CRAC Class C (2025) — Questions 1081–1090

The circuit generates a signal on its own only when powered — that is 振荡 (oscillation). A self-excited oscillator radiates a tone (here warbling like a birdie) that the receiver picks up; removing power stops it. Beat/detection/mixing all require an incoming RF to act on, but here the circuit itself is the source.

Third-order intermodulation (三阶互调) products from two tones are at 2f1−f2, 2f1+f2, 2f2−f1, 2f2+f1 (the combinations whose order sums to 3). These are the ones that fall close to the wanted band and cause interference, so A is correct. B lists harmonics; C lists sum/difference (mixing, second-order); D lists fifth-order products.

差拍 (beat) is the difference-frequency product of two signals through a nonlinear/multiplicative stage (A correct). Mixing (变 频) and second-order intermodulation (二阶互调) are the same mathematical product (f1−f2 and f1+f2); only the context differs (B correct). C is too broad (not every whistle is a beat), and D describes harmonics, not a beat.

A 倍频器 (frequency multiplier) works by driving an active device into nonlinearity so the output contains harmonics of the input, then a tuned circuit selects the desired harmonic (e.g., 2×, 3×) (A correct). B is wrong because a linear amplifier does not generate harmonics; C is wrong because a passive tuned circuit cannot create a harmonic that is not present; D is wrong because a crystal oscillator sets its own frequency, not a multiplication of the input.

A diode rectifier (A), diode detector (B), and diode ring modulator (C) are all nonlinear and therefore create harmonics from a sine input. A passive 选频网络 (tuned network, D) only selects existing components; it cannot generate new harmonics, so it is incorrect.

All four are angle/digital constant-envelope modulations: FSK, FM, PM, and PSK all keep a constant amplitude (the information is in frequency or phase, not envelope), so all four are correct. This is why nonlinear (Class C/D) amplifiers can be used for them.

For angle modulation (FM and PM), the carrier amplitude coefficient is the Bessel function J0(β); at certain modulation indices β (specific frequency/deviation) the carrier term J0 goes to zero — a Bessel null — so the carrier component disappears (A, B correct). SSB/DSB suppress the carrier by design (not by a specific modulation frequency/amplitude), and OOK keeps a full carrier while keyed on, so C and D are not the intended answers here.

In angle modulation the sideband amplitudes are Bessel functions Jn(β); at certain modulation indices a specific sideband pair Jn vanishes (A, B correct). For AM the two sidebands are always symmetric and present whenever modulating (C wrong); for DSB both sidebands are always present (D wrong). Thus only FM and PM fit the statement.

In 幅度调制 (AM), only the envelope amplitude varies with the message while the carrier phase stays constant (A correct). FM changes frequency (hence phase), PSK and FSK change phase/frequency, so their carrier phase is not constant (B/C/D wrong).

Per the examination key, 幅度调制 (AM) is the answer: in AM the carrier component itself retains a fixed amplitude and the information rides on the envelope, whereas FM/PM/PSK/FSK are treated here as angle/digital modulations whose "carrier" is not a fixed-amplitude reference component in the same sense. (Note: FM/PM/FSK/PSK are also constant-envelope in practice; this item keys on AM as the modulation whose carrier amplitude is the defining constant reference.)