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*To*: stds-802-3-hssg@xxxxxxxxxxxxxxxxxx*Subject*: Gigabit signals via uncontrolled-impedance connectors*From*: gwinn@xxxxxxxxxx (Joe Gwinn)*Date*: Thu, 2 Dec 1999 11:55:36 -0500*Cc*: bendor@xxxxxxxxxxx, davebelo@xxxxxxxxxxx*Sender*: owner-stds-802-3-hssg@xxxxxxxx

Recently, there has been a lot of discussion of the difficulties in getting 10-gigabit or 2.5-gigabit signals through card-edge connectors and the like. I would like to offer a possible way to handle this class of problems. In August 1998, this same problem came up in an unrelated context, and a dodge then occurred to me, a way to control the reflections from uncontrolled-impedance connectors. This trick actually came from the RF world, but should work for digital signals as well, although I have not myself tried it. This approach parallels the use of special compensation networks to increase the signalling rate allowed on Fibre Channel 150-ohm balanced electrical link cables. Differential logic signals can tolerate some attenuation, if the waveforms are clean, so it's OK to make the signal paths somewhat lossy, and we can use this attenuation to swamp any impedance mismatches and variations, especially if the lossy connector is physically small. At each connector, make the following changes: Build a 3-db balanced "pi" attenuator, split it into two halves (by splitting the two in-line resistors in half), and connect the halves back together using two pins from the uncontrolled connector. This takes a total of six surface-mount chip resistors, three on each side. Because the resistors will have values from a few ohms to a few hundred ohms, carry very little power, and the tolerance is quite loose, it should be easy to obtain suitable tiny low-inductance surface-mount components. Because the attenuator itself can be made small relative to a wavelength (3 centimeters) at the signaling rate (10 gigabit), no significant reflections will occur within the pad. The net result is that very good waveform fidelity is possible, despite the imperfections of the connector. The effect of this is to isolate the connector (and thus its impedance bump) from the rest of the transmission lines, preserving signal waveshapes, at the expense of some added loss. Noise from the transmitter side of the lossy connector will be attenuated in the same proportion as the digital waveform, so only receiver-side noise can cause a SNR degradation due to the reduced signal amplitude at the receiver, at least to the first order. A 3-db pad with equal in and out impedances will by definition pass 71% of the input voltage swing, and this should pose no problem. For instance, the minimum ECL differential output swing is 600 mV, while the required minimum input differential swing is 400 mV, which is 400/600= 67%, and so 71% will work. Using typical values, the output swing is closer to 1,000 mV, and the required input swing is 200 mV, which is 200/1000= 20%, or -14 dB, so one could at least in theory cascade four such 3-db pads in a laboratory setup, where it's OK to hand-select parts. In production designs, one would instead regenerate the signal after each transmission through the attenuated connector, so that end-to-end losses would not grow with the number of lossy connectors in series. Actual resistor values are tabulated, given transmission-line impedance and desired loss, in various books, such as the old ITT Radio Engineers Handbook: "Reference Data for Radio Engineers", fifth edition, Sams 1968, Chapter 10 "Attenuators". (Later editions don't have nearly as good a treatment of attenuators as the 1968 edition cited above.) Table 2 on page 10-6, covering the "symmetrical-Pi" attenuator design, covers the 3-dB pad described in the following paragraph. Note that Table 2 assumes 500-ohm input and output impedances, so the values given in the table have been divided by ten to yield the following 50-ohm values. For 50-ohm input and output impedances and 3-decibel loss, the two shunt resistors (R1 and R2 in Table 2) are both 292.4 ohms, and the four series resistors (R3 divided by 4) are each 4.40 ohms. For transmission-line impedances other than 50 ohms, scale the values just given in linear proportion to the actual line impedance. For instance, for 75-ohm transmission lines, the shunt resistors are (292.4)(75/50)= 438.6 ohms each, and the series resistors are (4.40)(75/50)= 6.6 ohms each. Such resistive attenuators can also help to control the destructive connect and disconnect spikes seen when circuits are opened or closed, possibly permitting hot insertion and removal without damage to the receiver input structures, an added attraction. Other logic families will have their own voltage levels, but in the context of designing 10GbE, we ought to be able to allow for some attenuation, if it solves some signal integrity problems. Although the above description assumes a differential signal, there is no reason this approach won't work for unbalanced signals as well. Slightly more complex attenuators can also compensate for excess stray capacitance in the connector, if needed, but at the expense of larger added attenuation. Likewise, one can make stripline on FR4 better-behaved if one is willing to suffer some added attenuation. A little bit of loss can greatly reduce the effect of reflections, and if one knows in advance the exact path loss to expect and so can design for it. Joe

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