The Great Gigabit Backplane Shootout - Question
#3
Dielectric loss, skin effect, impedance discontinuities (plated through holes for press-fit connectors, vias, the transceivers themselves), far-end crosstalk, and sometimes even near-end crosstalk.
The primary line impairments are the connector interface, the via discontinuity, board trace attenuation and crosstalk. Connector manufactures are addressing the connector related problems by redesigning connectors for higher throughput. One undesired effect of the connector redesign is decreased density for a given number of connections. Via discontinuities are being addressed by board manufactures with techniques like variable depth backdrilling. This technique is more costly and potentially less reliable. Trace attenuation is almost purely a function of dielectric. At this time, using a more expensive higher dielectric material or equalization techniques in the SERDES design solves this problem. The concern with both approaches is an increase in cost. The IC will also require more power to achieve the necessary equalization.
Connectors and board construction form the primary impairments, with board material and IC packaging contributing. Inter-Symbol Interference (ISI) is caused by channel bandwidth limitations due to via, connector and package parasitic, with both local (adjacent bit) and remote (internal reflections) impulse response impairments. Cross-talk is predominantly caused by connector structures, but can also result from poor track-to-track isolation in the board design. The board designer must take all of the parasitic effects into account and weigh them against other constraints involving cost, yield, reliability, density, thermal and power distribution.
The main impairments are: 1) Insufficient line bandwidth which results in attenuation and inter-symbol interference, 2) Impedance discontinuities at connectors which results in reflection and therefore signal distortion, 3) cross talk.
As signaling rates increase, and rise and fall times become faster, the amount of high-frequency transmit energy rises. This results in an increase in crosstalk and a dramatic change in the nature of the channel impairments. The low-pass channel characteristics, combined with the high-pass NEXT coupling, imply that higher signaling rates are subject to increased levels of temporal dispersion (ISI) and increased levels of ingress noise. At low signaling rates, the primary challenges include dielectric loss, skin effect losses, low level reflections, and low levels of intra-pair skew. These impairments primarily manifest themselves as direct signal loss, with some ISI. Well-known solutions exist to mitigate these losses including the use of transmit pre-emphasis/de-emphasis. Additionally, since the NEXT coupling is weaker at frequencies corresponding to low signaling rates, some capacity loss can be mitigated by trade-offs in signal power.
Above 5 Gb/s channel related impairments become much more significant. A 40" copper channel running over FR-4 and using standard connectors can easily experience greater than 20dB of attenuation (see Fig. 1.) In addition, as the signal content encroaches further into the low-pass region of the channel, variation in the signal amplitude and phase characteristics will increase, even among channels of the same length. This level of loss and channel variation requires the use of adaptive signal processing techniques such as decision feedback equalization for proper recovery. Even worse, at these higher speeds NEXT and FEXT caused by crosstalk inside, or in the vicinity of the connectors now become the primary channel impairments. Legacy linecards will also contribute to the crosstalk further aggravating the situation. In today's large routers and Ethernet switches, you have the possibility of having 1.25G, 2.5G, and 3.125G signals all running alongside next generation transceivers.
This cross-talk noise presents several significant problems for higher signaling rates. The coupling function is high-pass, thereby ensuring that higher signaling rates will suffer more ingress noise. This noise degrades channel capacity, even if whitened within the receiver. Increasing transmit signal power actually increases the ingress noise (NEXT/FEXT). There are significant benefits to keeping the signaling rates as low as possible.
The key to understanding today's backplane dilemma is
realizing that the speeds engineers are now considering (>5Gbps) produce
a new design problem with its own set of constraints. Increasing the speed
of legacy two level I/O or utilizing common copper cable transceiver architectures
does not work in this environment because these designs have been optimized
to overcome different impairments. The major issues faced by signal integrity
engineers in system design today are near-end crosstalk (NEXT), far-end
crosstalk (FEXT), limited power budgets, large signal attenuation, reflections
from impedance discontinuities, and the need for better bit error rates
(BER) as the data density within the box increases.
Cross-talk and jitter.
At the high speeds, crosstalk, reflections, dielectric loss, and ISI in connectors are some of the most significant line impairments.
There are two issues, loss and crosstalk. Loss, as expressed in S12, and Crosstalk, expressed in S32. The issues that impact loss are related to dielectric material, line width, and connector impedance. Impedance discontinuities from vias and layout issues also add loss, and create 'suck outs' on the S12 that impact signal integrity. The issues that impact crosstalk are related to shielding and signal routing.