF/IF signals. Different modulations require different detectors, but some devices are iconic: the diode detector for AM and the frequency discriminator for FM. Given Data / Assumptions: AM broadcast receivers commonly use diode envelope detectors. FM broadcast receivers use discriminators (Foster–Seeley, ratio detectors) or quadrature detectors/PLLs. Statements refer to common, not exclusive, usage. Concept / Approach: Statement 1: In consumer AM radios and many simple receivers, a diode is indeed the most common demodulating element. Statement 2: A discriminator is a classic FM detector topology converting instantaneous frequency deviations to voltage. Therefore both statements are correct as general truths. Step-by-Step Solution: Identify AM detection → diode envelope detector is ubiquitous.Identify FM detection → discriminator or ratio detector widely used.Conclude both statements are true in typical practice. Verification / Alternative check: Service manuals show diode detectors in AM paths and discriminators (or quadrature/PLL) in FM paths. Textbooks corroborate these standard choices. Why Other Options Are Wrong: Negating either statement contradicts widespread receiver designs; SSB demodulation generally uses product detectors, not simple diodes. Common Pitfalls: Assuming exclusivity: other detectors exist (synchronous AM, PLL FM), but the statements remain broadly true. Final Answer: Both the statements are true

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