CRAC Class A (2025) — Questions 511–520

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.

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.