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问题 问答题 简答题

为什么小功率系统的数字调制采用/4-DQPSK？大功率系统的数字调制采用恒包络调制？

答案

参考答案：实践证明， π／4－QPSK信号具有频谱特性好，功率效率高，抗干扰能力强等特点。可以在25kHz带宽内传输32kb／s数字信息，从而有效地提高频谱利用率，增大了系统容量。对于大功率系统，易进入非线性，从而破坏线性调制的特征。而采用恒包络调制正好能解决此问题。

选择题

下列氯化物不能用单质与氯气直接反应得到的物质是（　　）

A．CuCl2

B．FeCl2

C．HCl

D．NaCl
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单项选择题

Lobbying groups often try to disguise a financial self-interest by clumsily dressing up their arguments in the guise of concern for the public. You see this tendency in the pharmaceutical industry (21) in energy and lumber companies who like to tout their (22) of the environment. But (23) , two new books argue, are these tactics more (24) a cause for concern than in agribusiness.
Marion Nestle’s "Food Safety: Bacteria, Biotechnology, and Bio-terrorism’ looks at the way the American meat and biotechnology industries have (25) successfully on Capitol Hill (26) stricter federal regulation, which the author argues has undermined the safety of the food supply.
(27) , Maxime Schwartz’s "How the Cows Turned Mad" (28) the origins of mad-cow disease over more than two centuries, and reveals the fallout from the British government’s blind (29) that the disease could not be (30) to humans.
In 1999, Ms Nestle writes in her earlier book, Rosemary Mueklow, the executive director of the National Meat Association, lobbied against President Clinton’s (31) to establish a more thorough testing regime for E. coli 0157: H7, a potentially (32) pathogen.
Ms Muck low’s organization—which represents meatpackers and processors who (33) to discard or reprocess meat found to be infected under the new testing regime—argued on Capitol Hill that (34) microbial testing in meat could actually lead to a greater public health risk (35) confident consumers might relax their own safe-handling procedures at home.

A. However B. MeanwhileC. AccordinglyD. Worse still
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