问题 问答题 简答题

线性定常系统有哪些基本性质?简述同频性在动态测量中的重要意义。

答案

参考答案:

线性系统具有频率保持特性的含义是输入信号的频率成分通过线性系统后仍保持原有的频率成分。如果输入是很好的正弦函数,输出却包含其他频率成分,就可以断定其他频率成分绝不是输入引起的。它们或由外界干扰引起,或由装置内部噪声引起,或输入太大使装置进入非线性区,或该装置中有明显的非线性环节。

单项选择题
单项选择题

Robert Moody, 52, is an experienced police officer. Much of his work involves dealing with (16) and gang (团伙)problems in the schools of his community. Knowing that many kids often (17) trouble, he decided to do something about it. So in 1991 he began to invite small groups of kids to go fishing with him on his day (18) .

Those fun trips had a(n) (19) impact. A chance encounter in 2000 proved that. One day, (20) working security at a school basketball game, Moody noticed two young guys (21) . He sensed trouble between them. (22) one of them headed toward Moody and gave him a hug. "I (23) you. You took me (24) when I was in fifth grade. That was one of the (25) days of my life. "

Deeply touched by the boy’ s word, Moody decided to create a foundation (基金会)that (26) teenagers to the basics of fishing in camping programs. "As a policeman, I saw (27) there was violence, drugs were always behind it. They have a damaging (28) on the kids," says Moody.

By turning kids on to fishing, he (29) to present an alternative way of life, "When you’ re sitting there waiting for a (30) ," he says, "you can’t help but talk to each other, and such (31) can be pretty deep. "

"Talking about drugs helped prepare me for the peer(同龄人)pressures in high school," says Michelle, 17, who (32) the first program. "And I was able to help my little brother (33) drugs. "

Moody faces (34) in three years, when he hopes to run the foundation full-time. "I’ m living a happy life and I have a responsibility to my (35) to give back," Moody says. "If I teach a kid to

fish today, he can teach his brother to fish tomorrow. \

20().

A. once

B. while

C. since

D. until