CRAC Class C (2025) — Questions 381–390
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：
Exam tips · 考试提示
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.
U0737. In the amateur radio service, the speed of Morse code signals is often measured in WPM; the method of measurement is:
在业余无线电业务中，莫尔斯电码的信号速度经常用WPM作为 单位，其计量方法为：
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A standard word ("PARIS", 50 dot-units) defines the WPM scale, so WPM = (number of gap-free dots per minute) ÷ 50 (A). Dividing by 60 (D) would be a different (PARIS-less) scale; WPM is not simply English or Chinese characters per minute.
U0738. In the amateur radio service, the signal-timing standard used when calculating the WPM speed of Morse code (with the duration of one 'dot' signal as the reference) is: dot signal, dash signal, intra-character dot-dash spacing, inter-character spacing, and inter-word (group) spacing are respectively:
在业余无线电业务中，计算莫尔斯电码的WPM速度时采用的信 号时值标准（以一个“点”信号的时间为比较基准）为，点信 号、划信号、字符内点划信号的间隔、字符之间的间隔、单词 （组）之间的间隔分别为：
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The standard Morse timing, in units of one dot, is: dot = 1, dash = 3, intra-character gap = 1, inter-character gap = 3, inter- word gap = 7 (A). This is the universal timing that defines the code's rhythm.
U0739. The necessary emission bandwidth of a CW signal can be calculated using the formula Bn =
计算CW信号的必要发射带宽可以使用公式Bn=B×K。其中，B 为波特率。如果已知拍发电码的WPM值，则B可以计算为：
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Because one PARIS word = 50 dot-units and one dot = 1 bit at the keying rate, the baud rate B (bits/s) equals (WPM × 50)/60 = WPM / 1.2. Hence B = WPM/1.2 (A). The other arithmetic forms do not follow from the PARIS timing.
U0740. The necessary emission bandwidth of a CW signal can be calculated using the formula Bn =
计算CW信号的必要发射带宽可以使用公式Bn=B×K。其中， 常数K描述电码符号翻转时CW射频信号的边带滚降特性。K的 取值原则是：
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K is a dimensionless rolloff factor: on fading HF channels a wider K=5 is used to ensure copy, while on non-fading (line-of- sight/VHF) channels K=3 suffices (A). Options C/D wrongly assign baud values to K, which is not a baud rate.
U0741. The necessary bandwidth Bn of FM voice depends on the peak frequency deviation
调频话的必要带宽Bn取决于FM信号的峰值频偏D与最高调制频 率M。估算方法为：
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This is Carson's rule: Bn ≈ 2(D + M), i.e. twice the sum of peak deviation D and the highest modulating frequency M (A). The other formulas mis-weight D and M (e.g., Bn=2M+D or M+2D) and thus are incorrect.
U0742. The modulation methods used for slow-scan television (SSTV) and radio facsimile (FAX) in the amateur bands are both:
业余频段中的慢扫描电视（SSTV）和无线电传真（FAX）的调 制方式都是：
Exam tips · 考试提示
Amateur SSTV and radio facsimile (FAX) are normally transmitted using FM (frequency modulation) of the voice channel — the picture's audio baseband modulates the carrier (A). They are not AM, PM, or CW for the image transmission.