Issue June 2007
Backplane: Look into its eyes!
High signal integrity speeds up µTCA systems
The blu!box by Elma is a new MicroTCA shelf with
optimised backplane for demanding practical tests.
The implementation of real-time applications requires a powerful infrastructure
for the data transfer which has to
distinguish itself by large transmission
bandwidths at least possible delays and
minimum risks of loss. High data transfer
rates are required mainly by modern
telecommunication applications. This field
extends increasingly towards sophisticated
multimedia applications and demanding
industrial fields of application, such as
machine and process control. In particular
the
MicroTCA
standard
contributes
decisively
towards
this
development,
thanks to its high degree of modularity. In
this connection, signal integrity is of great
importance. As we will see in the following,
certain reflections on 3 GHz systems have
to be made that take into consideration the
signal behavior of up to 15 GHz!
The following contribution is meant to specify the “natural enemies“ of signal integrity and to
sensitise to the fact that with parsimony, you easily back the wrong horse in this particular field. Or,
put into positive words: It’s worth the effort to look for the right partner. After all, the complete
information chain that even includes peripherals is decisive for the quality and immunity to
interferences and fault tolerance.
Our considerations are based on serial data transmission. The standardisation committees at PICMG
and VITA realised at an early stage that for high-speed applications, you have to dismiss classical bus
structures and switch over to point-to-point connections with differential pairs. To avoid the
formation of long sequences of logical zeros and ones on serially transmitted data packets – which
complicate clocking and data regeneration – the 8B/10B encoding is successful in such applications.
8B/10B encoding
When using 8-bit words, you will get 256 (28) possible bit combinations, 10-bit sequences however,
result in 1024 possible bit combinations. For the formation of 8-bit words, the 8B/10B code only uses
such bit combinations that provide multiple state changes. These are words which contain a
maximum sequence of 5 zeros or 5 ones in a row. Thus it is possible to safely generate a clock signal
that is required for synchronisation from a coded signal with an adequate number of state changes.
Thanks to these advantages, the 8B/10B encoding is used in the Gigabit Ethernet, the 10-GigabitEthernet, with Fibre-Channel, StarFabric and generally with ATM (Asynchronous Transfer Mode)
interfaces.
Issue June 2007
MicroTCA, the latest development of the AdvancedTCA
standards which promises particular success, offers the
possibility to transmit data by Gigabit Ethernet, PCI-Express,
Serial Rapid I/O, Serial Attached SCSI or Advanced Switching.
At first, the question arises why to make highly crucial
reflections on signal integrity. After all, it’s a XAUI transmission
by means of an NRZ signal with a data rate of „only“ 3.125 Gbps
(XAUI = X Attachment Unit Interface, an interface used in the
10-Gigabit-Ethernet; NRZ = “Non-Return-to-Zero” signal, a
signal that does not return to zero at regular intervals). It is a
fact, however, that the edge steepness of the signals determines
the bandwitdth. Thus, at a basic frequency of 3.125 GHz, the
signal integrity for a bandwidth of at least 15.625 GHz has to be
guaranteed. This is due to the high-frequency, low-level
harmonic waves, strictly speaking to the 5th harmonic wave
according to Fourier.
With MicroTCA, Advanced Mezzanine Cards (AMC) are defined as
smallest modular assembly. For cost reduction reasons, this
technology uses contacting via connectors on the backplane and
gold pads directly on the AMC board. Due to their small
mechanical shape, 170 contacts have to be realized via these
connectors on a width of only 73.5 mm. To investigate the
performance of different printed conductor geometries, Elma
Trenew Electronic GmbH, as an experienced specialist in the
field of backplane and basic systems, carried out fundamental
metrological examinations. For this purpose, a MicroTCA test
backplane with appropriate high-quality test boards was
developped. It served to measure the physical properties by
means of a TDR (Time Domain Reflector) and VNA (Vector
Network Analyzer). These measurements assisted in deriving
decisive findings on important influencing quantities that affect
signal integrity.
The ISI effect: Maximum influence on signal integrity
The measurements showed that the Inter Symbol Interference
(ISI) effect decisively influences signal integrity. The ISI effect
describes how pulse-shaped signals are broadened compared to
the output signal through dispersion, skin effect, dielectric losses
and reflections. In addition, the inductive stub effects (with THT
connectors) or bone effects of up to 1.2 nH (that occur when
connecting SMT connectors to the inner layers of the backplane)
contribute to the ISI effect. Since the pulses can be broadened
through Inter Symbol Interference over several periods, this
effect limits the transmission bandwith that can be used
actually.
Issue June 2007
Each conductor made of copper has different line capacities and
inductivities – depending on the layout geometry, dielectrical
insulation and frequency. Because of this, pulse-shaped signals
are broadened. Such broadening increases with the physical
length of the transmission line. This results in different
propagation rates and thus in transit-time distortions for each
individual contactor which can be seen in the eye diagram as
jitter. For each differential pair, they have to be adjusted to
each other to few picoseconds (which corresponds to less than
1 mm of physical path). Especially for broad-band signals, an
optimisation has to be obtained by correctly selecting the basic
material to be used and by adopting a clever layout strategy.
This happens in several iterative steps since the individual
parameters have an influence on each other. This requires the
backplane design engineer to have well-founded experience so
that he is able to find a carefully selected compromise for which
all factors of influence are valued, and to minimize them
altogether.
Data transmission happens in the electric field!
With increasing frequency, the importance of the DC conductor
decreases, while the effects taking place in the electric field
become more and more dominant. In the case in hand, the
measurements also showed that the data transmission does not
take place in the conductor, but mainly in the field. Thus, we do
not have to observe the conductors and connector pins, but the
field in which the data transmission takes place as a whole. In
addition to this, it is necessary to reduce the influencing
quantities of dispersion, skin effect, dielectrical losses, stub and
bone effects by means of appropriate connector and layout
geometries and thus to positively influence signal integrity,
always while taking into consideration economic solutions using
the least expensive basic material.
But it’s not only one interference factor. Beside the factors
described above, another important item has to be mentioned:
cross-talking between differential pairs. Ideally, neighbored
pairs are separated from each other and shielded by ground
planes in which case only few “aggressors“ could influence the
individual pair. In practical operation, the high number of layers
of the board necessary for doing so, and thus the high costs
involved and the manufacturing difficulties speak against it.
Plus, the stub effect becomes more and more important with an
increasing board thickness.
Issue June 2007
The experiments also showed that in the backplane, the signal is
considerably invalidated due to attenuation and losses. X-talk
(crosstalk) on the other hand dominates in the connector. For
this reason, the relevant parameter for x-talk measurements is
the NEXT (Near Ended Crosstalk) parameter. Oddly enough,
FEXT (Far Ended Crosstalk) benefits from the attenuation on the
board and can be neglected provided that this influence now is
known.
What does this mean in practice?
To sum up, the following can be said: Thanks to the detailed
experiments carried out by ELMA regarding the signal behavior
on the transmission path it was possible to optimise the layout
strategy for MicroTCA backplanes. That way the number of
layers in the board can be reduced and in spite of that, all
requirements of the X-talk limits according to the MicroTCA
specification can at least be satisfied or even surpassed. (Thus,
ELMA succeeded for example in transmitting 6.25 Gigabits via
one of their AdvancedTCA backplanes in cooperation with their
partner Enigma, using an ingenious encoding. There still were
sufficient safe capacities so that an even higher data transfer
rate is realistic).
The measured parameters not only serve to determine once the
optimal layout strategy from an economic and technical point of
view. They rather can contribute to simulation programs. Thus,
it will be possible in future to investigate the complete package
of connectors plus board in HSPICE simulations regarding its
requirements on signal integrity (HSPICE® is an industrial
standard for the high-precision simulation of systems by a
complex analysis algorithm). This decisively reduces the
development process and supports users in reducing the timeto-market.
To avoid that this is all mere theory for potential users of the
promising MicroTCA technology, ELMA has supported this
market segment from the very beginning. With their blu!box by
Elma, the specialists situated in the town of Pforzheim,
Germany, provide an innovative, but most of all complete,
ready-to-run MicroTCA shelf for form factor testing and intense
board tests. The fully enclosed 19” subrack is 5 U high and
200 mm deep. The tested basic system is fully redundant thanks
to 2 x MCH (MicroTCA Carrier Hub), two power modules for
– 48 V / – 60 V and two cooling units. The speed of their high
performance-fans is PWM controlled.
Issue June 2007
For this purpose, and for the exact detection of the thermic
situation in the test structure, up to 20 temperature sensors can
be positioned in the shelf. The system can be enlarged with
maximally 12 AMC, 8 “full height” and 4 “half height” in
accordance with the form factor. All modules are Hot Swap
components and IPMI-controlled. The inserted backplane
benefits already from the results of the detailed series of
measurements described above. It has a 20 layer structure and
guarantees optimum signal integrity. Plus, it supports all FRU
(Field Replaceable Unit) functions. High speed routing is effected
via DualStar technology, the FCI connectors used are developed
especially for high-speed data transfer. Two options are
available for telecommunication and non-telecommunication
applications.
The reference signal: the eye diagram at the feeding point.
Eye diagram of the signal at a 15 mm slot distance.
Eye diagram of the signal deformed through the ISI efect
after 396 mm. Due to the suboptimal layout strategy, there
are 173 mask violations.
Compared to the eye diagram at 396 mm and
optimum layout strategy: no mask violations.
For further information concerning our products,
please contact our regional offices:
Germany
info@elma.de
Phone: +49 7231 97 34 0
Fax: +49 7231 97 34 97
Stuttgarter Strasse 11, D-75179 Pforzheim
Switzerland
sales@elma.ch
Phone: +41 44 933 41 11
Fax: +41 44 933 42 15
Hofstrasse 93, Postfach, CH-8620 Wetzikon
France
sales@elma-electronic.fr
Phone: +33 4 37 06 21 10
Fax: +33 4 37 06 21 19
ZA du Buisson Rond, F-38460 Villemoirieu
Romania
info@elma.ro
Phone: +40 21 3 10 79 98
Fax: +40 21 3 10 79 92
Str. Maltopol 16, RO-011048 Bucuresti
United Kingdom
info@elma-mektron.co.uk
Phone: +44 1 234 272 677
Fax: +44 1 234 761 335
Elma-Mektron, Unit 8 Bedford Business Centre,
Mile Road, Bedford MK42 9TW
Other Countries
Backplanes and System Platforms
info@elma.de
Phone: +49 7231 97 34 0
Fax: +49 7231 97 34 97
Stuttgarter Strasse 11, D-75179 Pforzheim
Enclosures & Components,
Switches, Knobs & LEDs
sales@elma.ch
Phone: +41 44 933 41 11
Fax: +41 44 933 42 15
Hofstrasse 93, Postfach, CH-8620 Wetzikon
ELMA Electronic is a global manufacturer of products for housing electronic systems.
The company provides everything from components such as modular enclosures,
cabinets
and backplanes, up to complete standard or custom system platforms.
ELMA also manufactures precision rotary switches. The company offers a fast, flexible
response to customer needs and extensive practical knowledge in tailoring solutions to
specific applications.