CRAC Class C (2025) — Questions 1091–1100

抑制载波单边带 (SSB) transmits only one sideband (upper or lower) plus no carrier, so it carries the information in a single sideband set (A correct). AM has two sidebands + carrier; CW is a keyed carrier with keying sidebands; DSB has two sidebands — none of those is single-sideband.

A 乘积检波器 (product detector) multiplies the incoming signal by a local-oscillator (BFO) signal to recover the baseband — exactly what CW and SSB need (A correct). FM needs a discriminator/quadrature detector; PM likewise needs a phase detector; "extracting speech from noise" is noise reduction, not the detector's function.

In a 超外差式接收机 (superheterodyne receiver), the 混频器 (mixer) multiplies the incoming RF with the local-oscillator signal to produce the 中频 (IF) — the difference frequency (A correct). B describes mere summation, not frequency conversion; C is the IF filter's job; D is unrelated.

In a 外差式发射机 (heterodyne transmitter), the mixer up- converts the modulated 中频 (IF) to the final 射频 (RF) by adding/multiplying with a local oscillator (A correct). B is baseband mixing (not the IF→RF conversion); C is filtering; D is an auxiliary output, not the mixer function.

A ring modulator is a four-quadrant multiplier: it multiplies the carrier by the audio baseband, and the product is exactly a 抑制 载波双边带 (DSB, suppressed-carrier double-sideband) signal — carrier canceled, two sidebands present (A correct). Addition (B) gives ordinary AM with a carrier; C/D misdescribe the operation.

A diode ring modulator by default produces a double-sideband suppressed-carrier (DSB-SC) signal because the carrier terms cancel. To obtain ordinary AM with a transmitted carrier, you bias the baseband with a DC offset so its sign never reverses; this adds a carrier component to the output (A). Options B (pilot/reversing carrier), C (forcing leakage via power), and D (swapping ports) do not yield proper AM.

Per the Barkhausen criterion, an oscillator needs an amplifier with loop gain greater than 1 plus a positive-feedback path that sets the frequency (A). Negative feedback (B) stabilizes but does not oscillate; C lacks the required gain; D omits the gain requirement entirely.

The classic warm-up "drift" of a simple LC oscillator is caused by component heating after switch-on, which changes the inductance/capacitance values and thus the resonant frequency (A). B (aging) is a long-term effect, not the immediate tone sweep; C and D describe feedback/power loss, not the pitch glide.

ppm stands for "parts per million" (百万分之几) and is the standard unit for 频率容限 (frequency tolerance) — e.g., ±2.5 ppm. B/C/D are unrelated expansions of the letters.

VFO stands for 可变频率振荡器 (variable frequency oscillator) — an oscillator whose frequency you can tune freely, typically via a knob, setting the transceiver's main carrier (A and B both describe it correctly). C is a crystal oscillator (XTAL/OCXO), and D is a VXO (variable-frequency crystal oscillator), not a plain VFO.