CRAC Class B (2025) — Questions 421–430
U0371. Whether FM or SSB modulation, applying an audio signal 10 times the specified input voltage to the microphone input of the transmitter, the most likely result is:
无论是FM还是SSB调制，向发射机的话筒输入端施加10倍于规 定输入电压的音频信号，最可能的结果是：
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Overdriving the mic input causes clipping/overmodulation, producing harmonics and intermodulation (IMD) that widen the emission (splatter) and reduce quality. It does not increase range (B/C false) and won't normally burn the PA (D false).
U0728. Amplitude modulation (AM) is a simple modulation because only the amplitude of the RF carrier varies following the applied modulating signal. The characteristics of AM are: (Choose all that apply.)
调幅（AM）是一种简单调制，因为仅有射频载波的幅度跟随施 加的调制信号而变。AM的特点是：
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In AM, only the carrier amplitude changes with the modulating signal: the carrier frequency stays fixed (A) and the instantaneous amplitude is proportional to the modulating signal's instantaneous value (B). C describes phase modulation, not AM; D is false because the necessary bandwidth (必要带宽) of AM is twice the highest modulating frequency, so it does depend on it.
U0729. An AM signal has not only the lower or upper sideband corresponding to the modulating signal, but may also contain a carrier. These can be subdivided and customized according to communication requirements. If AM signals are subdivided by type, which of the following descriptions is correct? (Choose all that apply.)
调幅信号不仅具有与调制信号相对应的下边带或上边带，而且 还可能含有载波。这些都可以根据通信要求细分定制。如将调 幅信号按种类细分，则以下描述正确的是：
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All four descriptions are correct: ordinary AM keeps both sidebands and the carrier, and CW (continuous wave) is indeed a keyed (on/off) form of AM (A); DSB-SC suppresses the carrier, leaving both sidebands (B); SSB-SC keeps one sideband only — LSB or USB (C); VSB (vestigial sideband) partly suppresses one sideband and is used for analog ATV (D). These are standard AM variants.
U0730. A voice signal is modulated by a USB transmitter into an RF signal. From a frequency-domain perspective, this modulation is:
一个语音信号经USB发射机调制成为射频信号。从频域观点 看，这种调制是：
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USB (upper sideband) modulation shifts the voice baseband spectrum upward and places it at the carrier frequency (upper sideband above the carrier) without inverting it — the spectrum is translated (A). LSB would invert the spectrum (option C). SSB is essentially a linear frequency translation, not a non-linear process producing infinite sidebands (D, which describes FM/PM).
U0731. A voice signal is modulated by an LSB transmitter into an RF signal. From a frequency-domain perspective, this modulation is:
一个语音信号经LSB发射机调制成为射频信号。从频域观点 看，这种调制是：
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LSB (lower sideband) modulation inverts the voice baseband spectrum and places it below the carrier (A). This is the mirror image of USB (option C, not inverted). SSB is a linear frequency translation, not the non-linear process in D.
U0732. Frequency modulation (FM) is a simple modulation because only the frequency of the RF carrier varies following the applied modulating signal. The characteristics of FM are: (Choose all that apply.)
调频（FM）是一种简单调制，因为仅有射频载波的频率跟随施 加的调制信号而变。FM的特点是：
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In FM only the carrier frequency varies while its amplitude stays constant (A), and the instantaneous frequency deviation (频偏) is proportional to the instantaneous modulating-signal value (B). C describes phase modulation (PM), not FM; D is false because FM's necessary bandwidth (Carson's rule) grows with both deviation and the highest modulating frequency.
U0733. Which of the following determines the frequency deviation of an FM signal?
下列哪一项决定了FM信号的频偏？
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For FM, the instantaneous frequency deviation (频偏) is proportional to the instantaneous amplitude of the modulating signal (A) — louder audio drives greater deviation. The modulating frequency sets how fast deviation swings, not its magnitude; phase relationship (D) is a PM concept.
U0734. Phase modulation (PM) is a simple modulation because only the phase shift of the RF carrier varies following the applied modulating signal. The characteristics of PM are: (Choose all that apply.)
调相（PM）是一种简单调制，因为仅有射频载波的相移跟随施 加的调制信号而变。PM的特点是：
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In PM the carrier amplitude is unchanged (A) and the instantaneous phase shift is proportional to the instantaneous modulating-signal value (B). C is the FM definition (deviation ∝ amplitude); D is false because PM bandwidth also depends on the highest modulating frequency. FM and PM are closely related (angle modulation).
U0735. For a given FM amateur station, the factor that determines its actual transmitted bandwidth is:
对于给定的FM业余电台，决定其实际发射带宽的因素是：
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By Carson's rule, an FM signal's necessary bandwidth (必要带宽) ≈ 2×(peak deviation + highest modulating frequency). Both higher audio frequency and larger amplitude (greater deviation) widen the occupied bandwidth, so A is correct. D is a nominal channel spacing, not the actual signal bandwidth determined by the modulation.
U0736. For a given SSB amateur station, the factor that determines its actual transmitted bandwidth is:
对于给定的SSB业余电台，决定其实际发射带宽的因素是：
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SSB (single sideband) bandwidth equals the bandwidth of the modulating (audio) signal — a wider audio passband gives a wider RF signal (A). Amplitude does not change SSB bandwidth (so C is wrong), and while 2.7 kHz (D) is the typical authorized phone bandwidth, the actual bandwidth follows the audio content; the highest frequency alone (B) is an incomplete description since the low cutoff also matters.