CRAC Class A (2025) — Questions 401–410

An engine-speed-dependent whine in a mobile installation is the classic signature of alternator (发电机) ripple/whine fed back through the DC power line (and sometimes radiated). The pitch tracks RPM because alternator commutation frequency is proportional to engine speed. Option A is correct. Spark-plug noise is a sharp crackle/pops, not a smooth whine; fuel pump and ABS are not RPM-synchronous tones.

Excessive FM deviation (频偏过大) means your modulation swings wider than the channel allows, spreading into adjacent channels. The fix is to use the narrowband (NFM) setting if available, or reduce the audio drive by speaking farther from the mic so the limiter isn't driven hard. Option A is correct. B (fresh battery) does not reduce deviation; C (more power) makes interference worse; D is wrong and risky — shouting over-drives the modulator.

Adjacent-channel interference (邻频干扰) can come from an off- frequency carrier or from excessive out-of-band / spurious emission (杂散发射) of your transmitter. The proper response is to verify the dial accuracy and that the transmitter meets its spurious-emission limits, and correct the fault. Option A is correct. Increasing power (D) worsens interference; switching mode (B) or calling the maker (C) does not address the root cause. This ties to the obligation to avoid harmful interference (有害干扰) under the Radio Regulations of the PRC.

On VHF (the 2-meter band / 144 MHz), signals are often line- of-sight and subject to multipath (多径效应): reflected paths combine destructively, causing rapid fading (衰落) and distortion. Moving a few steps changes the path geometry and restores the signal — a standard VHF technique. Option A is correct. B (CTCSS) only gates squelch, C is the other station's setting, D (battery chemistry) is unrelated.

A headphone (耳机) blocks ambient room noise and couples the received audio directly to the ear, dramatically improving the ability to copy (抄收) weak SSB/CW in a noisy environment. Option A is correct. A low-pass filter (B) is an RF/audio component, not a listening device; a video display (C) and boom mic (D) do not help receiving.

NB = Noise Blanking (抑噪): it detects short, high-amplitude impulse spikes (e.g. from automobile ignition) and blanks them for a few microseconds. SQL = Squelch (静噪): it mutes the audio entirely when the signal is below a threshold. They are distinct functions. Option A states both correctly; B, C, D wrongly merge them or swap their definitions.

Spark-plug impulse noise is a burst of sharp spikes. The noise blanker (NB / 抑噪) is specifically designed to detect and clip these impulses before they reach the AGC and audio, preventing the AGC from pumping the desired SSB/CW signal down. Option A is correct. Lowering squelch (B) only opens the audio gate wider (more noise); tuning off (C) or RIT (D) does not remove impulse noise.

MODE selects the emission/operating mode (工作方式) — FM, AM, SSB (LSB/USB), CW, RTTY, etc. Option A is correct. Squelch modes (B) are set elsewhere (CTCSS/DCS menus); receiver architecture (C) and audio monitoring (D) are not what the MODE knob does.

ATT = input attenuator (衰减器), a switchable pad at the receiver front end that reduces a very strong incoming signal so the first stages do not overload (过载/阻塞). Option A is correct. B is AT/TUNE, C is ALC, D is transmit audio AVC.

AGC = Automatic Gain Control (自动增益控制). In a receiver it samples the IF/Detection level and adjusts the gain so strong and weak stations come out at a steady audio level and the stages don't overload. Option A is correct. B describes audio AVC (not the standard AGC definition), C is AT/TUNE, D is transmit ALC.