Question 1

Three-point tracking in AM broadcast receivers To achieve three-point tracking (alignment at low, mid, and high ends of the band) in a superheterodyne receiver, which component is specifically adjusted or added in the oscillator section along with R…

Accepted Answer

Introduction / Context: Tracking ensures that the RF tuned circuit and the local oscillator maintain the correct frequency relationship across the tuning band. Three-point tracking refines this alignment at the low, mid, and high ends of the band to minimize dial error and maintain sensitivity/selectivity. Given Data / Assumptions: Single-conversion AM superheterodyne with a gang (ganged) tuning capacitor. RF and oscillator sections each tuned; small alignment capacitors are available. The goal is precise oscillator-to-RF tracking at multiple points. Concept / Approach: While the gang capacitor tunes both RF and LO simultaneously, fine correction is needed because coil stray capacitances and mechanical tolerances cause frequency-dependent errors. A padder capacitor (usually a series capacitor in the oscillator tank) is adjusted to set the low-end tracking, while trimmer capacitors are used to tweak the high-end alignment. Together, these provide three-point tracking across the band. Step-by-Step Solution: Set high-end alignment with oscillator and RF trimmers (on the gang capacitor).Adjust low-end alignment with the series padder capacitor in the oscillator section.Iterate adjustments to achieve minimal dial error at low, mid, and high frequencies. Verification / Alternative check: Alignment procedures in service literature specify using a padder for the low end and trimmers for the high end, achieving three-point tracking with the ganged tuning system. Why Other Options Are Wrong: “Gang condenser” is the main tuning element but is not the specific adjustment for low-end tracking. “Variable selectivity network” affects IF bandwidth, not tracking. “Any of the above” is incorrect because selectivity networks and AGC capacitors do not correct tracking. Common Pitfalls: Attempting to fix tracking errors by misaligning IF; proper method is padder (low end) plus trimmers (high end) with the gang capacitor. Final Answer: Padder capacitor

Question 2

In a superheterodyne transistor receiver, what is the main advantage of using a Field-Effect Transistor (FET) as the very first RF amplifier stage? Explain the practical benefit at the front end of the receiver rather than later stages.

Accepted Answer

Introduction / Context: At the front end of a radio receiver, the first active device largely determines the overall noise figure. A Field-Effect Transistor (FET) is often chosen as the first RF amplifier because of its high input impedance and inherently low noise when properly biased. This question probes why a FET is preferred at the very first stage rather than deeper in the chain. Given Data / Assumptions: Superheterodyne topology with a dedicated RF amplifier ahead of the mixer. The first stage dominates the cascaded noise figure. FET exhibits high input impedance and low gate current. Concept / Approach: Friis’ formula shows the overall noise figure is most sensitive to the first stage’s noise and gain. Putting a low-noise, moderate-gain FET first reduces the system noise temperature and improves sensitivity, particularly for weak signals near the noise floor. While selectivity and AGC matter, they are not the primary benefits of choosing a FET as the first device. Step-by-Step Solution: Identify the performance metric most affected by the first stage: noise figure and sensitivity.Recall that FETs, when designed correctly, offer low equivalent input noise at RF.Conclude the best advantage is reduced noise at the receiver front end. Verification / Alternative check: Practical receivers use low-noise JFETs or MOSFETs in the RF front end; measured minimum discernible signal (MDS) improves compared to bipolar-only solutions under similar matching and bias conditions. Why Other Options Are Wrong: Improve selectivity: primarily a function of RF/IF filtering and Q, not the device type. Improve AGC effectiveness: AGC is a control strategy; device choice does not inherently improve it. Reduce detector clipping: a detector issue, not a first-stage device choice. Increase LO deviation: unrelated to the RF amplifier device. Common Pitfalls: Assuming the front-end device affects selectivity directly; in reality, filtering topology dominates selectivity, while the first device dictates noise performance. Final Answer: Reduce the effect of noise (better noise figure) at the receiver front end

Question 3

In superheterodyne receiver alignment, a ‘‘padder’’ is used in the local oscillator section. What is a padder electrically, as used for low-end tracking adjustment?

Accepted Answer

Introduction / Context: Receiver tracking aligns the RF tuned circuit and the local oscillator (LO) so that their frequency relationship remains correct across the tuning range. A padder is added to the LO circuit to correct low-frequency tracking errors in superheterodyne receivers. Given Data / Assumptions: The padder sits in the LO tuned network. Used to correct low-end dial calibration/tracking. Component is adjustable during alignment. Concept / Approach: Tracking errors arise because the tuning capacitances and inductances do not vary perfectly with frequency. A padder capacitor (often in series) fine-tunes the effective capacitance at the low-frequency end of the band, aligning the LO with the RF input circuit across the dial. This improves image rejection and frequency accuracy. Step-by-Step Solution: Recognize the padder's role is in the LO tank, not in bias or AGC networks.Identify its nature: an additional adjustable capacitor in series/parallel.Select the electrical component: capacitor. Verification / Alternative check: Service manuals specify “padder” adjustments at the low end of the band while “trimmer” capacitors are typically used for the high end; both are capacitors. Why Other Options Are Wrong: Insulator/varistor/variable resistor: not used for LO tracking. Inductor: coils affect tracking but the named “padder” is specifically a capacitor. Common Pitfalls: Confusing the padder (low-end) with the trimmer (high-end) capacitor; both are capacitive adjustments but serve different parts of the dial. Final Answer: A capacitor (usually variable or adjustable) used for tracking

Question 4

Broadcast FM receivers operating in the 88–108 MHz band typically use which standard intermediate frequency (IF)?

Accepted Answer

Introduction / Context: In superheterodyne FM broadcast receivers, a fixed intermediate frequency (IF) simplifies filtering and demodulation. A widely adopted standard for VHF FM broadcast (88–108 MHz) is 10.7 MHz, balancing image rejection, selectivity, and practical filter technology (ceramic/LC filters). Given Data / Assumptions: FM broadcast band: 88–108 MHz. Single conversion receiver assumed. Standard consumer designs. Concept / Approach: Choosing IF trades off image rejection (improved with higher IF) against achievable selectivity for a given Q. 10.7 MHz has emerged as a sweet spot: sufficiently high for reasonable image spacing and supported by abundant, standardized filters and demodulators. Step-by-Step Solution: Identify common IFs: AM ≈ 455 kHz; FM broadcast ≈ 10.7 MHz.Select the standardized FM IF: 10.7 MHz. Verification / Alternative check: Commercial tuner ICs and service manuals consistently specify 10.7 MHz IF for FM broadcast reception. Why Other Options Are Wrong: 8, 9.9, 12.2 MHz: not the standard for broadcast FM. 455 kHz: typical AM broadcast IF, not FM. Common Pitfalls: Confusing AM and FM IF standards or assuming IF equals one fixed world-wide value across all services. Final Answer: 10.7 MHz

Question 5

For an ideal white-noise process used in communications engineering, how does the power spectral density (PSD) vary with frequency?

Accepted Answer

Introduction / Context: White noise is a fundamental model in receiver analysis, representing thermal noise and other wideband disturbances. Understanding its power spectral density (PSD) is essential for calculating noise power in a given bandwidth and predicting signal-to-noise ratios. Given Data / Assumptions: White noise is an idealization with uniform PSD across frequency. Real systems are bandlimited by front-end filters and components. Thermal noise approximates white within limited bandwidths. Concept / Approach: By definition, white noise has a flat PSD: Sn(f) = N0/2 (single-sided conventions vary). Total noise power is PSD multiplied by the receiver bandwidth. While 1/f noise (flicker) and device corner frequencies exist, these are separate phenomena and not part of the ideal white-noise definition. Step-by-Step Solution: Recall the definition of white noise: constant spectral density.Relate to noise power: Pn = (PSD) * B.Select “constant with frequency”. Verification / Alternative check: Receiver noise figure measurements assume a flat thermal-noise density kT over the measurement bandwidth, consistent with white-noise modeling. Why Other Options Are Wrong: Increasing/decreasing with frequency: describes colored noise, not white. 1/f: flicker noise, significant at very low frequencies but not white. Bandlimited to IF only: limitation of the receiver front end, not of white noise itself. Common Pitfalls: Confusing white noise (flat PSD) with pink or flicker noise that has frequency-dependent spectra. Final Answer: It is constant with frequency (flat spectrum)

Question 6

An FM signal with modulation index M_f passes through an ideal frequency doubler. What is the modulation index at the output of the doubler (assume the modulating frequency remains the same)?

Accepted Answer

Introduction / Context: Frequency multipliers are common in FM transmitters and test setups. They multiply the carrier frequency by an integer n and also scale the peak frequency deviation. Knowing how modulation index transforms is crucial for meeting deviation and spectral mask limits after multiplication stages. Given Data / Assumptions: Input FM has modulation index Mf = Δf / fm. A frequency doubler multiplies instantaneous frequency deviation and carrier frequency by 2. Modulating frequency fm is unchanged by the multiplier. Concept / Approach: For an n-times frequency multiplier, Δfout = n * Δfin, and fm,out = fm,in. Therefore, Mf,out = Δfout / fm,out = n * Δfin / fm,in = n * Mf,in. With n = 2, the output modulation index doubles. Step-by-Step Solution: Write Mf = Δf / fm.For a doubler, Δf → 2Δf; fm unchanged.Hence Mf,out = 2 * Mf,in. Verification / Alternative check: Spectral lines (Bessel components) shift and scale consistently with the multiplier, reflecting increased deviation (wider spectrum) proportional to n. Why Other Options Are Wrong: Mf or Mf/2: contradicts the standard transformation. Mf^2: not supported by FM theory. “Unchanged with limiter”: a limiter clips amplitude, not frequency deviation. Common Pitfalls: Assuming the modulating frequency doubles—only the carrier and deviation are multiplied by the frequency multiplier. Final Answer: 2 M_f

Question 7

In a single-tone frequency-modulated (FM) signal, at what spacing do the spectral sidebands occur relative to the carrier?

Accepted Answer

Introduction / Context: Understanding the FM spectrum is essential for bandwidth estimates (Carson’s rule) and interference control. A single-tone FM signal creates an infinite set of sidebands whose positions are harmonically related to the modulating frequency fm. Given Data / Assumptions: Single-tone modulation at frequency fm. Carrier frequency fc with deviation Δf. Standard narrow/wideband FM spectral structure. Concept / Approach: The FM spectrum contains components at fc ± k fm where k = 1, 2, 3, … . Amplitudes are given by Bessel functions of the first kind and depend on the modulation index, but the frequency spacing is strictly fm regardless of index. Step-by-Step Solution: Write sideband locations: fc ± k fm.Conclude constant spacing equals fm.Select option stating spacing at the modulating frequency. Verification / Alternative check: Fourier expansion of a single-tone FM signal explicitly yields harmonics spaced by fm around fc. Why Other Options Are Wrong: Four times, twice, half: incorrect spacing factors. Random spacing: amplitudes vary with index, but frequencies remain harmonic multiples of fm. Common Pitfalls: Confusing sideband spacing with bandwidth growth from larger deviation; bandwidth increases, but spacing remains fm. Final Answer: Exactly the modulating frequency (integer multiples of f_m)

Question 8

In a receiver front end, what is the chief purpose of adding a preselector stage ahead of the mixer? Choose the benefit most directly provided by the preselector.

Accepted Answer

Introduction / Context: A preselector is a tunable band-pass filter placed before the mixer in a superheterodyne receiver. Its role is to admit the desired RF signal while rejecting out-of-band energy, including image frequencies and strong interferers that can overload or create spurious mixer products. Given Data / Assumptions: Tunable RF front end with at least one resonant circuit. Mixer produces sum and difference products, sensitive to out-of-band signals. Image frequency is offset by 2×IF from the desired RF for a given LO injection. Concept / Approach: By sharpening the RF selectivity, the preselector reduces energy at the image and other undesired signals before they reach the nonlinear mixer. This improves dynamic range and reduces desensitization. While audio fidelity and PA linearity are important, they are not functions of the preselector. Step-by-Step Solution: Identify the problem: mixer susceptibility to strong off-channel signals and images.Apply solution: a tuned preselector that tracks the LO to pass the desired channel only.Therefore, the primary benefit is higher selectivity and image/interference rejection. Verification / Alternative check: Receiver block diagrams show preselectors or RF band-pass filters explicitly specified for image rejection and front-end protection from strong adjacent signals. Why Other Options Are Wrong: Audio fidelity: governed by IF/audio stages and demodulator, not the preselector. Power amplifier linearity: unrelated to RF input filtering. Wider passband: opposite of the preselector’s purpose. Eliminating AGC: AGC manages gain, not selectivity. Common Pitfalls: Assuming preselection is optional; in high-interference environments, it is essential to maintain sensitivity and prevent intermodulation. Final Answer: Provide higher selectivity and image/interference rejection before mixing

Question 9

In a superheterodyne radio, the local oscillator (LO) is commonly tuned above the incoming RF frequency. Why is the LO chosen higher in practice?

Accepted Answer

Introduction / Context: Local oscillator (LO) injection can be above (high-side) or below (low-side) the desired RF. High-side injection is widely used because it eases several practical issues in consumer receivers and many communication sets. Given Data / Assumptions: Fixed intermediate frequency (IF) is used. RF preselection has finite Q and limited attenuation far from the tuned channel. Receiver must track across a band with acceptable calibration and stability. Concept / Approach: With high-side LO, the image lies at fimage = fRF + 2·IF, which is farther above the passband than the desired channel by twice the IF. This generally improves image rejection for the same RF filter selectivity. Designers also find dial tracking and band coverage convenient with high-side injection over common broadcast bands, though exact benefits depend on the band and front-end design. Step-by-Step Solution: Consider image frequency placement: high-side pushes image farther from the RF passband.Tracking: it is often easier to achieve consistent LO–RF spacing across the dial.Coverage: LO range maps cleanly to RF coverage with fewer bands in some designs.Therefore, all listed reasons are valid motivations. Verification / Alternative check: Receiver alignment guides and service literature frequently specify LO above RF for broadcast FM and AM designs, citing image and tracking considerations. Why Other Options Are Wrong: “None of the above” contradicts standard practice. Common Pitfalls: Believing one LO side is universally superior; the optimal choice can vary with band, IF, and preselector design. Final Answer: All of the above

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