As the sophistication of commercial and military systems grows, RF/microwave designers are being asked to supply more functions in smaller modules.
Integration packs multiple microwave functions into a single housing. Military requirements, and to a less degree commercial systems, are driving increased integration of microwave functions into smaller and smaller footprints, often to accommodate a growing amount of digital hardware in these systems. In general, most companies that manufacture microwave components also offer some level of integration at a standard or custom level. A variety of approaches are being tried to achieve multiple-function microwave integration. Often, these different approaches may be found within the same subassembly or system.
Microwave multifunction integration has been in practice for about four decades, with early efforts usually involving basic microstrip or stripline function blocks, such as mixers, filters, oscillators, and amplifiers, bolted together into an assembly enclosed by a metal housing. At one time referred to as "supercomponents," these multifunction assemblies provided avionics and electronic-warfare (EW) system designers with a means of accounting for many of a system's required front-end functions in just a handful of parts.
Military and commercial systems have come a long way from those early days of supercomponents, however. Microwave designers are often asked to now integrate not only RF and baseband functions, but digital and memory circuitry as well. Long-time component suppliers such as M/A-COM (www.macom.com), which once relied solely on connecting microstrip, stripline, or even waveguide components within a common assembly, now employ a combination of discrete and monolithic or integrated-circuit (IC) technologies to fabricate miniature multifunction modules. The firm's model SA900001, for example, is a vector modulator with frequency range of 1.94 to 2.24 GHz designed for commercial wireless infrastructure applications. Measuring just 0.236 X 0.157 in., it features the flat amplitude (0.2 dB) and linear phase (0.5 deg.) responses that are benefits of the tight module packaging. The company also offers the MAMUSM0008 digital switched delay line for applications from 1.8 to 2.4 GHz. Suitable for cancellation loops in feed-forward amplifiers for commercial wireless systems, it controls a total delay range of 750 ps in 50-ps steps and handles input power levels to 1 W (+30 dBm).
High-frequency semiconductor integration made significant progress during the 1980s as a result of the DARPA-funded MIMIC program. That support produced such achievements as a complete receiver front end from Honeywell. Of course, the motivating force of modern commercial wireless-communications markets has spurred on the development of highly integrated radio integrated circuits. To meet the needs of multiple air-interface standards, manufacturers such as RF Micro Devices often assemble multiple ICs into a module assembly.
The Polaris 2 "Total Radio" module solution from RF Micro Devices, Inc. (www.rfmd.com), for example, is a complete radio consisting of transmitter and transceiver modules with as many as four wireless-communications frequency bands under the GSM, GPRS, and EDGE interface standards. These third-generation modules offer the advantages of requiring few external components and printed-circuitboard (PCB) space of less than 300 mm2.
The RF6026 quadband transceiver module includes a digital-conversion architecture and digital channel filtering. The RF3178 quad-band transmitter module integrates power-amplifier circuitry and fractional-N digital modulators to implement the advanced digital modulation formats required by these wireless standards. And Avago Technologies (www.avagotech.com) recently made its ACPM-7886 TDSCDMA-based power-amplifier modules available to the Chinese market. The modules are small enough to support the design of thin 3G cellular handsets with long battery life.
In some cases, integrated modules may provide not only electrical operation, but optical functions as well. In addition to supplying lines of broadband RF/microwave amplifier modules, iTerra Communications (www.iterrac.com) offers a wide range of electro-optical modules for broadband communications systems. The company's integrated encoder/optical-modulator drivers, for example, combine a LiNbO3 modulator driver with an NRZ-to-RZ encoder (iT6134) or NRZ-to-duobinary encoder (iT6144). These compact modules (Fig. 1) are supplied in the same compact housings to simplify system integration. The iT6134 handles data rates up to 11.1 Gb/s and the iT6144 up to 10 Gb/s. The iT6134 maintains peak-to-peak jitter of less than 10 ps and accepts input signals of 250 to 500 mV peak to peak (pp). It provides adjustment of RZ output voltage to 6 V pp. The iT6144's duobinary output level is adjustable from 6 to 10.5 V pp. It provides margin for loop control to track variations caused by temperature and aging. Single-ended clock and data inputs eliminate the need for phase-matched traces. The iT6144 supports applications employing an external Bessel filter and pre-filtered modulators. (For more on iTerra Communications, don't miss the Crosstalk interview this month with CEO Peter Walters.)
Modules and integration are of particular interest to DARPA and other military electronics designers because of the flexibility they allow at the system level. While the goal of many military systems integrators is an antenna connected to an all-digital receiver, the current speeds of analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) has not allowed the full elimination of RF/microwave circuitry for transmission and reception of signals. But progress on the ADC side is being made in the form of advanced multifunction modules by a number of suppliers, including Analog Devices (www.analog.com). For example, the company's model AD12401 ADC is a 12b, 400-MSamples/s device that is suitable for radar and satellite subsystems, phase-array systems, secure communications, and wideband commercial communications. It is supplied in a module measuring 2.9 X 2.6 X 0.6 in. (Fig. 2) and boasts a signal-to-noise ratio (SNR) of 63 dB full scale (FS) at 128 MHz. Its spurious-free dynamic range (SFDR) is 70 dB FS at 128 MHz. The module includes a transformer-coupled input path and digital post-processing circuitry.
One of the technologies that DARPA and other defense-minded researchers have investigated as a basis for higher-level module integration is low-temperature-cofired-ceramic (LTCC) circuit manufacturing technology. LTCC employs multiple layers of ceramic substrates with metallized circuit traces and ground planes. Metal pastes are screen printed on the different layers of unfired ceramic tape or foil used to create a module structure. The layers are stacked and fired or sintered in the final manufacturing step to form a compact structure with embedded components. For RF/microwave applications, LTCC features low line losses and more compact footprints than possible with conventional multifunction modules formed of stripline, microstrip, and coplanar-waveguide technologies. Unfortunately, traditional LTCC processes have exhibited some amount of shrinkage in the sintering process, requiring careful modeling of the full process to achieve usable yields.
A variety of high-frequency component manufacturers have adopted LTCC as an effective means of realizing miniature single-function components. MiniCircuits (www.minicircuits.com), for example, employs the process to manufacture lines of RF and microwave frequency mixers with excellent conversion loss and isolation performance. Other companies, such as CMAC MicroTechnology (www.cmac.com), Anaren (www.anaren.com), and Johanson Technology (www.johansontechnology.com), offer single-function components based on LTCC. Johanson, for example, supplies lines of chip-sized antennas for 1500-to-1600-MHz Global Positioning System (GPS) and 2.45-GHz Bluetooth applications.
The list of companies working in LTCC technology is a long one, and includes: American Technical Ceramics (www.atceramics.com); CETEK Technologies (www.cetektechnologies.com); DuPont (www.dupont.com); IMST GmbH (www.ltcc.de); Kyocera America (www.americas.kyocera.com); LTCC Lab (www.ltcclab.com); Minicaps (www.minicaps.com); Plextek (www.plextek.com); Sea Ceramics Technology (www.seaceramics.com); Selmic Micromodules (www.selmic.com); and TDK Corp. of America (www.tdkltcc.com). American Technical Ceramics, for example, offers a free LTCC Catalog on its website. IMST, in a joint project with materials supplier DuPont, has developed a 24-GHz automotive radar solution based on LTCC. The frequency-modulated-CW (FMCW) radar can measure distances to 30 m from the car and the velocity of obstacles around the car. It can measure objects with separation of only 10 cm and with resolution of better than 2 cm.
In contrast to LTCC, Labtech (www.labtechcircuits.com) designs its modules on thin dielectric material (TDM) using organic substrates on metal carries with a bonded cover. The firm offers amplifier gain modules from 2 to 18 GHz based on GaAs PHEMT technology. Temperature compensation and detection circuitry can be included on the modules. The firm also offers design capabilities for modules with switches, attenuators, and voltage-controlled oscillators (VCOs). Centellax (www.centellax.com) is another supplier of high-gain amplifier modules, with units operating as wide as 200 kHz to 30 GHz and 30 dB gain. The modules include DC blocks and coaxial connectors. Aeroflex (www.aeroflex.co) supplies multichip modules (MCMs), frequency converters, time-delay units (TDUs), and beamforming networks in modular form. The firm draws from a large library of modular circuit designs to provide fast turnaround on custom assemblies in modular form.
Spinnaker Microwave (www.spinnakermicrowave.com) offers a wide range of microwave modules based on high-frequency oscillators and synthesizers. The firm's designs can include VCOs, multiloop indirect synthesizers, and direct-digital synthesizers (DDSs) with frequency converters, microstrip filters, tuned amplifiers, micropower heaters, modulation ports, analog and digital conversion functions, and complex digital control circuitry.
The Multi-Mix® process developed by Merrimac Industries (www.merrimacind.com) is another organic-substrate-based module alternative to LTCC. The multilayer technology, which can support frequencies beyond 70 GHz and average power levels to 500 W, allows active semiconductor (diodes and transistors) die and passive circuit elements to be embedded within circuit layers, forming compact subassemblies. Originally applied to the manufacture of single-function components, such as hybrid couplers, the technology stacks layers of organic substrate material, such as polytetrafluoroethylene (PTFE) from Rogers Corp. (www.rogerscorporation.com), and fusion bonds the layers (without the need of lossy bonding films employed in other multilayer organic-substrate approaches) under carefully controlled temperature and pressure to form a compact module with weld-like strength between layers.
The fusion-bonded layers are edge plated to ensure good grounding throughout and provide high shielding effectiveness for electromagnetic interference (EMI). In addition to forming circuits using metal conductors, such as copper, passive and active components can be added to the Multi-Mix layers to realize any number of RF/microwave circuit functions, such as filters, power combiners, and amplifiers, as part of an integrated assembly. The firm has already applied the technology to such complex assemblies as beamforming networks and instantaneous-frequency-measurement (IFM) receivers and has developed a quick-turnaround platform for the design of semi-custom power-amplifier modules.