CRAC Class B (2025) — Questions 541–550

Because no station is actually on the uplink yet the repeater keys up with voice-like noise, the likely cause is third-order intermodulation: two strong off-frequency signals mix in the repeater receiver to produce a product exactly on the uplink frequency, opening the squelch (A). "Definitely malicious" (B) is too strong; parasitic oscillation (C, D) would not mimic voice from external signals.

If the repeater's own downlink leaks back into its receiver (poor isolation between uplink and downlink), the receiver keeps seeing a "signal" and holds the transmitter on — a self-locking loop (A). This is a technical fault, not necessarily jamming (B), and power-supply issues (C, D) wouldn't cause a sustained key- up tied to the uplink path.

After the legitimate uplink stops, if the repeater keeps transmitting with voice-like noise, an external signal mixing with the repeater's own downlink can generate an intermod product on the uplink frequency that re-opens the squelch (A). As in U0842, this is intermod, not necessarily jamming (B) or power issues (C, D).

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.

AGC only acts after the front-end amplifier; a very strong signal that arrives before the AGC-controlled stages can drive the first mixer/RF amp into nonlinear operation, causing blocking, cross-modulation and intermodulation (互调). The ATT pad sits ahead of that stage and tames the strong signal before it overloads the front end. Option A is correct. B (gain range), C (speaker protection), D (TX self-damage) are not the real reason.