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Patented Quadsplitter Technology

In current Fibre Channel systems, greater demand is put on cable and connector designers to address the high data rates already here and the ones fast approaching. At the moment, cable suppliers have produced excellent cable with tight tolerance characteristic impedance and low insertion loss. Connector suppliers, on the other hand, have been slower to respond to the need. The reason is in the unfamiliar interfacing. The most important concerns of users are the characteristic impedance and low reflection controls. Design equations for shielded differential pair lines exist but only in symmetrical-filled medium configurations, and they become inaccurate at cable entries. This is solvable with finite-element analysis where mixed mediums are calculable. 

The high-speed data rates require transmission systems that minimize reflections and can only be achieved through controlled characteristic impedance from source to load. In microwave systems, this is accomplished with waveguide or coaxial transmission lines. In both cases, the line geometry is the determining factor along with dielectric and conductor materials. Steps, bends, protrusions and so on will invariably cause reflections with subsequent loss of transmission efficiency. In two-wire differential-mode transmissions, this is acceptable at lower data rates. However, when data rates become higher, such as with Fibre Channel (into microwave frequencies), the line characteristic impedances become much more critical.

Impedance In these systems, the source and load differential impedances are usually high (100 to 150 O). Achieving these high impedances in coaxial transmission lines and connectors is size-prohibitive. As a result, a line configuration such as twinaxial where the signals carried between a pair of conductors (usually round) critically spaced from each other and surrounded by a conductive enclosure is used. In this "differential line," high impedances are more readily obtained because the mutual capacitance between the conductors is minimized, and differential impedance is approximately twice the pin-to-ground impedance.

Ideally, a matched system requires that the line characteristic impedance be equal to the source and load impedances. This condition is somewhat compromised in a connector, especially at the cable entry. It is further aggravated by captivation steps and spring junctions. Despite these conditions, excellent impedance control can be achieved by choosing constant diameter and effective dielectric-constant regions, setting these up as separate transmission line sections and applying the appropriate equations. This should yield predictable characteristic impedances. It is incumbent on the connector designer to configure his cable entry and contact spacing to avoid excessive flaring out of the cable conductors. Proper choice of cable and compensation techniques greatly reduces the effect of flaring.

Quadrax Development

A fairly recent development in differential transmissions is called "quadrax." Here, four conductors are enclosed by a single shield. The conductors are diagonally paired to form two differential pairs with mutually perpendicular electric and magnetic fields, thus nearly eliminating all crosstalk. This has the advantage of size and cost reduction.

Quadrax is often used through longer line runs up to 30 m or more with equalization. However, conversion to separated twinax cables must occur at some point. Because a connector pair usually exists from the equipment panel to the internal circuitry, the conversion is best achieved within the connector receptacle. Direct wiring requires the conductors to cross over, causing an unwelcome and inconsistent impedance disturbance, even with short leads. This is acceptable for low data rates, but when the data rate equivalent frequency approaches microwave and the lead lengths become a significant portion of a wavelength, this method compromises system performance by producing reflections and crosstalk.

The Quadsplitter
A better solution to this problem is use of a built-in feature within the receptacle member of the connector pair. This feature effectively divides the quadrax inner conductors into two separate twinax paths without disturbing the impedance any more than a stripline surface-launch microwave coaxial connector. A "quadsplitter" accomplishes this goal by employing multi layer stripline circuit boards carrying short runs of paired strip conductors between ground planes.

The attachment of the four contacts within the mating end of the connector is routed to the assigned circuit board while merely passing through the board assigned to the other pair. 

Figure 1 depicts a complete assembly using a MIL-DTL-38999 Series III size 11 receptacle with the rear cover removed. The twinax attachment is visible. Five layers are used to form the multi layer stripline circuit board (see Figure 2). By conductor coating one side of each board, three ground planes are established that provide two trace layers sandwiched between ground planes.

Each trace accesses the surface region or board edge through conductor-coated holes. The region where the front contacts interface with the board surface requires considerable attention, as sudden changes in diameter produce fringing capacitance. In any case, compensating dimension adjustment is straightforward. This condition also exists at the board edge, which again can be compensated. The internal trace dimensions and ground plane separation is calculable from standard stripline equations or available finite element software.

Figure 3 is a printout of a single quadsplitter differential response as taken on a Tektronix differential time domain reflectometer. The units and vertical scales are shown for differential and pin-to-ground characteristic impedance. The lower traces are for pin-to-ground (75 O) and the upper trace is differential (150 O).This is a fully compensated size 11 single quadrax assembly split into two twinax cables, as shown in Figure 1.

Figure 4 is a cross section of a four-quadrax to eight-twinax assembly. The exit location(s) to twinax cable is optional and could be on any edge, on several edges or straight out the back.

The external ground plane on the interface side is peripherally connected to the quadrax outer contact. On the exit end of the connector, the twinax conductors are simply soldered onto surface pads or into the conductor-coated holes. In the sample shown (Figure 1), a MIL-C-38999 Series III size 11 connector is used where the receptacle shell is the outer shield of the quadrax interface. This is already in common use with twinax attachment through open wiring. This can be tolerated for now for lower data rates, but will soon be replaced with a more sophisticated transition region such as a quadsplitter.

An extension of the single quadrax-into-two twinax example described becomes obvious. Multiple quadrax conversions within the same connector are totally feasible. A size 25 MIL-C-38999 connector can accommodate four quadrax cables with minimum penetration with the box, as shown in Figure 4. In this case, two separate circuit boards are shown, but combining them into one single multi layer board would not be a problem. The choice would be influenced by the preferred twinax exit location(s). This example reveals the need to avoid using the connector shell as the quadrax outer shield for more than one quadrax.

Example of rectangular quadrax

Figure 5 is a rectangular configuration for board-edge locations and blind-mate options. Again, the exit location(s) of the twinax cable is optional. 

Conclusion
By now, Fibre Channel system designers may have recognized an extended application for the stripline circuit boards. Today, packaging is the prime goal of any system designer and the useful surface area of these boards is evident even without extending the board beyond the regions occupied by the traces. An equalizer circuit could be accommodated. Active circuits could be included with radio frequency (RF) decoupled power supply wires fed through the case walls or additional pins at the connector interface. There is some question as to whether low-cost open-lead connectors such as DB-9 and similar configurations address the concerns of the system designer. In a growing field like Fibre Channel, it would seem that more attention should be paid to the high-frequency performance only achieved with controlled impedance and shielding effectiveness through the total path. Simply grouping contacts in a cheap standard connector may work for now, but with the coming data rates, the RF engineer must step in and raise the standard of these interconnect systems. The quadsplitter certainly improves the condition, but twinax interconnects deserve the same attention. The design approach should be to treat the interfaces and cable entries with the same RF/microwave discipline as microwave connectors. A standard twinax and quadrax Fibre Channel contact should soon evolve that would be interchangeable within all connector variations. Complex multi channel systems engineers should be the first to embrace this. 

Main Point: Conversion to separated twinaxial cables must occur at some point in quadrax differential transmissions. A solution to this problem is use of a feature that divides the quadrax inner conductors into two separate twinax paths without disturbing the impedance any more than a stripline surface-launch microwave coaxial connectors.