Mirror Symmetry Makes High-Performance Hybrids

This unique circuit-design approach can achieve high levels of performance for a wide range of components even with limited manufacturing tolerances and misalignments.

Dimensional tolerances are critical to both electrical performance and manufacturing yield of high-frequency circuits. Since every deviation in a printed-circuit-board (PCB) trace represents a significant portion of a wavelength and RF and microwave frequencies, circuit patterns must be tightly controlled with minimal irregularities to achieve consistent electrical performance. This can be done with the highest-caliber PCB photolithographic techniques and well-disciplined process control. Or it can be accomplished using a patent-pending circuit design and fabrication approach developed my B & H Electronics Corp. (Monroe, NY). This novel approach compensates for photolithography registration errors and manufacturing misalignments to provide high-performance RF and microwave circuits while improving the yield of even marginal manufacturing processes.

The approach involves imposing mirror symmetry about the circuit traces (transmission lines) that make up an RF or microwave component. The company has applied the technique to a new line of hybrid couplers (Fig. 1), which B & H Electronics Corp. sells through its passive components company, Hybrid Circuits (www.hybridcircuits.us), but the approach can also be applied to filters and other RF and microwave components. The patent is the brainchild of B & H Electronics founders Harvey Horowitz and brother Bernard Horowitz. As Harvey Horowitz relates, the approach has been patented as it applies to the company's hybrid couplers, but it is not limited to hybrids: "There are a set of geometric shapes that permit you to make microwave structures. By invoking mirror symmetry for structures with small misalignments, this approach cancels the effects of those misalignments on the RF performance."

The misalignments stem from normal RF and microwave PCB manufacturing process steps followed by any commercial board fabrication facility. According to Harvey Horowitz, "There is a whole class of shapes that can be used for this purpose" to improve circuit performance and even yield when working with a commercial PCB fabrication facility. The circuit design and manufacturing approach was developed with the aid of high-frequency simulation tools, including the three-dimensional (3D) electromagnetic (EM) simulator, High-Frequency Structure Simulator (HFSS) from Ansoft Corp. (www.ansoft.com).

The simulations showed that when applying the capabilities of a conventional PCB fabrication house, where tolerances or misalignments might typically be within a few mils, it was possible to achieve a 27:1 reduction in the effects of these misalignments on RF performance and on manufacturing yield. For example, for a component fabricated on a PCB with a 27-mil circuit-trace misalignment, the effects of this drastic a misalignment using the B & H Electronics technique would yield the same performance as another circuit fabricated without their approach and with circuit misalignment held within only 1 mil. Obviously, the cost and difficulty in achieving tolerances within 1 mil make the B & H mirror-symmetry approach a practical alternative worth considering for RF and microwave circuit designers working with the majority of PCB fabrication houses.

When it was time to translate the simulations to an actual fabrication project, the results were better than expected. As Harvey recalls, "The first time we applied this approach, we ran about 1800 hybrids at a PCB fabrication house in a batch process for a customer. We got a 99.9-percent yield. The pieces that died were the pieces that were fabricated on the sections of the PCB used to make connections to run the current for the plated throug-holes on the hybrids." The fabricated hybrids provided consistent, repeatable performance that was well within the customer's requirement performance limits. In short, the use of mirror symmetry in the design and fabrication of this customer's hybrids resulted in zero spoilage due to misalignments.

Harvey notes that the high yield required some further exploration into materials properties: "Before adopting this approach, we would sometimes get an entire board with no usable parts." The components were found to fail for various reasons, and B & H Electronics' engineers found themselves with a shrinking list of PCB fabrication houses willing to take on the tight-tolerance demands of their circuits. As Harvey notes, "Lots of microwave PCB houses were refusing to take some of our jobs. Even those that did, we sometimes found later that parts would fail. We had some boards fabricated for a military customer that initially met the requirements but would show poor yields when laying around for a few months." The boards were found to be hydroscopic and were absorbing moisture from the environment and failing after a period in storage.

The solution, said Harvey, was to develop a new substrate material. Harvey explains: "We formed our own substrates, a proprietary material that is essentially impervious to CO2 and H2O contamination. If you put these boards in salt water and then wash them down, they will not absorb any moisture in any amount that will degrade electrical performance."

Current hybrid products include the standard models HC1350 and HC1850, and a custom model HC2750 that can be designed to a customer's center frequency and bandwidth. The company shows measured data for the standard models on its www.hybridcircuits.us website. The test results are actual nonde-embedded S-parameters including the losses from the text fixture and its connectors, using the company's Test Jig model T1000. The test jig has four SMA connectors and 0.2 inches of microstrip on each port. The test fixture is based on 0.032-in.-thick of RO4000 PCB material from Rogers Corp.

The standard models, the HC1350 and HC1850, feature 3-dB coupling, although the firm also offers hybrids with coupling values of 10 dB, 20 dB, and 4.77 dB for making three-way combiners and dividers. In addition to the standard units with frequency response through 2.4 GHz, the company is working on a hybrid that will be well behaved from 2.7 to 30.0 GHz for higher-frequency applications.

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Measurements on the hybrids are performed on four-port HP8510 vector network analyzers from Agilent Technologies (www.agilent.com). As Harvey explains, the performance and repeatability of the new hybrids pushes the high-performance test equipment to its limits: "We were getting good long-term stability and we were getting repeatability that was well within the 0.01-dB measurement accuracy of the analyzers, which can be calibrated to NIST traceability."

The hybrids are rated for input power levels to 30 W, but have been tested at levels to 100 W average power across the full operating temperature range of

–55 to +125°C. According to Harvey, these new hybrids are based on a substrate material that can take the heat: "The material we developed for the hybrids can easily withstand +2001/2C, since it is processed at temperatures to +400°C." As Harvey reminds customers, however, the hybrids are designed with an excellent thermal path, but must be properly heat-sinked when testing at high power levels.

How well does this new circuit design and manufacturing approach deliver? The results can be seen in the performance of the models HC1350 and HC1850. Model HC1350 exhibits less than 0.3 dB insertion loss from 800 to 1800 MHz (Fig. 2) with typical insertion-loss performance of 0.15 dB. It is rated for 30 W average input power across the full temperature range of –55 to +125°C. Input return loss is better than 20 dB at 1800 MHz and better than 30 dB at 800 MHz (Fig. 3). Isolation between ports is better than 30 dB from 800 to 1200 MHz and better than 22 dB from 1200 to 1800 MHz.

Model HC1850 exhibits less than 0.26 dB insertion loss from 1200 to 2400 MHz (Fig. 4) with typical insertion-loss performance of 0.15 dB. Like the HC1350, this higher-frequency hybrid is rated for 30 W average input power across the full temperature range of –55 to +125°C, but it can also handle higher power levels without damage or electrical performance degradation. The input return loss is better than 20 dB from 1.8 to 2.4 GHz and better than 30 dB from 1.2 to 1.8 GHz (Fig. 5). The isolation between ports is a worst-case 25 dB at 2.4 GHz and better than 30 dB at 1.8 GHz and below (Fig. 6).

In addition to the hybrids, the company has applied to new approach to analog filters with response shapes resembling the best digital filters. In one case, as Harvey relates, a filter was needed for a wide-dynamic-range receiver being developed at B & H for a demanding government customer: "We needed to achieve a dynamic range of more than 80 dB, and we were getting more than 100 dB in production with filters developed with this new approach." Because the receiver was designed to extract low-level signals close to the system noise level, front-end filters were required to suppress spurious signals by 80 dB or more with sharp skirts and low signal loss in the passband. The combination of performance parameters made for an extremely difficult filter design for the high-performance receiver.

"We couldn't get anyone to agree to make these filters for us," Harvey recalls, "even if it involved cascading two filters to do the job." The solution, he explained, was in using the mirror-symmetry circuit design and fabrication approach, which is really an error-correcting method: "We ended up making the filters with the same kind of algorithms were used in designing the hybrids in order to suppress the registration errors. We were able to produce filters with consistent unit-to-unit VSWR and with unit-to-unit ripple that is within a few tenths of a dB." The final filter was developed for a compact VME receiver. The 11-pole filter suffered only 1.0 to 1.2 dB passband insertion loss with better than 78 dB rejection in a surface-mount structure measuring 1 in. long, less than 0.5 in. wide, and about 110 mils high. In addition to this receiver front-end filter, the firm has also fabricated a filter with 391-MHz center frequency and 75-MHz-wide 1-dB bandwidth that provided 90-dB rejection just 12 MHz away from the passband.

The company is perhaps best known for its wideband amplifiers, often specified for high-speed optical communications applications. But with this new approach for hybrids, filters, and other RF and microwave components, customers should soon recognize B & H Electronics as a diversified supplier of practical high-performance components and subsystems. The new hybrids, which are available in gold-plated and tin-plated versions, provide true 90 deg. signal splits that are within ±0.65 deg. In addition to the 3-dB coupling factor for standard model HC1350 hybrids, 10-dB, 20-dB, and other coupling factors are available. In addition, applications assistance is available for help with a specific or difficult application. Available as a bulk order or shipped in tape-and-reel format for use with automated assembly equipment.

B & H Electronics Corp., 308 Museum Village Rd., Monroe, NY 10950; (845) 782-5000, FAX: (845) 782-0470, e-mail: [email protected], Internet: www.bhelectronicscorp.com

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