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Re: [Fwd: 1000BASE-T PCS question], Importance of DC Balance


That, I must say, is a crass and ill-mannered comment.
You should apologise immediately.



Haim Shafir wrote:
> And we can safely assume that IBM's ownership of the
> 8B/10B patent does not impact you professional
> opinion at all.
> At 08:54 AM 6/3/99 -0400, widmer@xxxxxxxxxx wrote:
> >
> >How important is DC Balance? This question is best answered by the engineers
> >who
> >design the critical three circuits (Laser Driver, Receiver Preamplifier, Clock
> >Recovery), the persons who package the electrical and optical components, and
> >those who design the verification and production tests. Given an option, they
> >generally prefer a code with DC balance and a short run length. After
> >consultation with colleagues active in those endeavors, I can offer the
> >following list of circuit related advantages of a transmission code such
> as the
> >Fibre Channel 8B/10B code:
> >   Level settings of the laser driver bias point  and the receiver threshold
> >can
> >   be based on the average signal level which is simpler and more precise than
> >   using level restoring circuits. The receiver level restore circuits usually
> >   require some type of peak detection circuits which are difficult to
> >implement
> >   if the electronics is pushed to its limits. Peak detector noise may cause
> >   higher noise levels than otherwise expected because of the peak detectors
> >   tendency to capture occasional large noise excursions. The design of a peak
> >   detector which is both accurate and fast requires difficult inherent
> >   compromises.
> >   Thermal cycling of lasers or LED's is eliminated.
> >   Capacitive coupling  and level shifting is possible without
> complications to
> >   accommodate various package, grounding, and power supply configurations at
> >   the transmitter or receiver. At frequencies above 5 GHz it is hard to find
> >   capacitors which work well with unbalanced bit patterns for various
> reasons.
> >   Capacitive differential coupling at the front end of optical receivers with
> >   small integrated capacitors is more easily accomplished and provides better
> >   noise margins. At the lower data rates, designers may still include offset
> >   compensation circuits with a balanced code to reduce the capacitance values
> >   to a range compatible with integration in monolithic circuits. Such
> >   compensation circuits require less precision and complexity for a balanced
> >   code.
> >   Receivers at the end of computer or LAN links generally require a large
> >   dynamic range which is more readily achieved with a balanced code.
> >   Attenuation in electrical package interconnects is on the order of 1
> >dB/cm at
> >   the fundamental frequency of 10 GHz and much higher for the frequencies
> >   needed to transmit fast pulse edges. Any transmission line is easier to
> >   equalize for balanced codes because of the lower ratio of the maximum to
> >   minimum frequency content.
> >   It is desirable to set the low frequency cutoff of receivers as high as
> >   possible to remove noise from several   sources, such as: power supply
> >noise,
> >   low frequency modal noise arising from movements of multimode fibers,
> or 1/f
> >   noise of front end devices, especially GaAs devices not optimized for low
> >   noise analog operation . For low and moderate cost highly integrated
> designs
> >   it is usually not possible to pick the best devices which otherwise
> might be
> >   used.
> >   The shorter run length of a good code allows much relaxed
> specifications for
> >   the clock recovery circuit. The lower Q of the PLL enables it to cope with
> >   more external noise interference such as digital noise coupling from
> >   neighboring circuits, power supply variations,  or totally external
> >   electromagnetic interference. It is less problematic to place a  PLL with a
> >   lower Q on a large digital chip with limited isolation for a fully
> >integrated
> >   solution. The low pass filter of the PLL is more readily implemented
> with an
> >   on-chip capacitor or a totally digital solution (random walk filter)
> and the
> >   phase comparator is simpler for the coded version.
> >   For links carrying scrambled traffic, the link jitter budget expressed as a
> >   percentages of a baud interval  allows much less jitter for the transmitter
> >   which significantly complicates the design of the frequency
> synthesizer, the
> >   laser driver, and the connection between the driver and the laser.
> >   Simpler circuits consume less power in a critical area.
> >   Scrambled data requires nearly ideal circuit implementations in the areas
> >   discussed above. Cost considerations, design time  and skill limitations
> >make
> >   the attainment of near perfection for the 10 Gb Ethernet application an
> >   unrealistic goal. Less than perfect circuits have a greater hidden cost in
> >   terms of signal to noise ratio for scrambled data. For a given baud and
> >error
> >   rate, the coded link can span a longer distance with less sophisticated
> >   circuits.
> >   The design, performance simulation, test, and trouble shooting is
> simplified
> >   for a well constrained code. The robust operation of the coded link depends
> >   on no assumptions about the data pattern of the traffic. The
> performance can
> >   be proven with a few well defined worst case patterns tailored for
> stressing
> >   the major performance parameters. There is no exposure to hacking via the
> >   data pattern. This is in contrast with scrambled links for which
> performance
> >   in practice  can only be verified for a statistical consensus pattern and
> >   where it is always possible to come up with data patterns which cause the
> >   system to fail.
> >
> >
> Haim Shafir
> e9 Inc.
> PH 408-343-0192 cell 408-892-1838 fax 408-873-2642
> hshafir@xxxxxxx

Paul Gunning
Futures Lab
BT Laboratories

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