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| 型号： | AN-902 |
| 厂商： | National Semiconductor Corporation |
| 英文描述： | Twisted Pair FDDI Magnetics Overview and Recommendations |
| 中文描述： | 双绞线FDDI的磁性分析和建议 |
| 文件页数： | 3/10页 |
| 文件大小： | 209K |
| 代理商： | AN-902 |
6.0
PARAMETER MEASUREMENT
This section summarizes pertinent data as measured from
some of the key parameters mentioned previously. All tests
were performed using the same specially designed evalua-
tion platform. This platform consists of a multi-layer ‘‘ODL
replacement’’ emulation board fitted with a DP83223 trans-
ceiver in order to duplicate, as closely as possible, the per-
formance of a true TP-PMD application. Each of the four
magnetics solutions were tested against the same DP83223
transceiver in the same environment to ensure comparable
conditions. All tests were performed identically on each of
the magnetics solutions for both binary and MLT-3 encoded
data transmission unless otherwise noted. Again, it is very
important to understand that any data reported herein is
preliminary and is provided for reference. Each magnetics
vendor should be contacted for the latest performance in-
formation.
6.1
Insertion Loss
Insertion loss is measured in two steps. First, the magnetics
under test are replaced by shorting wires which DC couple
the transmitted signal to the digitizing oscilloscope and the
transmit waveform is calibrated to exactly 2V peak-peak dif-
ferential. Second, the magnetics under test are reinserted
and a second peak-peak differential measurement is per-
formed. The Insertion Loss resulting from scrambled FDDI
code is tabulated below.
Insertion Loss
(dB)
Bel Fuse
Coilcraft
Pulse
Valor
Scrambled FDDI
b
0.26
b
0.67
b
0.26
b
0.35
6.2
Return Loss
Although this parameter was not measured, the return loss
due to the magnetics alone should be minimal because
complex filtering is not required. Potential return loss may
be inferred by examining the magnetics vendor’s manufac-
turing tolerances.
6.3
Common Mode Rejection
This parameter was not tested. Refer to each vendor’s da-
tasheet for performance specifications.
6.4
Crosstalk
The virtual absence of Interchannel crosstalk between the
transmit and receive magnetics is due to sufficient physical
separation of the components as specified by National
Semiconductor. There will be some degree of crosstalk that
occurs between the transmit and receive channel outside of
the magnetics which will most likely occur within the media
connector and within the media itself. This effect can be
minimized by observing good high speed layout practices
and will not be increased by the use of the magnetics solu-
tions outlined herein.
6.5
Output Transition Time
The rise and fall times of a transmitted signal are a direct
indication of the bandwidth of the transmit channel. The
transition time specification depends somewhat on results
of EMI radiation testing and other performance tests. Slow-
er transition times can be achieved using different magnet-
ics components. To test the rise and fall times of the mag-
netics, the input of the magnetics were presented with the
2.0 ns transition times generated by the DP83223 TWIST-
ER. The output of each magnetics solution was then mea-
sured to determine the transition time performance.
Transition (ns)
Bel Fuse Coilcraft Pulse Valor
Binary Rise
2.41
3.25
2.37
2.34
Binary Fall
2.63
3.12
2.29
2.18
MLT-3 Rise (
b
1 to 0)
2.40
3.16
2.37
2.38
MLT-3 Rise (0 to 1)
2.67
3.32
2.59
2.43
MLT-3 Fall (1 to 0)
2.48
3.08
2.25
2.19
MLT-3 Fall (0 to
b
1)
2.76
3.32
2.49
2.36
(Refer to Figures 1 and 2.)
6.6
Overshoot
Overshoot, especially in MLT-3 mode, will decrease the
noise margin of the transmitted signal. Serious overshoot
may also contribute to unwanted bit errors in the received
signal. The overshoot at the output of the magnetics is mini-
mized because the input signal to the magnetics includes
the controlled transition times generated by the DP83223
TWISTER. Overshoot of less than 2% can be considered
negligible.
Overshoot (%)
Bel Fuse
Coilcraft
Pulse
Valor
Binary
k
2.0
k
2.0
k
2.0
k
2.0
MLT-3
k
2.0
k
2.0
k
2.0
k
2.0
(Refer to Figures 3 and 4.)
6.7
Baseline Wander
The effects of baseline wander can be directly inferred by
measuring the magnetics droop characteristic over a worst
case run length period of 480 ns for scrambled FDDI code.
The baseline wander is arrived at by doubling the percent-
age droop exhibited by a given magnetics solution.
Baseline
Wander (%)
Bel Fuse
Coilcraft
Pulse
Valor
480 ns Width
5.6
12.1
5.6
6.9
(Refer to Figure 5.)
3
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