特性阻抗和静电电容

1. 特性阻抗：Z0〔?〕和静电电容：C〔pF/m〕

a静电电容和特性阻抗成反比关系

(同轴电缆的实际测量例：参照下表)

一般同一绝缘体材料时，Z0决定于导体直径(d)和绝缘体外径(D)之比(D/d)。1. Impedance (Z0), Capacitance (C)

1)Impedance & Capacitance have an inverse proportion

relationship. (An example data on coaxial cable is illustrated below).

Fundamentally, if the insulating material is exactly the same, the ratio of Z0to C is determined by the diameter of

s Z0

Z0在频率大约为10MHz以上时趋于稳定。对于同轴电缆规定10MHz时的Z0，对于同轴之外的其它电缆，定义频率或测量法后规定Z0。Z0is stable when frequency gets to about 10 MHz & over. Coaxial cable regulates the Z0at 10 MHz, & cables other than coaxial regulate the Z0by defining frequency or measurement method.

d与介电常数( ε )的关系

除D/d、频率外，决定Z0和C的要素还有介电常数( ε )。这取决于绝缘体材料。

导体为同一时，ε 和Z0、C的关系如q、w。

q绝缘体外径不变，缩小ε时，

→C变小，Z0增大

(即可不改变结构尺寸地改变C和Z0)

w C和Z0不变，缩小ε 时，

→绝缘体外径变小

(即可不改变C和Z0地缩小外径)

一般来说PVC是受频率和温度影响ε 大幅度变化的绝缘材

料。因此，对Z0等电气特性的要求高时，应使用聚烯烃类、氟化乙烯树脂类等ε稳定的绝缘体材料。3)The relationship of dielectric contact (ε).

Another factor for determining Z0& C, besides D/d & frequency is dielectric contact (ε).

This is determined according to the insulating material.

q&w below show the relationship between ε, Z0& C when the conductors used are same size.

q When the external diameter of the insulating body is not changed & εis made smaller

→ C becomes smaller & Z0becomes bigger.

(In other words : without changing the size of the

structure, C & Z0can be changed.)

w When C & Z0are not changed, & εis made smaller

→the external diameter of the insulating body

becomes smaller.

(In other words : without changing C & Z0, the

external diameter of the insulating body can be

made smaller.)

In general PVC is an insulating material where unfortunately εexperiences large scale changes, according to frequency & temperature. Because of this, when electrical properties of Z0etc, are demanded polyoefin & fluorocarbon polymers etc. are used in the insulating material to stabilize ε.

〔补充说明：同轴电缆之外的Z0和C〕

即便是同一电缆，接线方法(信号传输方式)不同，Z0和C也大不相同。

(阻抗匹配电缆的实际测量例：参照下表)[Additional information on the Z0& C in cable other than coaxial cable.]

Even if the cable is the same, Z0and C can be very different by wiring.

(An example of a survey on impedance matching cable is illustrated below).

·不平衡接线：

测量信号线以外的接地线(单对)及屏蔽一并接地。

测量接地?测量信号线芯之间。

·平衡接线：

测量信号线以外均为开路。测量线对之间。

两者的关系大致如下式所示。

[不平衡接线Z0] × 1.4~1.6 ? [平衡接线Z0]

2. 延迟时间：Td [ns/m]

如果没有绝缘体(导体伸展于空中) ，信号将以光速传输。

但是，电线中有绝缘体，所以信号不是以光速来传输。(介电常数：ε的影响）

在电线中，信号传输的时间称为延迟时间。

ε和Td的关系如下表所示。

但除特殊用途之外，Td的绝对值不会成为问题。

·Unbalanced wiring : All GND wires, with the exception of the measured signal wire, & the shield are bundled together to earth. Measurement is between earth & the measured signal wire.

·Balanced wiring : With the exception of the measured signal pair, everything is open. Measurement is between pairs.

The relationship of both of the above is approximately shown in the equation below :

[Unbalanced wiring Z0] × 1.4~1.6

?[Balanced wiring Z0]

2. Time delay : Td [ns/m]

If there is no insulating body (when the conductor is an overhead cable - stretched up in the air) the signal is transmitted at light velocity. However, when the cable is insulated, the signal is not transmitted at light velocity.

(Dielectric contact : εinfluence).

The time taken for the cable to transmit the signal is called the ‘time delay’ (Td.)

The relationship between this ε& Td is as below :

But, except in particular uses, the absolute value of Td is not

a problem.

εTd 大小

大小

ε

Td big small

big small

3. 衰减量：α[dB/L] (L：试样长度)

用电缆传输信号时，因结构和电气特性上的能量损失，输出信号会比输入信号弱。α 就是表示输入和输出之比。

同一电缆时，基本上与长度成正比，且频率越高α 越大，尤其是在远距离、高频条件下使用时大多会出现问题。

3. Attenuation : α [dB/L] (L : absolute length)

When a signal is transmitted by a cable, because of the energy loss related to the structure & electrical properties ;the output signals become weak. The input/output ratio is shown as : α.

With the same cable, basically because in proportion to its length, the higher the wavelength becomes the bigger αbecomes, there are many problems when particularly long distances & high frequencies are used.

1000

100

101

101001000

频率

Frequency [MHz]

衰减量 [ d B / k m ]50?同轴电缆的实际测量例

An example of a survey on 50? coaxial cable.

A t t e n u a t i o n