CRAC Class C (2025) — Questions 1141–1150
U1310. During digital-to-analog conversion, to correctly reconstruct the analog signal the DAC's sample rate should be at least the input sample rate. But if interpolation is used in the DAC to obtain a higher sample rate, then: (Choose all that apply.)
进行数模转换时，为了将模拟信号正确还原出来，数模转换器 （DAC）的采样率应当至少等于输入样本率。但是如果在数模 转换时通过插值获得了一个更高的采样率，则：
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Oversampling/interpolation in a DAC pushes the image spurs to higher frequencies, giving a flatter in-band response (A) and making the residual images easier to remove with a simpler post-filter (B). C and D are impossible — interpolation cannot recover information already lost at sampling or undo aliasing.
U1311. Convert an analog signal of frequency f and maximum voltage U into a digital signal. If the digital signal must reflect amplitude detail equivalent to 0.1% of U and time detail equivalent to the 8th harmonic of f, the ADC must satisfy:
将频率为f且最大电压为U的模拟信号转换为数字信号。如果数 字信号需要反映相当于0.1%U的信号幅度细节和相当于f的8次 谐波的时间细节，则模数转换器应满足要求：
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To capture the 8th harmonic (8f) without aliasing, Nyquist needs a sample rate > 2×8f = 16f. To resolve 0.1% = 1/1000 of full scale you need at least 10 bits (2^10 = 1024 levels ≈ 0.1%). So A is correct; B samples too slowly for the 8th harmonic, C/D have insufficient rate or resolution.
U1312. When writing SDR software, hams often use an I/Q sequence centered at 0 Hz, using both positive and negative frequency regions to fully express the signal's baseband information. This form of digital signal is also called a "complex signal"; its advantages are: (Choose all that apply.)
编写SDR软件时，爱好者们经常使用一种中心频率为0Hz的I/Q 序列，利用正、负两个频率区域来完整表达信号的基带信息。 这种形式的数字信号也叫"复信号"，其优点是：
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I/Q complex (baseband) representation centers the spectrum at 0 Hz, so the sample rate need only cover the signal bandwidth (not the RF carrier) (A); magnitude/phase become simple complex math (B); and any modulation is easy to synthesize/analyze (C). D is false — you still need quadrature mixing (two paths or an RF front end) to get/emit the actual RF, not a single ADC/DAC at RF.
U1313. In an SDR transceiver, the main functions are all implemented by digital signal-processing algorithms, including: (Choose all that apply.)
SDR收发信机的主要功能均由数字信号处理算法实现，包括：
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In an SDR, mixing, filtering, modulation, and demodulation are all performed in software/DSP rather than with analog hardware, so A, B, C, and D are all correct.
U1314. Since analog filters already support amateur radio communication successfully, why do we still develop and use digital filters?
既然模拟滤波器早已支持业余无线电通信成功运作，我们为什 么还要研发使用数字滤波器？
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The key advantage of digital filters is flexibility — arbitrary passband shapes with repeatable, optimal performance, easily changed in software (A). B is not their role (HF preselection needs analog front ends); C is false (digital filters handle no RF power); D is false (they require DSP/math hardware).
U1315. In an SDR system, mixing (frequency translation) of I/Q signals can be done using the algorithm:
在SDR系统中，对I/Q信号进行混频可使用算法：
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Frequency translation of complex (I/Q) baseband signals is performed by complex multiplication with a complex exponential (A). Addition/subtraction just shift levels; square root is unrelated to mixing.
U1316. In an SDR system, demodulating an AM complex signal can be done using the algorithm:
在SDR系统中，解调AM复信号可使用算法：
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AM's envelope equals the magnitude of the complex baseband sample, so demodulation is the complex modulus |I + jQ| (A). The argument/phase (B) is used for FM/PM; cosine/exponential are not the AM detector.
U1317. Digital filters in an SDR system come in several types. The common ones are: (Choose all that apply.)
SDR系统中的数字滤波器具有多种类型。常见的有：
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The two mainstream digital-filter structures are FIR (finite impulse response) (A) and IIR (infinite impulse response) (B). C (Sallen–Key) and D (state-variable) are analog active-filter topologies, not digital-filter types, so they are not selected.
U1318. When building an SDR transmitter, the algorithms that can generate an SSB (single sideband) signal are: (Choose all that apply.)
制作SDR发信机时，生成SSB信号的算法可以是：
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SSB can be generated by the filter method (A), the phasing method (B), and the Weaver method (C) — all common in SDR. The limiting method (D) is used for FM/improving carrier purity, not for producing SSB, so it is not selected.
U1319. In an SDR system, the fast Fourier transform (FFT) can be used to: (Choose all that apply.)
在SDR系统中，快速傅里叶变换（FFT）可以用来：
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The FFT is used for spectrum analysis (A), designing FIR coefficients via frequency sampling (B), OFDM/FFT-based modulation and demodulation (C), and fast convolution (overlap-save/add) (D). All four are valid FFT applications in SDR.