CRAC Class C (2025) — Questions 721–730

A good receiver needs all four attributes: strong interference rejection (selectivity, 抗干扰能力) to ignore unwanted signals; enough overall gain (整机增益) to bring weak signals up to usable level; low internal noise (本机噪声) so weak signals are not masked; and low signal distortion (信号失真) so the demodulated audio/data is faithful. Each directly contributes to receiving capability, so all four options are correct.

Demodulation (解调) is the inverse of modulation: the demodulator (解调器) recovers the original information (voice/data) from the modulated RF/IF signal. Option B is amplification, C is filtering/bandwidth selection, D is carrier extraction (a sub-step, not the recovery of the message). The defining role is recovering the modulating signal — A.

The primary criterion for choosing a demodulator is fidelity — how faithfully it reconstructs the original modulating signal (忠 实还原). Amplification (B) is the job of amplifiers, boosting the carrier (C) is not the aim, and frequency-offset compensation (D) is the job of the receiver's frequency-control/AFC, not the demodulator's selection criterion. Hence A.

CTCSS (亚音频静噪 / sub-audible tone squelch, 连续单音编码静 噪) uses a low-frequency supervisory tone added below the normal speech band. The standard CTCSS tone set spans roughly 67 Hz to 250.3 Hz — these are "sub-audible" because they sit at the low edge of (or just below) normal hearing and are filtered out before audio output. Option B is the whole audio range, C is high audio, D is too high. So A is correct.

The shape factor / rectangular coefficient (通带矩形系数 / 形状 系数) of a bandpass filter compares its bandwidth at two attenuation levels (e.g., 60 dB bandwidth ÷ 6 dB bandwidth). A smaller number means steeper skirts — the response curve drops more sharply outside the passband. It describes the steepness of the filter's response-curve slopes (A), not the shape of a rectangular waveform (B, C) or the crystal cut (D). So A is correct.

The rectangular coefficient (60 dB BW ÷ 6 dB BW) measures skirt steepness. A smaller coefficient (3.8 vs 5) means sharper skirts, so receiver A attenuates signals just outside the 2.7 kHz passband much more strongly than B — i.e., better adjacent- channel (邻近频道) rejection. Image rejection (C) depends on the front-end/preselector, not the IF filter shape factor; ±10 kHz (D) is far outside the 2.7 kHz filter and both would be well attenuated, so the distinguishing conclusion is adjacent- channel rejection. Hence A.

A low-noise preamplifier (前置放大器 / LNA) placed early in the receive chain raises the signal above the receiver's own downstream noise figure, so the system noise is dominated by the (low) preamp noise rather than later stages. Its main purpose is to reduce the impact of internal receiver noise (降低 本机噪声影响), improving weak-signal sensitivity. It does not set audio output power (B), fidelity (C) or dynamic range (D) as its primary role. So A is correct.

In FM, information is carried by frequency deviation, not amplitude. The receiver's limiter (限幅电路) clips the IF amplitude to a constant level before the discriminator (鉴频器), so the recovered audio amplitude reflects only deviation (modulation), not received-signal strength. Therefore loudness cannot indicate signal strength — A is the precise, correct explanation. B is also "no" but gives a wrong reason (AGC does not normally set FM audio volume that way). C and D are false.

WFM (wideband FM, ~75 kHz deviation, used for broadcast) received in NFM (narrowband FM, ~2.5–5 kHz deviation, used for amateur/PMR) over-deviates the NFM discriminator. The discriminator still recovers audio, so you hear the signal, but large/high audio swings exceed the NFM deviation range and clip, producing obvious non-linear distortion. Hence A. C/D ("normal") are wrong; B ("cannot hear") is wrong because the carrier is still demodulated.

This is the reverse of U0511: an NFM (small deviation) signal received in WFM (large-deviation) mode under-deviates the WFM discriminator, so the recovered audio is weak/quiet but essentially undistorted — you hear it normally, just at low volume (A). Distortion (B) occurs in the opposite direction (NFM receiving WFM). Hence A is correct.