Measuring Ethernet Tap Capacitance Measuring Ethernet

Measuring Ethernet Tap Capacitance Measuring Ethernet
National Semiconductor
Application Note 757
Larry Wakeman
March 1991
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.
To summarize the maximum allowable capacitance specifications for both Thinwire and Thickwire Ethernet the following table is provided.
THE STANDARD’S REQUIREMENTS
To properly make the measurement, it is important to understand how the standard specifies 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.
TABLE I. Maximum Capacitance Allowed in IEEE802.3
Standard
Electrical
Circuitry
Mechanical
Connector
10BASE5
2 pF
2 pF
10BASE2
4 pF
4 pF
Note: Thickwire or Thick Ethernet refers to 10BASE5 and Thinwire or Thin
Ethernet refers to 10BASE2.
Measuring Ethernet Tap Capacitance
Measuring Ethernet Tap
Capacitance
TL/F/11163 – 1
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.
With the above in mind, the test is performed by first measuring the reflection caused by the attachment of a node.
Then the DTE is replaced with a reference variable capacitor, and the capacitor’s value is adjusted until the capacitance that causes the same size reflection is determined.
The capacitance of the node is therefore the same as the
reference capacitance value that causes the same amplitude reflection.
THE TEST WAVEFORM
In normal network operation the signal on the coax cable
has rise and fall times of 25 ns g 5 ns (defined by the
IEEE802.3 standard). With a purely capacitive load applying
signals with faster (or slower) edges cause larger (or smaller) reflections than would be seen on a typical network. If
the node were purely capacitive this would not affect the
measurement. The larger (or smaller) node reflection for a
given parasitic capacitance would track with the reference
capacitance’s reflection yielding accurate measurements.
However, the node is actually not a pure capacitance, but
has some series inductance associated with the network
connection as shown in Figure 1. The application of signals
with faster than 20 ns rise and fall times actually result in an
unrealistically low capacitance measurement. This is because the nodes capacitance is buffered by the stray series
inductances which reduce the reflection magnitude when
compared to the pure capacitance. This correlates to a lower than actual capacitance.
On the other hand applying very slow rise and fall times
(slower than 30 ns) result in the measurement of a larger
capacitance than actual. This is because the series inductance effects are less than would be seen with a nominal
waveform.
Since it is desirable to measure the capacitance in such a
way as to correlate to the effective capacitance seen when
IEEE802.3 signaling is used, the best compromise choice is
to select a 25 ns rise and fall times for this test. (This is the
reason for this choice in the actual standard.)
Again, the reason behind this decision is that although the
t 30 ns edges indicate larger capacitances a signal with
25 ns edge produces results that more correctly represent
the actual effect of the attached node’s capacitance.
TEST SETUP AND CABLE
An example test configuration which measures the capacitance of the Thickwire Ethernet is shown in Figure 2. The
waveform applied to the test node is an important consideration in setting up the test, as it will affect the resultant value
of capacitance. In particular the rise and fall times must be
carefully chosen to reflect the capacitance seen in an Ethernet network, as described in the next section.
The cable lengths and spacing between the scope input and
the transceiver’s connection are chosen to ensure that the
reflection due to the transceiver appears on the flat portion
of the test waveform. This allows accurate measurement.
The total cable length is equivalent to the full 10BASE5
length of 500m.
An oscilloscope is used to measure the voltage of the reflection. The scope, with a 1 MX input impedance, as shown
in Figure 2, is connected directly to the cable without a
probe. This eliminates any errors due to the probe. The distance between transceiver connection point ‘‘A’’ and the
scope is set so that the reflections will arrive at the scope
right after the signal rise and fall times. Moving point ‘‘A’’
any further makes the reflections smaller in amplitude (cable attenuation) and therefore harder to measure.
On the scope’s display measurements are made at the
point immediately after the rise time. Reflections are then
compared to the ones for known discrete capacitors.
TL/F/11163 – 2
FIGURE 2. Test Setup
2
TL/F/11163 – 3
FIGURE 3. Input Test Waveform
As shown in Figure 3, a low frequency trapezoidal signal is
used. This will keep the reflections from each edge of the
signal well away from the next edge enabling easier measurement. The 2 VPP test input signal is the typical voltage
swing on the coax cable in normal operation. In the case of
a discrete capacitor the voltage level of the signal may not
be important. However, due to the non-linearity of the node
and DP8392 capacitance a typical voltage signal should be
used following the same rational as was used for the signal
rise and fall times.
Thinwire
TL/F/11163 – 5
Thickwire
TL/F/11163 – 6
FIGURE 5. DP8392 Connection Diagram
TL/F/11163 – 4
INACCURACIES OF THE CAPACITANCE METER
As stated, in a real network, it is not the node capacitance
that creates a problem, but too large a reflection caused by
this capacitance. This reflection distorts the cable signal.
Therefore the best method of test is to measure the reflection under true network waveforms. By the same analogy
capacitance meters which have a test signal frequency that
does not correspond to 25 ns rise and fall time do not reveal
a true measurement of capacitance, and so capacitive measurements done only with a capacitance meter are usually
(almost always) inaccurate to the true effective capacitance
as seen by the network cable.
Note: This figure is conceptual. It does not show the waveform details.
FIGURE 4. Example of Reflection
TEST RESULTS
A special jig was built to connect the ICs to point ‘‘A’’ in
Figure 2. This greatly improves measurement repeatability.
Data repeatability of 0.01 pF is achieved.
Typical data for RXI and TXO capacitances are 1.0 pF and
2.0 pF respectively. Total node capacitance can be reduced
to around 1.6 pF with the addition of a small capacitance
diode in series with the TXO output, as shown in Figure 5.
For Ethernet applications two diodes in series can be used
instead.
3
Measuring Ethernet Tap Capacitance
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AN-757
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