CRAC Class A (2025) — Questions 611–620
U0610. In the system of legal units of measurement, what are the mathematical significance and textual meaning of the prefix p? ('x＾m' denotes 'x to the power of m'.)
在法定计量单位中，词头p的数学意义和文字含义分别为： （“x＾m”表示“x的m次方”）
Exam tips · 考试提示
The prefix p (pico, 皮) represents 10^-12. Option A is correct. B (tera, 10^12), C (nano, 10^-9), and D (giga, 10^9) are different prefixes.
U0611. Which of the following amateur radio abbreviations can express the term "radio frequency" or specifically refer to a certain frequency used for radio purposes?
以下哪个业余无线电通信缩语可以表述术语“射频”或特指无 线电用途的某个频率？
Exam tips · 考试提示
RF (radio frequency, 射频) is the abbreviation that denotes radio frequency or a radio-used frequency. HF (high frequency), AF (audio frequency), and MF (medium frequency) are band/signal designations, not the general term for radio frequency itself.
U0612. The term "audio frequency" (音频) refers to the frequency of sounds that people can generally hear. Which of the following descriptions is correct?
术语“音频”是指人们可以普遍听到的声音的频率。以下描述 正确的是：
Exam tips · 考试提示
音频 (audio frequency) is the range of sound audible to humans, approximately 16 Hz to 20 kHz (A). C gives a too-high and too-narrow range, and B/D wrongly place audio inside radio bands (VLF–MF, VHF) — audio is a baseband signal, not a radio-frequency band.
U0613. The commonly used nominal impedances of the transmission interfaces for radio communication and related test equipment, television equipment, and audio equipment are, respectively:
无线电通信及相关测试设备、电视设备和音频设备常用的传输 接口标称阻抗分别为：
Exam tips · 考试提示
Radio/RF and test equipment commonly use a 50 ohm interface, television (coaxial) equipment uses 75 ohm, and audio (balanced line) equipment uses 600 ohm. Option A lists them in the correct order: 50 Ω, 75 Ω, 600 Ω.
U0614. 1,805,000 Hz can be expressed as:
1,805,000Hz可以表述为：
Exam tips · 考试提示
1,805,000 Hz = 1.805 × 10^6 Hz = 1.805 MHz. Option A is correct. B (kHz) would be 1,000 times too small, C (mHz = 10^-3 Hz) is absurd, and D (GHz) is 1,000 times too large.
U0615. 2430 MHz can be expressed as:
2430MHz可以表示为：
Exam tips · 考试提示
2430 MHz = 2.430 × 10^9 Hz = 2.43 GHz. Option A is correct. B (243 GHz) is 100× too large, C (nHz) is vanishingly small, and D (kHz) is 10^6× too small.
U0616. A resistor of 1 ohm resistance is connected in parallel with a 13.8-volt power source; the power dissipated by the resistor is approximately:
将阻值为1欧的电阻与13.8伏电源并联，电阻所耗散的功率大约 为：
Exam tips · 考试提示
With the resistor directly across the 13.8 V source, P = V²/R = 13.8² / 1 ≈ 190.4 W, about 190 W (A). Options B and C give voltage and current (not power), and D (VA) is a volt-ampere figure inappropriate for a pure resistor.
U0617. 5 W can be expressed as:
5W可以表示为：
Exam tips · 考试提示
5 W = 5000 mW. In dBm: 10·log10(5000) ≈ 36.99 ≈ 37 dBm (A). For reference, 5 W = 10·log10(5) ≈ 7 dBW (so B is wrong), 17 dBm = 50 mW, and 35 dBμ = 3.16 mW — none equal 5 W.
U0618. 0.25 W can be expressed as:
0.25W可以表示为：
Exam tips · 考试提示
0.25 W = 250,000 μW. In dBμ (decibels relative to 1 μW): 10·log10(250000) ≈ 53.98 ≈ 54 dBμ (A). Note 0.25 W = 23.98 dBm (not 36 or 25 dBm) and 6 dBW = 4 W, so B/C/D are wrong. The exam uses dBμ here, not dBm.
U0619. 0.4 kW can be expressed as:
0.4kW可以表示为：
Exam tips · 考试提示
0.4 kW = 400 W = 400,000,000 μW. In dBμ: 10·log10(4×10^8) = 80 + 6.02 ≈ 86 dBμ (A). In dBm it would be 10·log10(400,000) ≈ 56 dBm, so B/C/D (400, 6000, 34 dBm) are all wrong magnitudes.