Question 1

In a simple automatic gain control (AGC) circuit used in radio receivers, which statement correctly describes how AGC responds as the received signal becomes stronger?

Accepted Answer

Introduction / Context: Automatic gain control (AGC) is a standard feature of superheterodyne radio receivers. Its purpose is to keep the audio output approximately constant despite large variations in received RF signal levels, improving listener comfort and preventing overload or distortion. Given Data / Assumptions: Simple diode-detector based AGC typical of broadcast receivers. AGC voltage is derived from the detected (envelope) amplitude after the IF or detector stage. AGC voltage is fed back to control the gain of RF/IF amplifiers. Concept / Approach: As the received signal strength increases, the detector output envelope becomes larger. The AGC network filters and rectifies this to a DC control voltage whose magnitude rises with signal level. This increased AGC voltage then biases gain-controlled stages (RF/IF) to reduce their gain, thereby leveling the overall output. Thus, stronger signals produce more AGC action (a larger control voltage). Step-by-Step Solution: RF signal → RF/IF amplifiers → detector/envelope → AGC rectifier/filter.As input field strength ↑, detector output ↑.AGC network outputs higher DC control voltage.Higher AGC voltage reduces RF/IF gain, stabilizing audio output level. Verification / Alternative check: Tuning from a weak to a strong station produces a measurable increase in AGC voltage and simultaneous reduction in IF gain—standard lab observation. Why Other Options Are Wrong: Highest AGC between stations: between stations the detector mainly sees noise; AGC is typically low to permit sensitivity.Faster time constant ⇒ “more accurate”: too fast produces pump-ing and distortion; proper time constants are a trade-off.Audio stage gain control: in most designs AGC acts on RF/IF stages, not the audio power amplifier.AGC does not selectively reduce noise; it reduces overall gain including both signal and noise. Common Pitfalls: Assuming AGC boosts weak signals (it generally reduces gain for strong signals rather than amplifying weak ones). Final Answer: An increase in signal strength produces more AGC (stronger control voltage)

Question 2

Superheterodyne receiver image frequency: In a superheterodyne receiver with an intermediate frequency (IF) of 455 kHz, if the tuned signal frequency is 1200 kHz, what will be the corresponding image frequency?

Accepted Answer

Introduction / Context: Image frequency is a key concept in superheterodyne receivers. It represents an undesired frequency that can also mix with the local oscillator to produce the same IF as the desired signal, leading to interference unless filtered. Given Data / Assumptions: Intermediate frequency (IF) = 455 kHz. Signal frequency tuned (f_s) = 1200 kHz. Local oscillator frequency (f_lo) = f_s + IF = 1200 + 455 = 1655 kHz. Concept / Approach: The image frequency f_i is given by: f_i = f_lo + IF = f_s + 2IF. Step-by-Step Solution: Compute f_lo = f_s + IF = 1200 + 455 = 1655 kHz.Image frequency f_i = f_lo + IF = 1655 + 455 = 2110 kHz. Verification / Alternative check: Another formula: f_i = f_s + 2IF = 1200 + 910 = 2110 kHz, confirming the result. Why Other Options Are Wrong: 1655 kHz: this is the local oscillator frequency, not the image. 745 kHz or 910 kHz: irrelevant lower frequencies. 2400 kHz: not related by standard image formula. Common Pitfalls: Confusing LO frequency with image frequency. Forgetting that image is separated by 2*IF from the desired signal. Final Answer: 2110 kHz.

Question 3

Broadcast superheterodyne receiver image frequency: A broadcast superheterodyne receiver without an RF amplifier has a loaded Q of the antenna coupling circuit equal to 100, with an intermediate frequency (IF) of 455 kHz. If the tuned signal is at 2…

Accepted Answer

Introduction / Context: At higher frequencies, image rejection becomes difficult without an RF amplifier stage. This question demonstrates how to calculate the image frequency for a tuned broadcast receiver. Given Data / Assumptions: Tuned signal frequency f_s = 25 MHz. Intermediate frequency IF = 455 kHz. No RF amplifier stage is present, so image response is significant. Concept / Approach: For a superheterodyne receiver: Local oscillator frequency f_lo = f_s + IF. Image frequency f_i = f_lo + IF = f_s + 2IF. Step-by-Step Solution: Compute f_lo = f_s + IF = 25.000 + 0.455 = 25.455 MHz.Image frequency f_i = f_lo + IF = 25.455 + 0.455 = 25.910 MHz. Verification / Alternative check: Alternative: f_i = f_s + 2IF = 25.000 + 0.910 = 25.910 MHz. Matches. Why Other Options Are Wrong: 24.09 MHz and 24.54 MHz: represent lower side frequencies, not correct images. 25.45 MHz: this is the LO frequency, not the image. 26.50 MHz: unrelated. Common Pitfalls: Forgetting that the image is separated by 2*IF, not IF. Confusing LO frequency with image frequency. Final Answer: 25.91 MHz.

Question 4

Noise in resistors: What is the type of noise generated due to random thermal agitation of charge carriers in a resistor?

Accepted Answer

Introduction / Context: Noise is an unavoidable phenomenon in electronic circuits. Resistors generate a specific type of noise due to the random motion of electrons caused by thermal energy. Given Data / Assumptions: Resistor at a finite temperature T. Noise voltage measured across its terminals. Concept / Approach: Thermal noise, also called Johnson or Nyquist noise, is proportional to the resistor value, temperature, and measurement bandwidth. Its power spectral density is uniform (white) over practical frequency ranges. Step-by-Step Solution: Formula: Vn_rms = √(4 * k * T * R * B).Here, k = Boltzmann constant, T = absolute temperature (K), R = resistance (Ω), B = bandwidth (Hz).Thus, noise is purely thermal in nature. Verification / Alternative check: Noise spectrum of a resistor shows flat (white) characteristic, confirming thermal noise is the correct answer. Why Other Options Are Wrong: Partition noise: occurs in current division at semiconductor junctions. White noise: describes spectrum shape, but specific origin here is thermal. Shot noise: occurs in semiconductor diodes and vacuum tubes. Impulse noise: external transient disturbances, not inherent to resistors. Common Pitfalls: Confusing white noise with thermal noise; while thermal noise is white, the correct physical term here is thermal noise. Final Answer: Thermal noise.

Question 5

AM detector circuit functions: What are the main functions of an Amplitude Modulation (AM) detector circuit in a radio receiver?

Accepted Answer

Introduction / Context: Demodulation is a fundamental stage in AM radio receivers. The detector circuit extracts the original audio information from the modulated RF signal. Given Data / Assumptions: Modulated input: AM wave containing carrier and sidebands. Desired output: audio frequency signal. Concept / Approach: An AM detector uses rectification followed by filtering to extract the envelope of the carrier, which corresponds to the audio message signal. Step-by-Step Solution: Step 1: Rectify the RF signal to remove negative half cycles.Step 2: Use a filter capacitor to smooth the carrier frequency, effectively discarding it.Step 3: Recover the varying envelope as the audio signal. Verification / Alternative check: Envelope detector operation matches these three steps, confirming that all listed functions are correct. Why Other Options Are Wrong: Each individual option (a, b, c) represents only part of the function; complete answer is all of the above. Common Pitfalls: Assuming detector only rectifies; in fact, it also filters and delivers audio output. Final Answer: All of the above.

Question 6

Common blocks in AM and FM receivers: Which of the following circuit blocks are the same in both AM and FM superheterodyne receivers?

Accepted Answer

Introduction / Context: AM and FM receivers share a similar overall architecture, particularly in the superheterodyne design, though the demodulator differs in implementation. Given Data / Assumptions: Both receivers use frequency conversion to a common IF stage. Standard blocks: RF tuning, mixer, IF amplification, demodulation, AGC. Concept / Approach: The IF stage provides selectivity and gain, common to both AM and FM receivers. AGC is also employed in both to regulate gain and improve reception quality. Demodulators exist in both, although their design differs (envelope detector for AM, discriminator for FM). Step-by-Step Solution: Identify circuit blocks common to both AM and FM: IF amplifier, AGC, demodulator stage.Confirm functional similarity: yes, all are present.Hence, answer is all of the above. Verification / Alternative check: Superheterodyne block diagrams of both AM and FM receivers confirm the same structural blocks, though tuned differently. Why Other Options Are Wrong: Individual choices are correct but incomplete. “None of the above” contradicts practical designs. Common Pitfalls: Thinking that FM receivers do not use AGC (they do, though implementation differs). Final Answer: All of the above.

Question 7

Intermediate frequency too low: In a radio receiver, what will happen if the chosen intermediate frequency (IF) is too low?

Accepted Answer

Introduction / Context: The choice of IF in a superheterodyne receiver involves trade-offs. Too low an IF introduces both advantages and disadvantages that affect performance. Given Data / Assumptions: Superheterodyne receiver design. Intermediate frequency selection influences image rejection, selectivity, and oscillator design. Concept / Approach: Low IF narrows the separation between the signal and its image, which improves image rejection. However, it also sharpens selectivity excessively and forces the oscillator tuning range to adjust accordingly. Step-by-Step Solution: Low IF → image frequency farther from desired signal → better rejection.Low IF → narrower filter bandwidth → overly sharp selectivity.Low IF → LO tuning range smaller → LO selectivity reduced. Verification / Alternative check: Standard receiver design handbooks confirm all these effects occur with low IF. Why Other Options Are Wrong: Each individual effect is valid, but complete answer is all of the above. Common Pitfalls: Assuming only one effect dominates; in fact, multiple occur simultaneously. Final Answer: All of the above.

Question 8

Broadcast superheterodyne receiver principle: Which of the following statements correctly describe the operation of a broadcast superheterodyne receiver?

Accepted Answer

Introduction / Context: Broadcast superheterodyne receivers are the most common radio architecture, using frequency conversion to simplify selectivity and amplification. Given Data / Assumptions: Receiver tuned frequency f_s. Local oscillator frequency f_lo typically = f_s + IF. Mixer requires inputs of both f_s and f_lo. Concept / Approach: The mixer stage combines the incoming RF signal with the local oscillator signal, producing sum and difference frequencies. To achieve proper conversion, the RF input must be tuned to the desired signal frequency f_s. Step-by-Step Solution: Identify RF amplifier stage → tuned to f_s.Mixer multiplies f_s and f_lo → generates f_lo ± f_s.Select IF = |f_lo – f_s|. Verification / Alternative check: Superheterodyne block diagrams confirm that the mixer input is always tuned to the signal frequency, while the LO is offset by IF. Why Other Options Are Wrong: Local oscillator below signal: uncommon for broadcast receivers. LO = 2*IF: not a general rule. RF amp above carrier by a few kHz: incorrect. Common Pitfalls: Confusing LO frequency placement (above vs. below signal). Final Answer: The mixer input must be tuned to the signal frequency.

Question 9

Superheterodyne receivers: What happens if the intermediate frequency (IF) lies within the receiver’s own tuning range?

Accepted Answer

Introduction / Context: In superheterodyne radio receivers, the incoming radio frequency (RF) is mixed with a local oscillator (LO) to produce a fixed intermediate frequency (IF). This IF is then amplified and filtered for selectivity. Placing the IF within the receiver's own tuning range creates multiple interference and usability problems that designers try to avoid. Given Data / Assumptions: Receiver uses superheterodyne architecture with a fixed IF (for example, 455 kHz in AM broadcast sets). The receiver's RF tuning range includes the IF itself. Standard mixer behavior produces sum and difference frequencies. Concept / Approach: If the IF lies inside the tunable RF band, the receiver can unintentionally tune to its own IF, and signals near the IF will interfere with normal operation. Beat notes (heterodyne whistles) and dead zones occur. Adjacent bands near the IF become unusable because any small leakage or LO harmonics can produce a loud tone or self-oscillation-like artifacts. Step-by-Step Solution: Mixer output produces f_lo + f_rf and |f_lo - f_rf| = IF.If IF is inside the RF tuning band, unwanted RF stations at or near IF may be amplified as if they were legitimate signals.Self-mixing and leakage around IF transformers create audible whistles when tuning across the IF region.The band immediately adjacent to IF becomes impractical to use due to persistent tones and overload. Verification / Alternative check: Receiver design handbooks recommend choosing IF values outside the receiver's RF coverage band or adding front-end RF selectivity explicitly to avoid these issues. Why Other Options Are Wrong: (a), (b), and (c) each describe real effects; the correct comprehensive choice is (d) All of the above. (e) None of the above contradicts known superheterodyne behavior. Common Pitfalls: Confusing image-frequency problems with the distinct issue of IF being inside the tuning range; both are different but harmful. Final Answer: All of the above

Question 10

Oscillators and operating bands: Which of the following oscillators is generally not used at Very High Frequency (VHF)?

Accepted Answer

Introduction / Context: RF designers choose oscillator topologies based on frequency range, stability, and ease of tuning. VHF spans roughly 30 MHz to 300 MHz. Popular RF oscillators (Colpitts, Clapp, Hartley, and Armstrong variations) are widely used in this band. Given Data / Assumptions: Target operating range: VHF (approximately 30–300 MHz). Candidate oscillator types: Colpitts, Clapp, Armstrong, and an 'Ultra Audio' oscillator (audio-frequency region). Assume standard component quality and practical RF layouts. Concept / Approach: Colpitts, Clapp, and Armstrong (including Hartley) are resonant LC oscillators frequently adapted for VHF because they employ tank circuits that can be built with small, low-loss inductors and capacitors suitable for high frequencies. 'Ultra Audio' describes very low-frequency applications and is not a recognized RF VHF oscillator class. Step-by-Step Solution: Identify VHF-friendly LC oscillators: Colpitts, Clapp, Hartley/Armstrong → widely used at VHF.Identify non-VHF option: 'Ultra Audio' → audio-frequency domain, not suitable for VHF generation. Verification / Alternative check: Basic RF textbooks show Colpitts and Clapp as preferred for higher frequencies due to their excellent frequency stability and ease of implementation. Armstrong/Hartley designs also scale into VHF with proper components. Why Other Options Are Wrong: Colpitts, Clapp, and Armstrong are established VHF oscillators. Hartley (listed under option e) is also used at VHF with correct design. Common Pitfalls: Assuming any audio-labeled oscillator can be used in VHF by simply changing component values—parasitics and device transit times limit such scaling. Final Answer: Ultra Audio oscillator

Question 11

Effect of choosing too high an intermediate frequency (IF) in a superheterodyne radio receiver: Which statement is correct?

Accepted Answer

Introduction / Context: The selection of intermediate frequency (IF) in superheterodyne receivers is a key design compromise. Higher IF values impact selectivity, tracking, and adjacent-channel behavior. Understanding these trade-offs is essential in communication system design. Given Data / Assumptions: Receiver uses a single, fixed IF. We are evaluating the effects of choosing an excessively high IF. Quality factor (Q) of tuned circuits and practical filter limitations apply. Concept / Approach: Bandwidth BW for a tuned circuit is approximately f_center / Q. For a given practical Q, a higher center frequency (higher IF) increases bandwidth, which reduces selectivity and worsens adjacent-channel rejection. Tracking—the ability of RF and LO tuned circuits to follow each other across the band—tends to be easier with higher IF because frequency offsets are proportionally larger and front-end alignment becomes less critical. Step-by-Step Solution: Assume fixed Q in IF filters.If IF increases → BW = f_if / Q increases → selectivity degrades → statement (a) is true.Higher IF typically eases tracking constraints in the RF/LO front-end → statement (b) is broadly acceptable (tracking difficulties are least).Adjacent-channel rejection depends on selectivity; if selectivity is poorer, adjacent-channel rejection becomes worse → statement (c) is false. Verification / Alternative check: Standard receiver design texts note that increasing IF improves image rejection but degrades selectivity and adjacent-channel performance unless very high-Q filters (or crystal/SAW filters) are used. Why Other Options Are Wrong: (b) alone is not the best single answer; although often true, it does not address the main trade-off asked here. (c) is incorrect because poorer selectivity causes worse, not better, adjacent-channel rejection. (d) All of the above is false since (c) is false. (e) None of the above is false because (a) is true. Common Pitfalls: Confusing image-frequency rejection (which improves with higher IF) with adjacent-channel rejection (which degrades without higher-Q filters). Final Answer: Selectivity will be poor

Question 12

In communication systems, what is the function of a duplexer device in relation to transmitters, receivers, and a common antenna?

Accepted Answer

Introduction / Context: A duplexer is an important device in modern communication systems, particularly in radar and radio transceivers, where both transmission and reception must occur using a single antenna. This avoids the need for separate antennas and simplifies system design while ensuring isolation between transmit and receive paths. Given Data / Assumptions: One antenna is shared by transmitter and receiver. Transmitter produces high power RF signals. Receiver requires protection from transmitter output while remaining sensitive to weak incoming signals. Concept / Approach: The duplexer is a switching or filtering device that alternately connects the antenna to the transmitter during transmission and to the receiver during reception. In radar, fast switches like TR (transmit–receive) cells or circulators are used to achieve this. Step-by-Step Solution: Requirement: single antenna for both Tx and Rx.Duplexer ensures isolation of receiver when transmitter is active.During reception, it connects antenna to sensitive receiver while isolating transmitter. Verification / Alternative check: Radar block diagrams confirm that duplexers sit between antenna, transmitter, and receiver. Why Other Options Are Wrong: Feeding multiple receivers: that is done by a power divider, not a duplexer.Connecting two transmitters: requires a combiner, not a duplexer.Multiple antennas: unrelated to duplexing function. Common Pitfalls: Confusing duplexer with diplexer (frequency multiplexer).Assuming duplexer amplifies or processes signals—it only switches/isolates. Final Answer: Connect a receiver and a transmitter to the same antenna

Question 13

In a superheterodyne receiver with an intermediate frequency (IF) of 450 kHz, if the receiver is tuned to a signal frequency of 1200 kHz, what is the corresponding image frequency?

Accepted Answer

Introduction / Context: The concept of image frequency is central to understanding the operation of superheterodyne receivers. The image frequency is an undesired signal that can also be converted to the same intermediate frequency (IF), potentially causing interference if not rejected by the RF front-end. Given Data / Assumptions: Signal frequency fs = 1200 kHz. Intermediate frequency fi = 450 kHz. Local oscillator frequency chosen as flo = fs + fi = 1650 kHz (high-side injection). Concept / Approach: The image frequency fsi is defined as fsi = flo + fi or fsi = flo − fi depending on injection side. For high-side injection (LO above signal), fsi = flo + fi = 1650 + 450 = 2100 kHz. Step-by-Step Solution: flo = fs + fi = 1200 + 450 = 1650 kHz.fsi = flo + fi = 1650 + 450 = 2100 kHz.Therefore, image frequency = 2100 kHz. Verification / Alternative check: Alternative formula: fsi = fs + 2fi = 1200 + 900 = 2100 kHz (same result). Why Other Options Are Wrong: 750 and 990 kHz correspond to differences, not images.1650 kHz is the LO frequency, not the image.1350 kHz is arbitrary, not related to IF image equation. Common Pitfalls: Confusing LO frequency with image frequency.Using wrong formula: fsi = fs − 2fi (applies only for low-side injection). Final Answer: 2100 kHz

Question 14

In radio receivers, RF amplifiers are included at the front end primarily to achieve which of the following benefits?

Accepted Answer

Introduction / Context: Radio frequency (RF) amplifiers are often used as the first stage in radio receivers. Their purpose extends beyond amplification; they also contribute to selectivity, sensitivity, and interference suppression, making them a critical part of the receiver design. Given Data / Assumptions: RF amplifier stage placed before the mixer. Connected directly to antenna input. Operates at the frequency being tuned, not at IF. Concept / Approach: RF amplifiers provide gain to weak signals but also serve additional roles. They improve image frequency rejection by filtering, reduce adjacent channel interference, and isolate the local oscillator to prevent radiation back through the antenna. Step-by-Step Solution: Improved image frequency rejection → achieved by RF stage selectivity.Improved adjacent rejection → due to tuned RF filters.Prevent oscillator radiation → RF stage provides isolation between antenna and mixer/oscillator. Verification / Alternative check: Receiver design texts confirm the three standard purposes of RF amplifiers. Why Other Options Are Wrong: Options (a), (b), and (c) are individually true but incomplete; the correct answer must be 'all of the above'. Common Pitfalls: Believing RF amplifiers exist only to increase sensitivity—ignores their key role in selectivity and LO isolation. Final Answer: All of the above

Question 15

In frequency modulation (FM), how does the transmitted carrier power vary with respect to the power of the modulating signal?

Accepted Answer

Introduction / Context: In frequency modulation (FM), unlike amplitude modulation (AM), the carrier amplitude remains constant regardless of the strength of the modulating signal. This unique feature of FM provides inherent noise immunity and efficiency advantages. Given Data / Assumptions: FM system uses constant amplitude carrier. Information is carried in frequency deviation, not amplitude. Transmitter power depends on carrier amplitude, not modulation depth. Concept / Approach: The total transmitted power in FM remains constant because the carrier amplitude does not vary with modulation. Frequency deviation changes, redistributing power between carrier and sidebands, but the total RF output power remains the same regardless of modulating signal strength. Step-by-Step Solution: FM signal: v(t) = Ac cos(2πf_ct + β sin(2πf_mt)).Ac (carrier amplitude) remains fixed.Average transmitted power P = (Ac^2) / (2R).This does not depend on β (modulation index) or modulating signal power. Verification / Alternative check: Practical FM transmitters (broadcast, two-way radios) deliver constant carrier power independent of modulation depth. Why Other Options Are Wrong: Increase or decrease with modulating signal: describes AM, not FM.Dependence on modulation index: affects spectral distribution, not total transmitted power.'None of the above' is incorrect because there is a specific correct answer. Common Pitfalls: Confusing FM with AM, where power is proportional to modulation depth.Assuming carrier amplitude changes with modulating signal in FM—it does not. Final Answer: Is independent of the modulating signal power

Question 16

In a superheterodyne receiver, if the signal frequency is fs and the intermediate frequency is fi, what is the formula for the image frequency fsi that must be rejected by the RF front end?

Accepted Answer

Introduction / Context: The image frequency is a well-known challenge in superheterodyne receivers. It arises because both the desired frequency and another undesired frequency can produce the same intermediate frequency (IF) after mixing. To avoid distortion, image frequency must be rejected by RF filters. Given Data / Assumptions: Superheterodyne receiver with IF = fi. Signal frequency = fs. Local oscillator frequency = fs + fi (high-side injection assumed). Concept / Approach: The image frequency is defined as the frequency that is separated from fs by twice the IF. Mathematically, fsi = fs + 2fi for high-side injection. For low-side injection, fsi = fs − 2fi. Step-by-Step Solution: Local oscillator flo = fs + fi.Mixing: |flo − fs| = fi.For an undesired frequency fsi: |flo − fsi| = fi.So fsi = flo + fi = (fs + fi) + fi = fs + 2fi. Verification / Alternative check: Example: fs = 1200 kHz, fi = 450 kHz ⇒ fsi = 1200 + 900 = 2100 kHz. Why Other Options Are Wrong: fs − 2fi: applies to low-side injection only, not general formula.fs ± fi: corresponds to LO or IF, not image.2fs − fi: unrelated expression. Common Pitfalls: Forgetting the ±2fi relationship; confusing LO with image frequency. Final Answer: fsi = fs + 2fi

Question 17

Advantages of superheterodyne receivers: Compared to tuned radio frequency (TRF) receivers, which of the following is the main advantage of using superheterodyne receivers?

Accepted Answer

Introduction / Context: The superheterodyne architecture is the dominant receiver design in radio communications. It was developed to overcome the limitations of tuned radio frequency (TRF) receivers, particularly when operating at higher frequencies where selectivity becomes a challenge. Given Data / Assumptions: TRF receivers rely on multiple RF tuned stages. Superheterodyne receivers use frequency conversion to a fixed intermediate frequency (IF). IF stages provide consistent filtering and amplification. Concept / Approach: By converting all signals to a common IF, superheterodyne receivers simplify the design of sharp filters. This improves selectivity, especially at high frequencies, while TRF receivers struggle to maintain consistent tuning and narrow bandwidths across the spectrum. Step-by-Step Solution: TRF receivers → multiple RF stages tuned to the signal → bandwidth varies with frequency.Superheterodyne → converts signal to fixed IF (e.g., 455 kHz) → high-Q IF filters provide stable selectivity.Result → much better selectivity, especially for high-frequency signals. Verification / Alternative check: Historical adoption of superheterodyne receivers in AM/FM broadcasting and communications demonstrates their superior selectivity as the main advantage. Why Other Options Are Wrong: High gain and sensitivity: possible with TRF as well, not unique to superhet. Stability: LO drift can cause instability; not a primary advantage. Noise suppression: depends more on modulation/demodulation, not core architecture. Lower cost: TRF sets were actually simpler and cheaper. Common Pitfalls: Confusing sensitivity with selectivity. Assuming superhet suppresses noise inherently (it does not). Final Answer: Better selectivity at high frequencies

Question 18

Padders in receivers: In a radio receiver, what is the function of padders?

Accepted Answer

Introduction / Context: Padders are small capacitors used in the local oscillator or tuning circuits of radio receivers. They play a crucial role in ensuring the oscillator and RF stages track each other correctly across the frequency band. Given Data / Assumptions: Receiver uses superheterodyne design. Tracking means maintaining a fixed IF difference between the RF and LO frequencies across the entire band. Concept / Approach: Because the tuning capacitance vs. frequency relationship of RF and LO circuits may not match perfectly, padders are added to adjust (pad) capacitance in the LO circuit, ensuring proper alignment and tracking across the band. Step-by-Step Solution: Without padders: RF and LO frequencies drift apart at higher/lower ends of the band.Add padders: adjust capacitance curve of LO circuit.Result: tracking is restored, IF remains constant. Verification / Alternative check: Radio servicing manuals confirm that padders are adjusted during alignment to ensure RF and LO stages track properly. Why Other Options Are Wrong: Discard the carrier: function of detector, not padders. Filter input signal: done by RF filters. Suppress noise: handled by AGC and filtering. Provide impedance matching: padders are not for matching. Common Pitfalls: Confusing padders with trimmers (used for fine adjustment). Final Answer: Facilitate tracking

Question 19

Most common intermediate frequency (IF): For broadcast receivers that tune the medium frequency (MF) band from 540 kHz to 1650 kHz, what is the most widely used IF value?

Accepted Answer

Introduction / Context: The intermediate frequency (IF) is a critical design choice in superheterodyne receivers. The chosen IF must balance selectivity, image rejection, and ease of filter implementation. Given Data / Assumptions: Receiver covers MF broadcast band: 540–1650 kHz. Conventional IF choices are in the 400–500 kHz range. Concept / Approach: The most common IF for AM broadcast receivers is 455 kHz. This value is high enough to provide good selectivity with LC filters and low enough to avoid excessive image problems within the band. Step-by-Step Solution: Set IF near 455 kHz.This allows image frequency spacing = 2 * IF = ~910 kHz, manageable within MF band.Filters for 455 kHz are easy to manufacture with stable characteristics. Verification / Alternative check: Textbooks and service manuals confirm 455 kHz as the de facto standard IF for AM broadcast sets. Why Other Options Are Wrong: 433 kHz and 465 kHz: occasionally used, but less standard. 545 kHz and 555 kHz: too close to broadcast band, risk interference. Common Pitfalls: Confusing IF of AM receivers (455 kHz) with FM receivers (~10.7 MHz). Final Answer: 455 kHz

Question 20

Ratio detector characteristics: Which of the following statements about a ratio detector is correct?

Accepted Answer

Introduction / Context: The ratio detector is a frequency demodulator used in FM receivers. It is derived from the phase discriminator but has important advantages in terms of automatic amplitude limiting. Given Data / Assumptions: Ratio detector is used in FM demodulation. Operates without a limiter stage in some designs. Concept / Approach: A ratio detector generates an output voltage proportional to the frequency deviation of the input FM signal. Unlike the phase discriminator, it inherently cancels amplitude variations, providing stabilization against signal strength fluctuations. Step-by-Step Solution: FM input signal is fed into tuned transformer secondary.Two diodes detect and charge capacitors asymmetrically.Amplitude variations cancel; frequency variations remain as output. Verification / Alternative check: Radio receiver design manuals confirm ratio detectors improve performance by rejecting AM noise and signal strength changes. Why Other Options Are Wrong: (a) Linearity is not worse; it is acceptable. (b) Output is not exactly double; misleading. (d) Diode reversal alone is not the defining feature. (e) Output is not constant—it varies with frequency deviation. Common Pitfalls: Assuming ratio detector improves linearity—it actually improves amplitude stability. Final Answer: Stabilization against signal strength variations is provided

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