CRAC Class C (2025) — Questions 1071–1080

阻抗匹配 (impedance matching) means the load's reactance cancels the source reactance (A correct, conjugate match) and the load resistance equals the source's optimal resistance (B correct, for maximum power transfer). C and D describe constant-current/constant-voltage operation, which is not the meaning of matching.

By the maximum-power-transfer theorem, when a source of internal resistance R is matched to a load R, the load voltage equals half the open-circuit voltage, so the source and load each dissipate equal power (A correct). B/C/D are false; matching applies to both DC and AC.

非线性失真 (nonlinear distortion) specifically means the output contains frequency components not present at the input — new frequencies (harmonics, intermodulation) are created by the nonlinear transfer characteristic (A correct). B (gain change), C (amplitude change), D (phase shift) can occur in linear systems too and are not the defining feature of nonlinear distortion.

A diode detector rectifies the envelope (nonlinear squaring/commutation, A), a ring modulator multiplies two signals (a nonlinear product generating sum/difference and new components, B), and a rectifier produces pulsating DC with rich harmonics (C) — all are nonlinear transformations. D is merely Ohm's law (I = V/R) with a series resistor, a linear relationship, not a waveform-nonlinearity transformation.

By Fourier series, a pure sinusoid/cosine of frequency f is a single spectral line at f; any complex periodic signal of fundamental f decomposes into harmonics at integer multiples (f, 2f, 3f, …) of f (A correct). Fractional multiples (B) occur only for non-periodic/inharmonic signals; C/D misstate the spectra.

A single infinitely narrow (impulse) pulse has a Fourier transform that is flat across all frequencies — a continuous spectrum with equal amplitude at every frequency (A correct). A comb of discrete lines (B) belongs to a periodic pulse train, not a single pulse.

Only a pure sinusoid (简谐信号, simple harmonic signal) has a single spectral line. A square wave has odd harmonics, a damped pulse has a broadband spectrum, and a pulse train has a comb of discrete lines — all contain multiple frequencies (B/C/D wrong).

Across the whole spectrum, only a single infinitely narrow impulse has a perfectly flat (uniform) continuous spectrum (A correct). A sinusoid is a single line (B), a square wave is discrete odd harmonics (C), and a damped pulse is a continuous but not uniform band (D).

The headphones reproduced the SSB voice, meaning the circuit recovered the audio from the RF — that is 检波 (detection/demodulation). The transmitted SSB was being picked up and detected by the nearby circuit (e.g., a diode in the headphone lead acting as an envelope/products detector).

CW has no audio of its own; an audible tone appears only when the received carrier beats against a local oscillator in the circuit, producing an audio 差拍 (beat frequency) equal to the difference. The nearby circuit therefore contains a local oscillator and a beat/mixer stage (a BFO/heterodyne), so the function is beat.