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.