Abstract: There have been many papers about the types of circuits to realize Digitally Controlled Delay Elements（DCDE）.But most of them are about qualitative analysis and theoretic computing.Few of them deal with the practical issues such as how to choose the size of a certain MOSFET.In this paper，we bring two types of typical DCDEs by abstracting the common used DCDEs.By circuit simulating and theoretic computing，we sum up the features of these DCDEs and point out the practical issues that should be paid attention to when they are realized.At the same time，we give the way by which the size of the MOSFET should been chosen.

Key words: delay elements, Digitally Controlled Delay Elements（DCDE）, delay line, digitally controlled current supply

摘要： 目前已有很多种实现数控延迟单元的电路结构，有关这方面的研究文献也层出不穷，但是这些文献大部分都是对某种电路结构进行的定性分析或者理论计算，很少有涉及到具体实现时应该注意的问题以及具体的晶体管尺寸选择问题。在对常用延迟单元结构进行总结的基础上，抽象出了两种典型的数控单元实现方式。根据理论分析及电路模拟结果指出了这两种结构各自的优缺点以及进行具体实现时应该注意的问题。同时，通过对电路模拟结果的总结和分析提出了一种简单实用的晶体管尺寸选择方法，从而简化了电路设计的复杂程度。

关键词: 延迟单元, 数控延迟单元, 延迟线, 数控电流源