Multi-Company Teamwork Fashions Compact Filters

Large multi-company organizations can sometimes deflate any attempts at inter-company partnering. In contrast, the teamwork developing among three of the Dover Corp's Electronic Technologies companies-Dielectric Laboratories (DLI), K&L Microwave, and Vectron International-is showing that the whole can be much greater than the sum of the parts, especially in the design and development of advanced RF and microwave subassemblies.

Last month, the second installment in this three-part article series highlighted the work of DLI (www.dilabs.com) and Vectron International (www.vectron.com), along with non- Dover partner Mimix Broadband (www.mimixbroadband.com), in the design and development of a 10-GHz narrowband voltage-controlled oscillator (VCO). The VCO, which includes resonators from DLI and active monolithic microwave integrated circuitry (MMIC) from Mimix Broadband, represents a new high-frequency mark for Vectron's engineers, especially noteworthy for a company where the majority of previous products had operated below 1 GHz.

DLI and K&L Microwave began working under the Dover corporate umbrella to meet the needs of customers trying to fit a multitude of RF functions into densely packed systems. For the two companies, the project (see Part 1, Microwaves & RF, September 2007, p. 120) involved the development of miniature switchfilter banks for total coverage spanning UHF through Ku-band frequencies. Although DLI brought considerable expertise to the project in the way of their high-qualityfactor (high-Q) resonators, and had already developed a portfolio of high-performance microwave filters at frequencies from 100 MHz through 67 GHz, K&L Microwave lends long experience in filter design and higher-level subsystem integration, notably in achieving a maximum number of RF functions in the smallest size possible.

The switch-filter-bank assemblies were conceived by DLI's chief scientist David Bates as an assembly with multiple bands spanning approximately 100 MHz to 18 GHz modules. In each module, thin-film microstrip filters are fabricated using proprietary high dielectric- constant (K), high-Q ceramic materials, although with different materials for lower and higher band modules (see Table 1). In order to achieve miniaturization with the lower-frequency filter module, DLI's CG material (with K of about 68) provides considerably size savings compared to filters fabricated with thin-film circuitry on alumina (with K of 9.8). The high dielectric constant of the substrate material also makes it possible to achieve high isolation with closely spaced lines and circuit spacings, while confining the electromagnetic (EM) fields sufficiently to make low module cover heights practical. In fact, the use of such high K substrate material makes it possible to realize a module having a single housing with top and bottom cavities each containing multiple-filter circuits. The CG material exhibits a loss tangent of 0.0010 with temperature stability of better than 30 PPM/C over the full military operating temperature range. One version of the module measures a mere 3.6 X 3.0 X 0.400 in. and incorporates a commercial GaAs MMIC switch from Hittite Microwave, a single- pole, eight-throw (SP8T) nonreflective device capable of operating from DC to 3.5 GHz. The switch and filters are interconnected by means of grounded coplanar-waveguide (CPW) transmission lines fabricated on 8-mil-thick RO4003 substrate material from Rogers Corp. (www.rogerscorporation.com). The module includes TTL/CMOS-compatible control lines. The housing is an aluminum/silicon composite material with good coefficient of thermal expansion (CTE) match to the various printedcircuit- board materials used in the lowband module. Another version depicted in the figure measures 2.5 X 1.75 X 0.150 in.

Because of its higher-frequency filters, the higher band modules can achieve small size using CF material with K of about 23. As with the lower band module, it achieves small size for the filters compared to conventional alumina substrate material with K of 9.8 and incorporates a commercial M/A-COM switch to select filter bands. The switch is a single-pole, sixth-throw (SP6T) PIN diode switch with very good isolation and fast switching speed. The filters and switch are interconnected by grounded CPW fabricated on 10-mil-thick alumina. A thin two-layer PCB (such as Kapton) is bonded to the back surface of the alumina for the control functions, such as the switch drivers. The highband module measures 2.2 X 2.0 X 0.180 in. and is also designed without SMA connectors.

In spite of the small size of the filters, DLI's vice president, Mike Busse, admits "that their customers still want more functionality in a smaller size with continual reductions in weight and size. That was how we initially got started into doing these more complex assemblies."

Part of the solution for switched filter banks comes from the design experience at K&L Microwave. Like DLI, K&L Microwave designs and manufactures switch-filter banks. One key difference, however, is that K&L develops proprietary diode switches and manufactures their filters without the benefit of DLI's temperature-stable ceramic substrate materials. Busse confided that the two companies were certainly aware of each other's capabilities: "We approached K&L with our technology and filters two years ago. We felt that our filters were in a market that they weren't in at the time. K&L was aware of our ceramic technology, but was also well invested in their discrete filter designs and proprietary switches." Busse points to customer's demanding requirements on miniature switch-filter assembly as a catalyst to bringing the two companies together: "The challenges of meeting difficult customer demands on switchfilter modules helped foster true teamwork between the companies for the first time." Busse explains that having the switch expertise of K&L Microwave may contribute to shrinking the assembly further: "In some instances, we may be able to use custom designs from K&L that would enable shrinking the design even further. As Michele Garigiliano, product line manager at K&L Microwave explains: "We have been making switchfilter assemblies for some time and, typically, we are designing PIN diode switches for those assemblies. And not only are we making our own PIN diode switches, but we make are own driver circuits as well. By doing this, we can optimize our switches for transient performance in terms of rise time, fall time, and overall transition time."

Garigiliano feels that teaming with DLI for this product line will lead to other opportunities: "From K&L's perspective, this allows us to broaden our product line. We've been making switched filter banks since the late 1980s, designing and manufacturing our own filters whether they are lumped-element, ceramic, cavity, or interdigital types. With DLI's capabilities, now we can add even more filters to our base, either singly or in combination with what we do." She elaborates that "we can now partner with DLI to bring their miniature, temperature-stable filters to our designs to make them even smaller." Busse adds, "Dover wants to offer the best technology in the industry, and if we have it within our companies, we'll combine our strengths to offer that to the industry."

Busse notes that partnering with companies within the industry is not exclusive to the Dover group: "We are doing some work now with a local software company in the Syracuse area, Sonnet Software (www.sonnetsoftware.com), to develop new software for our ceramic materials. The new software will allow us to be more efficient in the design process." This software (in beta test) will be available to the general public with the next Sonnet release. It will include new anisotropic models based on measurements of DLI's temperature-stable materials as well as components built from those materials. Busse adds that DLI is "also beta testing cluster computer systems assembled by Sonnet for use with their EM simulation software." He explains, "These eight-processor computer systems (available now) break up a problem into six different segments with dual processors and takes the number crunching time down in some cases from eight hours to about 15 minutes."

DLI president, Brian DuPell, perhaps best summarizes the new spirit of cooperation among the Dover companies: "The thinking now is that if K&L runs across a specification that they can't hit, they'll turn it over to us. We'll do the same with K&L, and similarly with Vectron International. But when there is a need to combine capabilities, we will work together. One of the areas where we will be working closely will be on these miniature switch-filter banks."

Busse adds that the teamwork leads to dramatic improvements in technology. "DLI has moved from capacitors to filters to a switched filter bank," he observes. "And the same growth is true for Vectron, coming from working almost exclusively below 2 GHz and now moving to an area where we can build a 40-GHz oscillator at fundamental frequencies," Busse points out. In the end, it is the customer who will benefit from the fruits of the companies' teamwork.