Passive RF/microwave components rarely grab headlines like their active counterparts (they are passive, after all). But without the functions provided by components like attenuators, couplers, dividers, filters, and terminations, there would be no military radar systems or modern commercial wireless systems. And while system-level designers often take the performance of passive components for granted, suppliers of these components continue to improve their products by trimming size, weight, and even insertion loss with frequency. A number of factors contribute to those improvements, including the availability of evolving computer-aided-engineering (CAE) simulation software and higher-quality printed-circuit-board (PCB) materials that make possible better performance levels from proven stripline and microstrip passive circuits.
High-power passive components with minimal loss are still built around waveguide housings, while extremely broadband passive components typically reside in coaxial housings, with SMA, 3.5-mm, or some other form of high-frequency connectors. But in recent years, the number of passive components in smaller packages, such as drop-in or surface-mount-technology (SMT) housings, has grown. Circuit designers can use modern CAE programs to analyze the effects of different transmission-line structures without building them, and they have access to improved, low-loss circuit materials that contribute to higher performance at high frequencies.
1. This cross-guide coupler employs WR75 waveguide for operation from 10 to 15 GHz, with low insertion loss and high directivity. (Photo courtesy of ARRA.)
Many high-frequency component suppliers, such as ARRA and Microwave Communications Laboratories, Inc. (MCLI), offer products in both coaxial and waveguide housings. Waveguide passive components can handle extremely high power levels but operate within the limited frequency range of the waveguide transmission lines and flanges, such as 3.3 to 4.9 GHz for WR229 rigid waveguide and 10 to 15 GHz for WR75 rigid waveguide. While the power-handling capabilities of coaxial connectors is somewhat less than those of waveguide connectors, passive components with coaxial connectors can achieve broad bandwidths. As an example, 90-deg. hybrid model HB-57 from MCLI operates from 1.2 to 12.0 GHz with SMA female connectors. It measures just 3.150 x 1.000 x 0.504 in. but can handle 50 W average power and 2 kW peak power in part because of low insertion loss of 1.5 dB or less.
A passive component that combines coaxial and waveguide connections, model 75-603-A-6-7-30FSF from ARRA, is a cross-guide directional coupler available with both coaxial and waveguide coupled ports (Fig. 1). This particular model, designed with WR75 rigid waveguide (10 to 15 GHz), is available with coupling values from 20 to 60 dB and at least 15-dB directivity.
Most system-level specifiers want components that are smaller and can handle higher power levels, whether for portable commercial electronics or military backpack radios. In passive-component structures based on transmission-line technologies such as stripline, microstrip, and coplanar-waveguide (CPW) approaches, the improved thermal characteristics of circuit-board materials enables higher power levels from smaller circuits. Most of the leading PCB material suppliers now offer thermally enhanced circuit materials that can dissipate higher power levels at higher frequencies, in support of increased power densities for RF/microwave circuits.
At least one of these firms, Rogers Corp., offers free-of-charge guidance on selecting materials for fabricating passive components with increased power density, in the form of a presentation (“Using High Frequency PCB Laminates For Improving Thermal Management Issues”). Available as a free download, this presentation reviews laminate properties critical to handling high power levels and how to achieve proper thermal management in high-frequency passive components. A number of PCB material properties are necessary for handling high power levels, including high thermal conductivity, high glass transition temperature Tg, low dissipation factor, and even a smooth copper surface profile for the conductive layers. As passive components become smaller and are used for higher power levels, these thermally enhanced PCB materials become even more essential for properly dissipating heat from smaller components.
2. There is still much need for traditional LC filters such as this coaxial component, when low passband loss and high out-of-band rejection are needed. It measures just 2.0 x 0.5 x 0.5 in. and is suitable for airborne applications. (Photo courtesy of Trilithic.)
In addition, many passive-component designers have experienced the benefits of low-temperature-cofired-ceramic (LTCC) circuit materials, especially for components where high-power-handling capabilities must be combined with small size. One company that has pushed LTCC across a total range of DC to 44 GHz is Anaren, including in its miniature Xinger® surface-mount passive components. As an example of high power in a small package size, model 1G1304-30 is a 30-dB directional coupler with frequency range from 800 to 1000 MHz for cellular communications. With 0.25 dB maximum insertion loss and 15-dB minimum directivity, it is only 0.56 x 0.35 in. but achieves impressive 150-W CW power-handling capability. (For more on directional couplers, click here.)
To create smaller passive components, some firms have taken advantage of traditionally active-component technologies. For example, the miniature attenuators from Mini-Circuits featured in this issue’s Cover Story provide precision fixed attenuation values and power-handling capabilities of several watts in transistor-sized packages. They are fabricated with a gallium arsenide (GaAs) monolithic-microwave-integrated-circuit (MMIC) semiconductor process.
Yet, they represent but one passive component product line among the many now being offered by companies generally regarded as semiconductor suppliers, such as M/A-COM Technology Solutions, SkyWorks Solutions and TriQuint Semiconductor. And while they may not all apply their semiconductor technologies to their passive product lines, all are competitive in terms of offering extremely small passive components capable of handling high power densities.
For example, model MAFL-010256-CB0AD0 is a 75-Ω bandpass filter from M/A-COM Technology solutions developed for Multimedia over Coax Alliance (MoCA™) applications from 1125 to 1550 MHz. This lead-free, RoHS-compliant component achieves 1.7 dB typical passband insertion loss with 53-dB stopband rejection from 5 to 300 MHz, 44 dB from 300 to 800 MHz, 40 dB from 800 to 1002 MHz, and 35 dB from 2250 to 3000 MHz. Because it is intended for competitive cable-television (CATV) applications, the low-cost filter is supplied in a miniature housing of only 15 x 15 x 4 mm that can easily be integrated into other set-top equipment enclosures.
SkyWorks Solutions is another company associated with semiconductor-based products, and another firm with an extensive line of miniature surface-mount passive components, including fixed RF/microwave attenuators, 90-deg. hybrid couplers, and directional couplers in SOT-6 housings. For many of these passive components, the company applies semiconductor technologies to fabricate miniature circuits with excellent electrical characteristics. As an example, its model DC25-73LF is a monolithic directional coupler with typical insertion loss of only 0.2 dB from 2.30 to 2.60 GHz. It delivers 33-dB typical port-to-port isolation and handles as much as 4 W continuous-wave (CW) input power in an SOT-6 housing measuring just 2.8 x 2.9 x 1.19 mm.
TriQuint Semiconductor is yet another “semiconductor” company with an extensive line of compact passive components, including Lange couplers and Bessel filters. Model TGB4001 is a Lange coupler with only 0.25 dB insertion loss from 18 to 32 GHz; a number of other versions are available for use from 12 to 21 GHz and 27 to 45 GHz. Designed to handle power levels to 1 W, it is about the size of a semiconductor chip, at 1.0 x 3.0 x 0.1 mm. Even smaller, model TGB-2010-10 is a Bessel filter that is 0.49 x 0.49 x 0.10 mm and provides a ±0.5-GHz passband from DC to 10 GHz.
Another semiconductor-driven company perhaps best known for its active circuitry, Analog Devices, made news a few years ago by introducing a pair of wideband passive frequency mixers—models ADL5811 and ADL5812—for communications applications from 700 to 2800 MHz. Although lacking the bandwidth of active mixers, these passive mixers deliver low-noise performance and excellent linearity over their operating range, with a single-sideband (SSB) noise figure of 11 dB and input third-order-intercept point of +24 dBm. The passive mixers are well suited for software-defined radios (SDRs), cellular picocells, and wireless infrastructure equipment.
An advantage of realizing passive components by means of monolithic fabrication processes is the option to integrate functions when necessary, such as a power divider and a pair of filters. In addition, semiconductor processes also offer active devices, making it possible to combine amplifiers with any needed passive components. Of course, this has been done for years with larger components, in the form of microwave integrated circuits (MICs), and many of the companies working with LTCC substrates have realized the benefits of that material’s excellent thermal characteristics for mixed active and passive component assemblies. And numerous long-term suppliers of MIC components also can combine multiple functions into a single assembly. Narda Microwave, for example, has continued to enhance its integrated microwave assembly (IMA) technology over the years to combine active and passive components into smaller and smaller modules.
But while passive components produced with semiconductor processes are small, suppliers of more traditional distributed-element-type passive components are finding ways to reduce the size of their products, using housings often just large enough for mounting coaxial connectors. For instance, Trilithic, a long-time supplier of coaxial attenuators, directional couplers, fixed and tunable filters, and power dividers/combiners, recently introduced a series of inductive-capacitive (LC) bandpass filters for Iridium applications (Fig. 2).
These filters measure just 2.0 x 0.5 x 0.5 in. even with connectors making them ideal for airborne applications. They pass signals from 1616.0 to 1626.5 MHz with 2.6-dB insertion loss or less and achieve at least 15 dB isolation of Global-Positioning-System (GPS) L1 signals and at least 70 dB isolation of GPS L2 signals. They also provide 45 dB isolation of signals from 1710 to 1850 MHz, and more than 55 dB isolation of signals beyond that through 10 GHz. The company also offers PCB-mount versions without the connectors.
Similarly, Planar Monolithics Industries, which designs and manufactures lumped-element filters across a total frequency range of 1 to 6000 MHz, has developed a GPS bandpass filter with 1575.42 MHz center frequency and 140-MHz bandwidth measuring a mere 0.75 x 0.50 x 0.50 in. with female SMA connectors. Passband insertion loss is 1.80 dB or less while suppression of unwanted signals is 32 dB at 1065 MHz and 60 dB or more at 2500 MHz. With an operating temperature range of -40 to +85°C, the miniature filter meets MIL-STD-202F requirements for humidity, shock, vibration, altitude, and temperature cycling.
For many years, filters at RF and lower microwave frequencies have achieved miniaturization through the use of surface-acoustic-wave (SAW) technology, and a good number of firms still offer filters based on that technology in compact housings, including Amplitronix, EPCOS/TDK, Murata, RF Monolithics, Sawtron, TriQuint Semiconductor (which acquired SAW filter company Sawtek), and Vectron International. SAW filters are widely used in commercial, industrial, medical, and military applications, and their excellent filter characteristics make them popular for cellular communications, GPS, Industrial-Scientific-Medical (ISM), and wireless-communications uses.
SAW filters rely on the motion of mechanical waves along the surface of a piezoelectric substrate. They are formed with a pair of interdigital transducers at the input and output to convert an applied voltage to mechanical waves at the input and then back to a voltage at the output. They can achieve reduced size and weight across their frequency range compared to other filter technologies and are very reliable. As an example Vectron’s model TFS 403 SAW bandpass filter has a tight 1.5-MHz passband around a 403.5-MHz center frequency. The passband insertion loss is nominally 4.6 dB, while the rejection of unwanted signals is 32 dB just 9 MHz outside of the passband. The miniature filter is only 5 x 5 x 1.8 mm.
One more circuit approach seeing increased use in higher-frequency passive components is microelectromechanical-systems (MEMS) technology; this is essentially the use of micromachined three-dimensional (3D) mechanical structures in electronic circuits. Radant MEMS and RFMD are just two of the companies currently offering small RF/microwave switches and other passive components based on MEMS technology, with tremendous promise for this technology based on the interest and investments from such organizations as the US Defense Advanced Research Projects Agency (DARPA) and Raytheon. In spite of the obvious interest for military applications, MEMS components have great appeal for designers of commercial cellular handsets and other compact RF/microwave products, and it is a technology that will only spread further through passive component design.