由准同步视频检波的特点及其存在的问题引出锁相环（ PLL）完全同步视频检波电路。
The PLL full synchronization video detector circuit is educed by feature and troubles of quasi-synchronization video detector.
这一环节既可以复习前面部分问路的知识（地图上标出动物园所在），二可以引出如何正确过马路。
Review:Show out a map , then ask the students to say how can Jim and Kate get to the zoo.
隔板、垫圈位于螺栓上，其中钢丝环引出端从环的切线方向引出后被固定于隔板及垫圈上。
When the screw bolt is rotated, the washer is forward pushed out, and thereby, the steel wire ring is tightly connected to realize the purpose of tightening ring.
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