AN-757| Application Note
Measuring Ethernet Tap Capacitance
Measuring Ethernet Tap Capacitance
INTRODUCTION When a node is added to an Ethernet network its nodal capacitance changes the impedance of the cable at the point of connection to the cable The impedance change causes a reflection of the Ethernet waveform which distorts the waveform The more the capacitance the greater the distortion and eventually with large enough node capacitances the Ethernet signal could become so distorted that the packet data would become corrupted when decoded by a network node For this reason the IEEE802 3 standard specifies a maximum value of capacitance that a node may add to the network as well as a minimum node to node distance spacing Since the capacitance of a node includes stray inductances the effective capacitance of a node connection cannot be measured simply by using a capacitance meter This note presents the method for measuring capacitance of an Ethernet tap for 10BASE5 or a BNC ``T'' for 10BASE2 THE STANDARD'S REQUIREMENTS To properly make the measurement it is important to understand how the standard sp
ecifies the capacitance of a node To quote the IEEE802 3 standard 8 3 1 1 Input Impedance The shunt capacitance presented to the coaxial cable by the MAU circuitry (not including the means of attachment to the coaxial cable) is recommended to be no greater than 2 pF The resistance to the coaxial cable shall be greater than 100 kX The total capacitive load due to MAU circuitry and the mechanical connector as specified in 8 5 3 2 shall be no greater than 4 pF These conditions shall be met in the power-off and poweron not transmitting states (over the frequencies BR 2 to BR) The magnitude of the reflection from a MAU shall not be more than that produced by a 4 pF capacitance when measured by both a 25 ns rise time and 25 ns fall time waveform This shall be met in both the power-on and power-off not transmitting states
National Semiconductor Application Note 757 Larry Wakeman March 1991
To summarize the maximum allowable capacitance specifications for both Thinwire and Thickwire Ethernet the following table is provided TABLE I Maximum Capacitance Allowed in IEEE802 3 Standard 10BASE5 10BASE2 Electrical Circuitry 2 pF 4 pF Mechanical Connector 2 pF 4 pF
Note Thickwire or Thick Ethernet refers to 10BASE5 and Thinwire or Thin Ethernet refers to 10BASE2
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FIGURE 1 Simple Model of the Parasitics Presented to the Ethernet Cable THE TEST METHOD Due to the nature of the capacitance of a DTE (Data Terminal Equipment) rather than perform a simple capacitive measurement using a meter the capacitance of the network node is more accurately measured by testing it in an environment where the actual signal reflection caused by the capacitance of a node attachment is measured when applying a typical Ethernet signal The magnitude of the reflection is then correlated to an equivalent capacitance This is the most appropriate method since it is the signal degradation due to the capacitive load that is the important consideration in defining the above specifications
AN-757
C1995 National Semiconductor Corporation
TL F 11163
RRD-B30M75 Printed in U S A
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