Match the items in communication systems: (A) Ring modulator, (B) VCO (Voltage-Controlled Oscillator), (C) Foster–Seeley discriminator, (D) Mixer — with their primary functions: (1) Clock recovery, (2) Demodulation of FM, (3) Frequency conversion, (4) Summing of two points, (5) Generation of FM, (6) Generation of DSB-SC.
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AA-1, B-3, C-2, D-4
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BA-6, B-5, C-2, D-3
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CA-6, B-1, C-3, D-2
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DA-5, B-6, C-1, D-3
Answer
Correct Answer: A-6, B-5, C-2, D-3
Explanation
Introduction / Context:Communication systems rely on distinct building blocks, each serving a well-defined role in modulation, demodulation, frequency translation, or timing. This matching exercise checks whether you can associate common RF/communication components with their hallmark functions: ring modulators for suppressed-carrier generation, VCOs for frequency generation/modulation, Foster–Seeley discriminators for FM demodulation, and mixers for heterodyning (frequency conversion).
Given Data / Assumptions:
- (A) Ring modulator
- (B) VCO (Voltage-Controlled Oscillator)
- (C) Foster–Seeley discriminator
- (D) Mixer
- Possible roles: (1) clock recovery, (2) FM demodulation, (3) frequency conversion, (4) summing, (5) FM generation, (6) DSB-SC generation
Concept / Approach:
A ring modulator (a balanced modulator) multiplies two signals and cancels the carrier, producing a double-sideband suppressed-carrier (DSB-SC) output. A VCO produces a frequency proportional to a control voltage; by driving that control line with the modulating signal, it generates frequency modulation (FM). The Foster–Seeley discriminator is a classic frequency-to-voltage detector used for narrowband FM demodulation. A mixer multiplies two sinusoids, producing sum and difference frequencies, enabling up/down conversion in superheterodyne receivers and transmitters.
Step-by-Step Solution:
Step 1: Ring modulator → DSB-SC generation: choose (6).Step 2: VCO → FM generation: choose (5).Step 3: Foster–Seeley discriminator → FM demodulation: choose (2).Step 4: Mixer → frequency conversion (heterodyning): choose (3).Verification / Alternative check:
Textbook transmitter chains: balanced modulator for DSB-SC, optional carrier reinsertion for AM; FM transmitters employ VCOs or indirect modulation loops. Receiver IF strips commonly use Foster–Seeley (or ratio detectors) for FM detection. Mixers are ubiquitous in front ends (RF→IF) and in transmit chains (IF→RF).
Why Other Options Are Wrong:
(1) Clock recovery is typically a role of PLLs/CR circuits, not ring modulators or mixers directly. (4) Simple summing is performed by adders/combiners, not ring modulators. Swapping VCO and discriminator confuses generation with detection.
Common Pitfalls:
Assuming any multiplier equals a “mixer” in all contexts; forgetting that balanced modulators suppress the carrier; confusing PLL-based FM synthesis with the discriminator’s detection role.
Final Answer:
A-6, B-5, C-2, D-3