布线技巧:屏蔽性能指标的含义
欢迎进入网络技术社区论坛,与200万技术人员互动交流 >>进入 1.表面转移阻抗(Surface Transfer Impedance) 按IEC61196-1测试同轴电缆的方法,测试带屏蔽的平衡电缆,短路8根芯线后用50Ω信号源激励。被测试线长1米,测试频率30MHz,频率越高,线长越短导体
欢迎进入网络技术社区论坛,与200万技术人员互动交流 >>进入
1.表面转移阻抗(Surface Transfer Impedance)
按IEC61196-1测试同轴电缆的方法,测试带屏蔽的平衡电缆,短路8根芯线后用50Ω信号源激励。被测试线长1米,测试频率30MHz,频率越高,线长越短导体表面转移阻抗。主要用于评估连接硬件的屏蔽效率,其实测值不超过以下计算值。
ZTcable=37+4f+4f1/2+5f1/3
ZTcable:表面转移阻抗,单位mΩ/m
f:信号频率,单位MHz
2.转移阻抗(Transfer Impedance)
转移阻抗与屏蔽电缆和连接硬件的屏蔽效率相关,其数值可通过实验室高频密封箱测量屏蔽插入损耗,计算得出。
Ri1=Ri2=50Ω――网络分析仪特性阻抗
R1=50Ω――馈电电阻
R2=50Ω――终端电阻
U1=信号发射电压(V)
U2=信号接收电压(V)
Uc=被测设备两端电压
Zcond=连接器特性阻抗(Ω)
Zt=转移阻抗(Ω)
Zt=1/l电缆长度?Ri1/Ri2?(R2+ Ri2)?U2/ U1=100/l电缆长度?U2/ U1
由于屏蔽插入损耗(αs)为20?lg (U2/ U1)dB,转移阻抗(Zt)也可以表示为:
Zt=100?10α/20(Ω)
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