4.3 Common Mode Rejection
ing in baseline wander. An increase in baseline wander con-
tributes directly to increased jitter. In general, the higher the
OCL (open circuit inductance), the lower the low frequency
pole for the magnetics bandpass region and the less severe
the Baseline Wander.
This is the ability of the magnetics, either transmit or re-
ceive, to reject common mode energy which may exist in
the transmission signal. Also, the ability of the magnetics to
not impart any common mode energy to the signal. Com-
mon Mode Energy can be described as some potential ex-
isting equally (in phase) on each side of a differential pair
with respect to some fixed potential such as ground. As an
example, some twisted pair conductors are routed through
typical office locations which contain significant ambient
energy. This can inject as much as 30V AC (in some cases
even higher) of common mode potential to the twisted pair.
If this common mode voltage is not blocked, the line receiv-
er, which may be powered by a single 5V rail, will fail to
receive a signal that is well outside of its specified operating
range.
4.8 Conducted Power Spectrum
This is the power spectrum of a properly terminated PMD
transmitter (including the magnetics) as measured by direct
connection into a spectrum analyzer. This spectrum analy-
sis is a convenient method of comparing the results of dif-
ferent signalling techniques. The degree of randomness
within the data stream as well as the differences between
binary and MLT-3 are easily compared via conducted emis-
sions.
4.9 Radiated Emissions
This is the radiated power spectrum of a properly terminated
PMD transmitter (including the magnetics) as measured by
a near field antenna within a strictly controlled environment.
Although this application note does not report on the radiat-
ed emissions results of the recommended magnetics it re-
mains a very important parameter. It is the responsibility of
the systems vendor to ensure that the performance lies
within mandated limits set forth by the various and appropri-
ate regulatory agencies.
4.4 Crosstalk
This is the amount of energy coupled from the transmit
channel to the receive channel within the magnetics. The
effects of this type of crosstalk are virtually eliminated due
to the physical isolation between transmit and receive mag-
netics as shown in subsequent connection diagrams.
4.5 Output Transition Time
This is the standard ‘‘rise and fall time’’ as measured from
10% to 90% of full amplitude. With MLT-3, it is important to
measure both rise times and both fall times of the three
level signal. Again, due to the controlled output transition
time of the DP83223, additional wave shaping filters re-
quired by some implementations are unnecessary.
4.10 EMI Susceptibility
This is a measure of the tolerance of a working TP-PMD
receiver to a controlled ambient field of radiation imposed
on the twisted pair cable carrying the scrambled FDDI line
code. The receive-end magnetics can be supplemented
with some degree of high frequency filtering to afford great-
er immunity to susceptibility.
4.6 Overshoot
Given a square wave, overshoot may be defined as the
amount of energy above or below the intended final high or
low voltage level(s) as expressed in percent. Overshoot
may result from unintentional emphasis of some high fre-
quency harmonics and or transitions coincident with reflec-
tions. Due to the controlled transmit transition times of the
DP83223, the potential for overshoot is reduced by the in-
herent decrease in high frequency energy of the transmitted
transition times.
5.0 RECOMMENDED MAGNETICS
This application note highlights specific magnetics from four
vendors. It is important to understand that this note does
not suggest preference to any one vendor or magnetics so-
lution. The results herein are made available strictly as a
means of objective comparison intended to assist the sys-
tem designer in making the best possible choice for a given
implementation. Due to the relative immaturity of Twisted
Pair FDDI, this application note reports on only a limited
number of magnetics solutions. Future updates or adden-
dums to this application note will include a larger selection
of magnetics suggested for use with National Semiconduc-
tor PMD solutions. The four magnetics solutions are listed,
in alphabetical order, by company name followed by product
number. (Contact information for each of the vendors is lo-
cated at the end of this applications note.)
4.7 Baseline Wander
In an AC coupled digital transmission system, baseline wan-
der is the variation in the DC content of the transmitted
datastream at any point in time. This phenomenon is depen-
dent on the digital content of a given data stream and the
low frequency cutoff of the magnetics. The scrambled FDDI
line code generated by a twisted pair FDDI PMD can result
in run lengths (no transitions) of up to 480 ns. If the magnet-
ics low frequency pole is not sufficiently low to allow, with-
out attenuation, a 480 ns static condition, then the attenua-
tion at the critical frequencies will result in a ‘‘droop’’ or ‘‘tilt’’
of the waveform during the run length. This droop will effec-
tively offset the baseline reference of the datastream result-
Ý
Bel FuseÐ 0556-3899-04
Ý
CoilcraftÐ Q3950-C
Pulse EngineeringÐ PE-65620M
Ý
Ý
ValorÐ ST6021
Please contact each individual magnetics vendor for the lat-
est product information and part numbers.
2
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