Experimental Analysis of Common Mode Currents on
Fibre Channel Cable Shields due to Skew Imbalance of
Differential Signals Operating at 1.0625 Gb/s
James L. Knighten
Norman W. Smith
Joseph T. (Ted) DiBene II
Lothar 0. Hoeft
Convergence Design
9948 Hibert St., Suite 205
San Diego, CA 92131
Consultant, Electromagnetic
Effects
9013 Haines Ave., NE
Albuquerque, NM 87 112
NCR Corporation
17095 Via de1 Campo
San Diego, CA 92127
Abstract: The spectral nature of common mode currents induced on
high speeddifferential cables operating at 1.0625 Gb/s was investigated using specially constructed shielded test boards. The source
test board provided a source with a selectableamount of delay skew.
The load test board provided a simple 150 Ohm differential load.
The two boards were placed in separateshielded enclosures with a
one meter Fibre Channel cable connecting them. Common-mode
cable shield current and radiated emissions at 3 meters were measured as a function of delay skew. At the fundamental frequency of
53 1.25 MHz, common-mode current and radiated emissions increasedat a rate of approximately 9 dB/decade of skew. At skew
values much lower than the rise time of the signal, common-mode
current increased nearly linearly with skew. The second harmonic
was present on the cable shield due to duty cycle distortion and rise
and fall time differencesinherent in the driver transceiver.
In a shielded transmission line, the common-modecurrent returns to
the transmitter via the inner surface of the shield. This current becomes an EM1 issue as it egressesthe shield by meansof the common-modesurfacetransfer impedancesof the cable and connectors.
High speed transmission lines used in digital circuitry are usually
designed to minimize imbalance as the characteristic impedancesand
path lengths of both sides of the differential pair are designed to be
the same. While manufacturers of high speed digital differential
devices take care to minimize asymmetries,imbalance may be produced by asymmetriesin semiconductor drivers as voltage swings,
rise and fall times may not be equal. In addition, variabilities in
seemingly identical passive components may also contribute to imbalance. Skew is a term used to describeone of the common types of
differential imbalance. Delay skew denotes effective path length
differences between the sidesof a differential pair.
INTRODUCTION
Digital computing systemssometimesemploy high speedserial interconnects to transfer data. A massively parallel processing (MPP)
system is designed for scalable data warehousing applications and
employs a high speed serial data interconnect based on the Fibre
Channel protocol at 1.0625 Gb/s [l]. Such a system may employ a
large number of cablescarrying high speedserial data.
Common-mode currents, rather than normal operating currents, are
often primary generators of EM1 [2]. Common-mode currents are
usually created by asymmetries,both mechanical and electrical and
are often not apparentfrom examinations of electrical schematicdiagrams.
High speed digital signaling is often implemented via dzj‘kential
signaling on two-conductor transmission line structures. (This is not
the differential-mode current described in [2], but is a transmission
line structure of two signals, propagating in the odd-mode, that are
equal and opposite with a virtual ground reference.) In the ideal
case, differential signaling produces no net current down the transmission line. When an imbalance in the differential signal pair occurs, the resulting modesof propagation on the transmission line are
both odd-mode and even-mode. The even-mode, where the two
wires carry equal signals, representsthe common-modesignal on the
transmission line.
0-7803-5057-X/99/$10.00 © 1999 IEE
Previous work, along with experience, indicate that cable radiation
due to skew imbalance of the differential digital signals in the cable
can be a dominant EM1 mechanism(vs. common-modeboard noise
internal to the chassisexiting via the cable shields) [3],[4]. This can
be the casewhen high speedboards are effectively designed to minimize the generation of conventional common-mode currents and
placed in effective shielded enclosures.
Calculations from analytical models of differential amplitudes, waveshapes,rise and fall times, and repetition rates show that the amplitude of common-modecurrents generatedon the shields of cables, at
the fundamental frequency, are primarily dependenton delay skew of
the differential signals within the cable. Dependenceon the risetime
of the signals is of secondaryimportance [3].
This paper describes experimental investigations conducted to corroborate analytical conclusions of [3]. Figure 1, taken from [3], illustrates signals on each line of the differential pair, the differential
signal, and the resulting common-modesignal for a specific example
of risetime and delay skew. The example in Figure 1 employs idealized signals with exponential rise and decay and no amplitude imbalance.
195
Line
between them at a height of approximately one meter above the floor.
The entire apparatus was placed in a fully anechoic shielded chamber. The 20-to-80% risetime of the launched differential voltage
signal was on the order of 250 ps. Figure 2 illustrates both the common-mode current and radiated field measurement setup. When radiated field measurements were performed, the current probe and the
spectrum analyzer were removed from the shielded chamber containing the test setup.
2
I
Line 1
Figure 3 illustrates the method employed to achieve deliberately induced delay skew on the printed circuit board. Soldered jumpers
were used on both sides of the differential line, to equalize skew effects from the soldered jumpers themselves. One side of the differential transmission line had a “straight through” connection and
served as a reference for the other side of the line. The “skewed” side
exhibited path length differences with respect to the “straight
through” side that ranged from 0 to 4.23 inches. The printed circuit
board was constructed from FR-4 material with a dielectric constant
of approximately 4.0. The differential traces were formed in a “microstrip” configuration, e.g., with FR-4 material below the traces and
air above the traces. Trace width was specified as 5 mils of I/ oz.
copper. The intent was to achieve differential lines of 150 Ohm differential impedance, 75 Ohms to ground.
-at,““‘,“‘,“““,‘,,“‘1
O.OEiOO
5.OE-10
1 .OE-O9
Time,
L
1.5E-09
2.OE-09
2.5E-09
Seconds
Figure 1: Typical line-to-ground differential and common
mode time domain waveforms (170 ps lo-90% risetime,
100 ps delay skew [3]
EXPERIMENTALSETUP
Two test boards were prepared, one with a battery-powered HP
HDMP-1536A Fibre Channel transceiver source and the other with a
150 Ohm differential load with no connection to signal ground (pure
differential termination). The source board was designed with selectable amounts of deliberately induced delay skew imbalance (from 0
to 754 ps in six increments by means of soldered jumpers). The bit
pattern produced by the transceiver was jumper selectable and set to a
continuous repetitive 0101010101 pattern. The fundamental frequency of this bit pattern is 53 1.25 MHz. Both boards were outfitted
with DB-9 board-mounted connectors and each was installed in a
shielded enclosure with a one meter, 150 Ohm differential cable,
suitable for Fibre Channel use at 1.0625 Gb/s connecting the enclosed boards. The shielded enclosures were (23 cm H x 34 cm W x
75 cm D) arranged so that the one meter cable was stretched taut
The test cable used had two untwisted signal pair, with an internal
foil shield around each pair and an outer braid over the entire bundle.
The cable was intended for use in Fibre Channel-like environments,
i.e., Gbis data transmission and low values of cable skew and loss.
The cable was terminated on both ends with a DB-9 connectors and
fully enclosed one piece metal can backshells.
For each value of induced delay skew, the common-mode current on
the outside of the cable shield was measured, using a Fischer F-2000
current probe, a Hewlett Packard 8593EM Spectrum Analyzer, and
an HP 84471) Pre-Amplifier. In the sequence of measurements made,
some were above the 1.3 GHz calibration limit of the pre-amplifier.
Skewed Line, Selectable Amounts
Antenna
-T
3m
Source
Current
Probe
Load
I
Soldered
Jumpers
/
Straight Through Line
Figure 3: Deliberately-induced
delay skew was accomplished by
means of soldered jumpers used to increase transmission line
length on one side of a differential pair. Jumpers were utilized
on both sides of the line.
Figure 2: Experimental
setup for measurement of commonmode shield currents and radiated fields from a 1 meter length
cable carrying a 531 MHz repetitive lOlO... signal
196
Table 1: Peak common-mode current measured along the length of one-meter cable versus induced delay skew for three harmonic
frequencies
Table 2: Radiated electric field at a three-meter distance from a one-meter cable versus induced delay skew for three harmonic
frequencies
For these measurements, a calibration measurement was performed to
determine the gain at these frequencies to assure accurate measurements.
Measurements of common-mode current on the cable shield were
performed inside a fully enclosed anechoic shielded chamber so that
external ambient noise signals would be minimized. Shield current
measurements were made both at the source and load ends of the
cable and along the cable length to observe the maximum current.
(All settings of skew and all frequencies measured produced standing
waves on the cable shield. The maximum current may not have been
observed at the source end.) All currents reported in this paper are
peak values of current observed along the cable’s length.
Figure 5 provides an illustration of the skew behavior for values of
delay skew significantly less than the risetime of the differential signal. For these values of delay skew, the peak common-mode current
at the fundamental frequency, 531.25 MHz, is nearly proportional to
delay skew, rising at approximately 0.04 dB/ps of skew and radiated
electric field rising at approximately 0.033 dB/ps of delay skew. For
larger values of delay skew, the common-mode current increases at a
slower rate. Neglecting the largest value of skew, a regression line
1--C
i%ield Q 3 m +
Common-Modes1
Radiated electric fields were measured at three meters by placing the
test configuration in a fully anechoic chamber and measuring the
field with an EMCO 3143 BiConilog antenna, along with the spectrum analyzer and pre-amplifier from the previous test configuration.
25
For both current and field measurements, the frequencies of 531.25
MHz, 1062.5 MHz, and 1593.75 MHz were chosen, representing the
fundamental frequency and its second and third harmonics.
RESULTS
Tables 1 and 2 provide the peak current measured along the cable
length and radiated electric field at a three meter distance, along with
noise floor measurements for each frequency and value of delay
skew.
Fundamental
10.00
Frequency- 531.25 MHz
*II, 0
0 ,
100.00
1000.00
Delay Skew (ps)
Figure 4 shows both the peak value of the fundamental frequency
component of the common-mode current and the radiated electric
field at a three meter distance versus delay skew. An examination of
the figure shows that common-mode current and electric field increase monotonically with delay skew.
197
Figure 4: Radiated electric field and peak commonmode current at 531.25 MHz on a one meter cable for
various values of induced delay skew
-O-E-Field
@ 3 m +Cawwn-Mode
inherent imbalance in the interconnect path and source circuit. Subsequent investigations of the printed circuit board showed that the
individual traces in the differential pair did exhibit some voltage level
imbalance, which itself can create a common-mode current. In addition, the cable is an undoubted source of skew imbalance, even
though it was a cable designed for transmission of high speed differential signals, with a low skew value.
Current
Figure 6 shows the ratio of radiated electric field to peak measured
common-mode shield current at 531.25 MHz for all values of skew
examined. In linear units, for all values of delay skew, the average
ratio of radiated field at 3 meters to cable shield current was approximately 3.6 pV/m per pA at 531.25 MHz. [5] postulates a ratio
of 10 between radiated electric field in mV/m at 10 meters to current
in mA. A 3 meter distance represents approximately 5.3 wavelengths
at this frequency, so the antenna may be considered to be approximately in the far field. Multiplying the factor of 3.6 by 3/10 to compare with ratio in [S] yields a factor of 1.1. [51’s ratio appears to be
the result of a worst-case calculation for a resonant dipole. This data
represents radiation from a longer than resonant dipole (and may not
represent true maximum radiation as neither the antenna nor the EUT
were moved exhaustively in search of the peak). While the comparison with [5] is not close, for this type of measurement at this high
frequency, this is not bad comparison. This transfer function is interesting in that it allows estimation of the maximum level of commonmode current permissible to stay within regulatory limits of radiated
emissions.
25
10
5
5
o,t~l,,~,~t,t,t
0
0
100
200
300
Delay Skew (ps)
Figure 5: Radiated electric field and peak common-mode
current at 531.25 MHz for values of skew less than the
risetime of the differential signal
Higher Harmonics
through the common-mode current data points rises at nearly 9
dB/decade of skew. The radiated field rises at nearly the same rate.
Notice in Figure 5 that there is measurable common-mode current
with no deliberately induced delay skew. This is an indication of an
40
Frequencies other than 531.25 MHz were examined. Figure 7 shows
the first three harmonics of the fundamental frequency commonmode currents that were measured on the cable. It should be noted
that the existence of the second harmonic, 1.0625 GHz, was not predicted in the idealized analytical model used in [3]. The reason for
this is the apparent asymmetry in the common-mode current pulse.
To avoid even ordered harmonics in a repetitive pulse, half wave
symmetry must be observed, i.e.,
f(t) = - f(t+T/2)
where, t= time, and
T = period.
(1)
Figure 1 shows clearly the presence of half-wave symmetry in the
common-mode waveform. This is seen by inspection. Hence the
spectrum of the waveform exhibits a lack of even numbered harmonics. However, in less ideal circumstances half-wave symmetry can
easily be lost.
25
5
10
Common-Mode
15
20
Shield Current, dBbA
Figure 6: Radiated electric field at a 3 meter distance
versus peak common-mode current at 531.25 MHz for a
one meter cable
Non-ideal semiconductor devices seldom provide the symmetry required to avoid even-ordered harmonics. As an example of this behavior, [6] provides an oscilloscope trace of a clock pulse on the
parallel I/O side of the same HDMP-1536 Fibre Channel transceiver
which exhibits some typical duty cycle distortion, along with an instrnment-calculated Fourier Transform spectrum that very clearly
shows the presence of even-ordered harmonics. Note that device
asymmetries that produce duty cycle distortion are common in these
devices.
In a non-ideal high speed circuit, lack of symmetry (balance) can be
due to many factors. The risetime of the signal pulse may be different than the falltime, and the duty cycle may be less than 50% (one
198
-531.25
+
MHz
. ..m... 1062.5 MHz
1593.75 MHz
a2 lo
iii 5
. 264
m
current
RIsetime
p5
Falltime
Duty cycle(l)
u p-p(1)
0
284 ps
47.6%
921.7
""
Figure 8: Launched differential waveform showing duty cycle
distortion and differing rise and fall times (0 ps induced
skew.
-5
IO
100
1000
Delay Skew (ps)
Figure 7: Peak common-mode currents at three lowest harmonic frequencies on a one meter cable
half of the pulse is slightly shorter in duration than the second half of
the pulse). The launched waveforms and spectra from the test board
without cable attached, shown in Figures 8 and 9, illustrate this phenomenon. Asymmetries may be due to behavior of the semiconductor transceiver, the printed circuit board, and/or the cable.
+(FT)=
8.95.dBra
X(FT)=
-23.50
dBn
b = -32.45
da
The measurements in Figures 8 and 9 were made with an HP 54720D
digital oscilloscope. (Some risetime degradation is observed due to
bandwidth limitations of the oscilloscope probes.) Clearly, in this
example, neither rise and fall waveshapes are identical, nor is the
duty cycle 50%.
The second harmonic component of the current, 1062.5 MHz, exhibits a behavior that is much different than the monotonic behavior of
the fundamental frequency. As discussed previously, the second
harmonic is made possible by duty cycle distortion and other asymmetries. The study of the process of differencing of two asymmetric
waveforms is complex and is beyond the scope of the present effort.
However, the third harmonic, 1593.75 MHz begins to decrease after
the skew reaches some intermediate value. This was puzzling until
the characteristic impedance of the printed circuit board was examined. Indeed, the board was fabricated with an etch process that was
not closely controlled. Under-etching produced traces with a singleended characteristic impedance very close to 50 Ohms, instead of the
desired 75 Ohms (the traces were wider than 5 mils).
The third harmonic behavior may be attributed to an increase in reflection coefficient as a function of transmission line electrical length.
Due to layout disparities in the impedance of the board level transmission lines and the cable transmission lines, an impedance mismatch is present. For very long wavelengths and short transmission
lines the reflection coefficient will approach zero and no backward
traveling wave is created. As the line length and/or frequency of the
signal increases, the phasor relationship of the reflection coefficient
l/AX
=
531 iiHz
1.063
LiHZ
531 IiHZ
1.882
"5
Figure 9: Fourier spectrum of the launched differential
waveform, as computed by an instrument-generated FFT
begins to become important, increasing the reflected wave. This
increase in the reflected wave decreases the transmitted signal onto
the cable transmission line (assumed to be matched at its opposite
end).
At 1593.75 MHz, the maximum skewed trace length, the 4.73 inch
trace length represents approximately 0.64 1. The next lowest
skewed trace length of 1.91 inches represents 0.26 a. Clearly, the
skewed side of the differential pair is long enough to exhibit mismatched transmission line behavior, whereas the 0.5 inch straight
through side probably does not, due to its short length.
Hence, larger skew values produce larger common-mode currents and
simultaneously larger reflections cause net common-mode current to
decrease. Thus, for higher harmonics, an increase in delay skew can
cause the common-mode current to decrease due to the loss of
transmitted signal onto the cable. This is illustrated in the data of
Figure 2 for the 31dharmonic. This is the behavior that the 1593.75
MHz current exhibits in Figure 7.
The transmission line behavior described in the preceding paragraph
is probably evident, but to a lesser degree, at the second harmonic,
1062.5 MHz. The 4.73 inch maximum length skewed side of the
199
CONCLUSIONS
-+-531.25
MHz -a--
1062.5 MHz +
1593.75 MHz
1.
Imbalance in differential
signals on cable shields.
2.
The second harmonic was present due to duty cycle distortion
and rise and fall time differences inherent in the driver transceiver.
3.
At the fundamental frequency of 531.25 MHz, common-mode
current and radiated emissions increased at a rate of approximately 9 dBldecade of skew.
4.
At skew values much lower than the rise time of the signal,
common-mode current increased nearly linearly with delay
skew.
5.
For all values of delay skew examined, the average ratio of radiated electric field (at 3 meters) to cable shield current was approximately 3.6 pV/m per pA at 53 1.25 MHz. In terms of linear
units, this represents a ratio (electric field in pV/m to pA) of 3.6
at a three meter distance and a ratio of 1.1 at a ten meter distance.
25
10
100
1000
Delay Skew (ps)
Figure 10: Radiated electric fields at a 3 meter distance from
a one meter cable carrying differential signals with induced
delay skew
signaling produces common-mode
ACKNOWLEDGEMENTS
trace represents 0.43 a, and the 1.91 inch skewed trace represents
0.17 h. The impedance mismatch reflection, combined with the
complex generation mechanism of the second harmonic creates a
situation which is difficult to analyze.
In retrospect, due to the unforeseen low characteristic impedance of
the printed circuit board traces, common-mode current data at the
harmonics above the fundamental are difficult to draw conclusions
from, with the exception of the existence of the second harmonic.
Figure 10 shows radiated electric fields at a three meter distance for
each of the harmonic frequencies.
It should be noted that no adjustment was required on antenna height to peak the reading, i.e.,
raising or lowering the antenna had little effect on field strength. The
fundamental and third harmonics behave in a manner similar to the
peak current seen in Figure 7.
The second harmonic radiated field, however, does exhibit a monotonically increasing behavior, contrary to the non-monotonic behavior of the cable current. An examination of the data shown in Tables
1 and 2 shows that the measurements of radiated electric field at
1062.5 MHz were very close to the noise floor, whereas the current
measurements were many dB above the noise. Therefore, it is possible that some of the radiated field measurements, particularly the
readings for the three smallest values of delay skew, may be contaminated with noise.
The authors express their appreciation to their employers, colleagues,
and families for the encouragement and support received during the
pursuit of this work. In addition, the authors wish to thank Molex,
Inc., which contributed to the success of this work by supplying the
cable assembly used.
REFERENCES
1.
McMillan,
R.J., ‘The BYNET ~2.0 Interconnection Network,”
Hot Interconnects 6, August 1998, pp. 116 - 135.
2.
3.
4.
5.
6.
200
Paul, C.R., “A Comparison of the Contributions of CommonMode and Differential-Mode Currents in Radiated Emissions,”
IEEE Transactions on Electromagnetic Comuatibility, Vol. 3 1,
No. 2, May 1989, pp. 189-193.
Hoeft, L.O., J.L. Knighten, J.T. DiBene II, M.W. Fogg, “Spectral Analysis of Common Mode Currents on Fibre Channel Cable Shields due to Skew Imbalance of Differential Signals Operating at 1.0625 Gb/s,” IEEE International Svmnosium on Electromagnetic Comoatibilitp, Denver, CO, August 1998, pp. 823 827.
Xu, M., S. Radu, J. Knighten, J. DiBene II, J. Drewniak, T.
Hubing, and L. Hoeft, “Signal Induced EM1 in Fibre Channel
Cable-Connector Assemblies,” IEEE International Svmoosium
on Electromagnetic Comnatibilitv, Seattle, WA, August 1999.
Goedbloed, Jasper, Electromagnetic Compatibility, Prentice
Hall, New York, 1990.
DiBene, J.T. II and J.L. Knighten, “Effects of Device Variations
on the EM1 Potential of High Speed Digital Integrated Circuits,”
IEEE International Symposium on Electromagnetic Comoatibili&, Austin, TX, August 1997, pp. 208 - 211.