One of the challenges in the layout of multifunction microwave assemblies is the crossing of microwave transmission lines while maintaining isolation requirements. In designs of limited complexity, this crossing-over has been done using a vertical transition. The transition comprises RF feedthroughs that are used to route signals to the backside of the housing and back up again in order to maintain the high isolation between signal paths. Although this design technique is standard, it takes up a lot of space. As the units increase in complexity and packaging density, there is not enough room to allow for this type of connection. An example of this type of design is shown in Fig. 1.

An example of a subassembly containing four RF crossover boards, which allow all RF circuitry to be planar on one side, is shown in Fig. 2. Due to packaging density issues and the volume required for the control circuitry on the backside of the unit, RF feedthroughs could not be used in this example. Instead, a planar solution was needed. All of the channels use silver-epoxied, 10-mil-thick Rogers Corp. 5880 DUROID dielectric microstrip. These microstrip channels can now pass over each other at right angles using the crossover boards. Wirebonds to the boards consist of three wires to lower the inductive discontinuity of the connections.

Crossover-Board Design
The crossover board consists of two stripline layers that are made up of four 10-mil sheets of ARLON Corp. CLTE dielectric. Two sheets form a stripline layer. The four microstrip launches around the board connect the straight through path and up and over stripline paths. The up and over path has two vertical transitions. They are each formed from five via conductors, which are composed of four grounds and a center conductor.

A picture of a crossover board in a test fixture along with a conductive gasket, which covers the board, is shown in Fig. 3. The crossover board's overall dimensions as shown in Fig. 4 are 0.486 x 0.221 in. Some additional machining to widen the channels is required to fit the board in, as the channel widths are only 0.125 in. Physically speaking, this crossover board is the smallest one that could be manufactured at the time by the board manufacturer. The total nominal thickness adds up to 50 mils or 0.05 in. when metallization thickness is included.

The design of the crossover board consists of four "U"-shaped microstrip-to-stripline transitions. The microstrip tapers down to the width of the stripline. These transitions also are known as microstrip launches. They are critically toleranced to assure good VSWR at all microstrip connections to the board. To maintain good VSWR match, the tapered part of the microstrip line must always remain outside the stripline dielectric region. The end of the taper must therefore be 6 ± 5 mils from the stripline section of the crossover board. The minimum distance of the taper will then be 1 mil from the stripline dielectric region. The machining for this must be done with high precision in order to ensure good return loss performance for all the microstrip launches.

The vias are plated throughholes connecting all of the internal and external groundplanes except for the layer-transition vias. They have been machined part way down to disconnect them from the top and bottom groundplanes as shown by the 0.060-in. counter bores in Fig. 4. At first, a connection to these groundplanes was required to allow the via hole to be electroplated along with all of the ground vias.

A 3-mil-thick sheet of 5025E silver epoxy is used to attach the bottom of the board to the floor housing or test fixture. That sheet also is used to attach the DUROID dielectric microstrip lines leading up to it. The top of the board is covered with a 40-mil conductive gasket, which is compressed down by the housing cover to the continuous groundplane connection, from the cover, and through the board via to the housing floor. Such an approach was needed to account for the tolerances in the crossoverboard material thickness and the machined pieces.

The gasket material is a Parker-Hannifin Corp. CHOMERICS CHO-SEAL 1285 conductive rubber gasket. It is made up of silver-plated aluminum particles in silicone. It can be compressed by up to 15 percent of its thickness, although 10 percent is the recommended amount. The purpose of this material arrangement is to create a high-isolation Faraday box around the stripline paths in the crossover board. This configuration resulted in greater than 70-dB isolation performance.

Crossover-Board Measurements
The crossover board was placed in a test fixture for S-parameter measurement evaluation. The measured S-parameters were then compared to the Ansoft HFSS crossover-board, two-port data model within an overall Agilent Corp. ADS software simulation of the fixture and board. The fixture consists of SMA connectors and 10-mil DUROID dielectric microstrip. Figure 5 contains plots of the measured and modeled insertion losses along with the return losses of the critical "up and over" vertical transition path. The measured insertion loss is less than 1 dB up to 18 GHz. The return loss is greater than 15 dB. The modeled ADS simulator plot shows good agreement with measured return loss but less insertion loss. The reason is that the SMA-connector losses were not fully accounted for in the ADS simulation.

Figure 6 is a plot of the isolation between the two perpendicular RF paths in the crossover board. Greater than 70 dB isolation at frequencies up to 18 GHz was measured in this fixture. If the gasket was removed, the isolation drops more than 30 dB. The microwave energy will then propagate over the top of the crossover board between all four ports.

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Crossover-Board HFSS Model
A complete Ansoft Corp. HFSS EM Simulator model has been made of the crossover board shown in Fig. 7. The model is a three-dimensional finite-element analysis including all material electrical properties. The frequency range of the analysis is from 1 to 20 GHz at 500-MHz increments. The HFSS model shows all structures within the board. They also can be seen in the 3D projection drawing.

The four microstrip launches have 24.6-mil-wide lines, which taper down at 45 deg. to 12.2-mil-wide stripline. The taper is placed back from the stripline wall to adjust for the parasitic capacitances from the nearby groundplanes as well as from the taper itself. The exposed stripline section is slightly inductive, which compensates for the parasitic capacitance.

The entire crossover-board model is located within a metal shield, which represents the housing channel. Only the ends are open to allow for the propagation of microwave energy into and out of the board along the up and over path. The straight-through path going underneath was not part of this simulation. The top of the board within this metal shield is covered by a 40-mil-thick conductive gasket. It is modeled in HFSS as a conductor with a resistivity of 0.008 Ohm-Cm.

All vias are simulated by 12-line-segment cylindrical polygons. The curved areas of the launches are represented by the same type of straight-line segments that are found on the vias. Each segment covers about 30 deg. of arc on a circle.

The ground vias are represented by solid-metal cylinders while the layer-transition vias are hollow. The reason is that the ground vias form a continuous shield, which does not allow any field energy to be coupled into them even though they are hollow. The layer-transition vias are open to field energy within the board. As a result, they have to be represented electrically as hollow-metal cylinders. The shorter machined-layer transition vias are 14 mils in diameter. Internally, they are connected to 32-mildiameter, stripline-layer transition pads instead of internal grounds. The four ground vias surrounding each of these layer-transition vias are also 14 mils in diameter. All of the remaining ground vias are 20 mils in diameter. These ground vias internally connect all grounds together.

Applications
Complex broadband-microwave-frequency applications within a limited available space would require crossover boards of either this type or of a larger design, which would be specific to a given application. This particular board is small, however. It can be used as a general-purpose design component.

The crossover-board design works best using 10-mil-thick, DUROID dielectric microstrip line connections with a dielectric constant of 2.1 to 4.5. The DUROID dielectric thickness could vary from 7 to 15 mil. But this would create a step in the groundplane that could degrade the return loss.

Although the recommended connecting channel width is 125 to 150 mils, it could be adapted to a wider channel with some added machining. Yet a wider channel would begin to limit the frequency range of the crossover board because of propagating waveguide modes. The channel also needs to be shielded on top with soft copper to fill in all of the voids between the housing and cover. As a result, microwave energy will not be able to radiate between machined channels and degrade the isolation performance. The board itself requires 40 mils of conductive gasket material plus metal shims to fill in the space from the top of the board to the top of the channel plus 10 percent of the gasket thickness for correct compression. The soft copper also would cover this area of the housing.

Conclusion
A wide range of possibilities now exists for making more complex planar subassemblies utilizing crossover boards at all frequencies up to 20 GHz. The critical high-isolation requirements have all been met using a low-cost stripline package. The low insertion losses and VSWR ripple allow these crossover boards to be added to microwave circuitry with no adverse effect on electrical performance. These boards are being used successfully in multi-function assembly products.

ACKNOWLEDGEMENTS

The authors would like thank to Paula Arinello, Tony Gonsalves, and Mark Bogdhan for test support and Doug Abramson for Thermal Stress Screening.

M/A-Com, Inc., 1011 Pawtucket Blvd., Lowell, MA 01853; Internet: www.macom.com.

Trademark Acknowledgments:
DUROID is a trademark of Rogers Corp.
ADS is a trademark of Agilent Corp.
ANSOFT HFSS is a trademark of Ansoft Corp.
CLTE is a trademark of ARLON Corp.
CHOMERICS CHO-SEAL is a trademark of Parker-Hannifin Corp.