CRAC Class C (2025) — Questions 371–380
U0367. Which of the following modulation modes can be classified under amplitude modulation (AM)?
下列哪种调制方式可以纳入调幅（AM）的范畴？
Exam tips · 考试提示
SSB is a derivative of AM (it is amplitude modulation with carrier and one sideband suppressed), so it falls under the AM category. Chirp, DSSS, and PSK are distinct (frequency/spread/phase) modulations not classified as AM.
U0368. In the amateur radio service there is a communication mode abbreviated CW. Its meaning is:
在业余无线电业务中，有种通信方式的缩写是CW。其含义为：
Exam tips · 考试提示
CW means continuous wave (等幅电报) — on/off keying of a constant-amplitude carrier to send Morse code. It is the emission mode, not the Morse code itself (B), nor ASK (C) or FSK (D).
U0369. Which modulation mode has the narrowest emission bandwidth?
以下哪种调制方式拥有最窄的发射带宽？
Exam tips · 考试提示
CW occupies only a few hundred Hz (the narrowest), narrower than SSB phone (~2.4–2.7 kHz), FM phone (~12.5 kHz), and ATV (hundreds of kHz). This is why CW is the weak-signal DX favorite.
U0370. Amateur stations generally do not use FM phone for Morse code contacts. This is because:
业余电台一般不用调频话方式进行莫尔斯电码通联。这是因 为：
Exam tips · 考试提示
The real reason is regulatory/bandwidth: FM's wide channel far exceeds the narrow necessary bandwidth for Morse (CW), violating the allocation rule to minimize bandwidth. FM technically can carry Morse (B is false), and C/D are not the official rationale.
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：
Exam tips · 考试提示
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：
Exam tips · 考试提示
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：
Exam tips · 考试提示
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：
Exam tips · 考试提示
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：
Exam tips · 考试提示
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：
Exam tips · 考试提示
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.