CRAC Class B (2025) — Questions 771–780

FM quieting/Noise: with no signal, the receiver sees random noise (QRN plus internal noise). Because FM information is in phase/frequency, the random phase fluctuations of this noise are demodulated by the discriminator as a "shhh" (hiss). Random amplitude fluctuations are removed by the limiter and thus do not contribute to the audio hiss — exactly A. Options B/C/D wrongly attribute the noise to amplitude variations (proportional to V, V², √V), which is how AM noise behaves, not FM.

An FM signal is demodulated by a frequency discriminator (鉴 频器), which converts frequency deviation into voltage. A limiter (B) only flattens amplitude (prep for the discriminator); a product detector (C) demodulates SSB/CW (AM-family); a mixer (D) does frequency conversion. So only A demodulates FM.

鉴频 (frequency discrimination) is the demodulation of an FM signal — recovering the original audio from frequency deviation. B and C describe frequency monitoring/measurement, not demodulation; D is detection (检 波) of AM, not 鉴频. So A is correct.

An AM signal is demodulated by a detector (检波器), typically an envelope detector. A limiter (B) is for FM, a discriminator (C) is for FM, an inverter (D) simply flips phase. So A is correct.

检波 (detection) is the demodulation of an AM signal. B and C describe monitoring functions, and D describes 鉴频 (FM demod), not 检波. So A is correct — this is the deliberate counterpart to U0515.

AGC (automatic gain control, 自动增益控制) samples the IF (or detector) level and adjusts receiver gain to keep strong signals from overloading subsequent stages — A is correct. For FSK/PSK (e.g., RTTY, FT8) weak-signal decoding, a slow/fast AGC can pump the audio and hurt the decoder, so disabling AGC can improve decoding — B is correct. D is wrong because AGC controls RF/IF gain, not audio volume (and not to protect the speaker). Per the official answer key, C is not selected for this item (although in practice AGC is often disabled for noise- figure/solar-noise measurements, the exam key marks only A and B).

A DC receiver is a Direct-Conversion receiver (直接变换): the incoming RF is mixed directly to baseband/audio (or near-zero IF) without the usual superheterodyne intermediate frequency — no image-frequency issue but it receives both sidebands. DC here is not Direct Current (B), a capacitor (C), or digital conversion (D). So A is correct.

A mixer (混频器 / frequency converter) multiplies two frequencies (the incoming signal and a local oscillator) to produce sum and difference frequencies — performing frequency conversion (频率变换). It is not audio mixing (B), motor speed control (C), or a multi-frequency amplifier (D). So A is correct.

Up/down conversion is defined by the relationship between the input (RF) frequency and the resulting IF: if the IF is higher than the input frequency, the signal was up-converted; if the IF is lower, it was down-converted. This is the convention used in the Chinese exam — A. Note this is defined by IF vs input, not by the local-oscillator relationship (C/D) or merely input vs IF (B). So A is correct.

The selectable IF filter bandwidth rejects signals close to (but outside) the wanted channel — i.e., adjacent-frequency (邻近频 率) interference. Image-frequency interference (B) is handled by the preselector before the mixer, not by the IF bandwidth; C and D are not what IF bandwidth selection addresses. So A is correct.