CRAC Class C (2025) — Questions 781–790

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U0557. For amateur communication that must receive very weak signals, such as EME contacts, the receiver noise figure Fn (expressed as a ratio) is an important technical indicator. Some basic knowledge about Fn is:

对于需要接收微弱信号的业余通信，例如EME通联，接收机的 噪声系数Fn（以比值的形式表示）是一项重要技术指标。关于 Fn的一些基本常识是：

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Because any real receiver degrades the SNR, Fn is always > 1, and a value closer to 1 means less added noise, hence better (A). Options claiming Fn < 1 (B, D) are impossible for a passive- or-active real receiver; "larger is better" (C) is wrong.

U0558. For amateur communication that must receive very weak signals, such as EME contacts, the receiver noise figure NF (expressed logarithmically) is an important technical indicator. It refers to:

对于需要接收微弱信号的业余通信，例如EME通联，接收机的 噪声指数NF（以对数表示）是一项重要技术指标。它是指：

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噪声指数 (noise figure, NF) is simply the noise figure expressed in decibels: NF = 10·log10(Fn) = 10·log10[(Si/Ni)/(So/No)] (A). It is the dB form of the SNR-degradation ratio; B/C/D describe unrelated noise readings.

U0559. For amateur communication that must receive very weak signals, such as EME contacts, the receiver noise figure NF (expressed logarithmically) is an important technical indicator. Some basic knowledge about NF is:

对于需要接收微弱信号的业余通信，例如EME通联，接收机的 噪声指数NF（以对数表示）是一项重要技术指标。关于NF的 一些基本常识是：

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Since Fn > 1, NF = 10·log10(Fn) is always > 0, and a value closer to 0 dB means less added noise, hence better (A). Options with NF < 0 (B) or NF > 1 in dB terms (C) or "between 0 and 1" (D) misstate the dB relationship.

U0560. In radio communication, the "noise temperature" indicator is often used to describe the internal-noise magnitude of signal sources, amplifiers, receivers, or other equipment/system components. Taking a receiver as an example, the meaning of equivalent noise temperature Te is:

在无线电通信领域中，描述信号源、放大器或接收机等设备或 系统组件的内部噪声大小时常用"噪声温度"指标。以接收机为 例，等效噪声温度Te的意义是：

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等效噪声温度 (equivalent noise temperature, Te) represents the receiver's internal noise as the physical temperature a matched input resistor would need to have to generate the same thermal noise power (A). It is an input-referred equivalent, not an output heating temperature (B, C) nor an ambient operating temperature (D).

U0561. For an ideal amplifier or receiver that produces no internal noise, the noise figure Fn, noise figure NF, and noise temperature Te are respectively:

不产生任何内部噪声的理想放大器或接收机的噪声系数Fn、噪 声指数NF和噪声温度Te分别为：

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An ideal noiseless device adds no noise, so its input and output SNRs are equal: Fn = 1; NF = 10·log10(1) = 0 dB; and Te = 0 K (absolute zero) (A). Values of Fn = 0 (B, C, D) are impossible.

U0562. Suppose an antenna amplifier used for the amateur-satellite service operates at standard temperature (17 °C), with its input already connected to a well-matched antenna. If the internal noise produced by the amplifier is equivalent to the input thermal noise, then the amplifier's noise figure Fn, noise figure NF, and noise temperature Te are respectively:

假设一个用于卫星业余业务的天线放大器工作在标准温度 （17℃）下，其输入端已连接良好匹配的天线。如果放大器所 产生的内部噪声与输入的热噪声等效，则该放大器的噪声系数 Fn、噪声指数NF和噪声温度Te分别为：

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If the amplifier's added noise equals the input thermal noise, the total output noise is doubled relative to the ideal, so Fn = 2; NF = 10·log10(2) ≈ 3 dB; and Te = (Fn−1)·T0 ≈ 1×290 K = 290 K (A). The "standard temperature" reference is T0 ≈ 290 K (≈17 °C), which is why Te lands at 290 K.

U0969. Modern transceivers are mostly designed and built on DSP technology. Because their main functions are implemented in software, they are called software-defined radio (SDR) equipment. Compared with analog transceivers, the advantages of SDR equipment are: (Choose all that apply.)

现代收发信机大多基于DSP技术设计并制造。因主要功能由软 件实现，便获得软件无线电（SDR）设备的称谓。相比模拟收 发信机，SDR设备的优势是：

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SDR (软件无线电) moves functions into software/DSP, giving lower noise (mainly quantization noise), higher precision with less distortion and better (arbitrary) filtering, and freedom from analog-component aging drift (A, B, C). Digital processing typically adds some latency rather than reducing it, so D is not a correct advantage per the key.

U0970. Most SDR transceivers provide a band spectrum display; its basic uses are: (Choose all that apply.)

SDR收发信机大多提供波段频谱显示器，其基本用途是：

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The panadapter/spectrum display of an SDR serves to spot band activity and tune quickly (A), to assess a signal's strength/quality/spectrum for reporting (B), and to watch fading as a propagation indicator (C). Option D describes subjective "timbre" and equalizer coaching, which is not a basic function of the display, so it is excluded.

U0971. Some SDR transceivers can provide encoding/decoding functions for signals such as CW, RTTY, or FT8. This is because:

有些SDR收发信机可以提供CW、RTTY或FT8等信号的编解码 功能。这是因为：

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By definition, an SDR (软件无线电) performs modulation/demodulation in software on an internal or attached computer, so it can add CW/RTTY/FT8 codec modes by software update rather than hardware. Options B (plug-in modules), C (cloud/AI), and D (screensaver) misdescribe how SDR codecs work.

U0972. Compared with analog transceivers, why can most SDR transceivers provide more flexible filtering?

相比模拟收发信机，为什么多数SDR收发信机可以提供更加灵 活的滤波功能？

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SDR filtering is implemented digitally, so filter shape and bandwidth are set in software and can be changed freely with adequate DSP computing power — no physical crystal/mechanical filters needed (A). B describes analog filter banks, C confuses phase-shift methods, and D invokes nonexistent cloud filtering. 无线电测向 Radio Direction Finding (ARDF)