Leveraging Component Savvy Into Assemblies

Often at the request of customers, component suppliers have learned to take the next step in system integration by combining multiple functions into integrated microwave assemblies.

Systems integrators rely on the high-frequency electronics industry for a wide range of components, both active and passive and with or without connectors. But often a time comes when a higher level of integration is needed, for example to fit an electronic assembly into an avionics system or within the tight confines of a 3G/4G cellular base station. In such cases, microwave integrated circuits (MICs) have traditionally provided a solution, packing multiple functions into the housing for a single component. But technologies change, and the industry's approach to MICs has changed, and with it great benefits to the system integrators seeking more functionality in smaller and lighter packages.

Last month, for example, Narda Microwave-East offered a glimpse of how its integrated microwave assemblies (IMAs) could combine to form a compact satellite communications (satcom) transceiver in a single small package. Narda's approach to integration builds upon traditional MIC design approaches, but takes advantage of multilayer printed-circuit-board (PCB) fabrication to produce single multilayer boards that incorporate microwave circuitry on the top side of the board and bias, control, and digital circuitry on the bottom. Connections between layers are made by means of viaholes, also known as plated through holes (PTHs).

Designing and fabricating an IMA is not as trivial as simply interconnecting separate component circuits on a common motherboard. Impedance matching of different component functions at the board level can be complex and depend not only on the choice of conductor material and thickness but also on the dielectric substrate material and thickness, dielectric constant, and consistency of the dielectric constant across the length and width of the PCB. In addition, many IMAs, such as receiver or transmitter subsystems, may include both signal sources and signal-processing components, such as mixers and filters. When placed in close proximity, isolation between circuit functions plays a key role in achieving overall target IMA performance levels, such as spurious-level performance. For example, by mounting multiple oscillator or synthesizer circuits, selected by switches, within a common housing, poor isolation between the circuits can invite unwanted modulation of one source by another, and a net increase in output spurious-signal levels.

Also, many IMAs mix both RF/microwave components and digital hardware, such as microprocessors, analog- to-digital converters (ADCs), and digital-to-analog converters (DACs). In some cases, such as in direct-digital synthesizers (DDSs), the DAC may be an integral part of an RF/microwave component function. The noise from digital components must be filtered and isolated from sensitive RF/microwave components to minimize the assembly's noise floor. The solution for many designs, as applied in Narda's IMAs, is to isolate the RF/microwave and digital components by mounting them on different sides of a multilayer PCB.

Although not all manufacturers of components offer IMAs, the number of IMA suppliers has grown steadily over the years and generally includes many of the companies associated with high-reliability or military-grade RF/ microwave components. For example, ET Industries has built a strong reputation for the quality and performance of its lines of power combiners/ dividers, but has also responded to the special needs of customers by creating assemblies that house multiple components, such as switched filter banks, active beam-forming networks (Fig. 1), and digital phase shifters. As an example of the digital phase shifters, model DPS- 03-8 serves intermediate-frequency (IF) applications with a 10-MHz bandwidth centered at 30 MHz. It controls phase shifts from 0 to 360 deg. in 255 steps with phase accuracy of 3 deg. at the center frequency. Insertion loss is held to 3 dB maximum and control is by means of 8-b negative TTL signals. For higher frequencies, the firm also offers its model DPS-28-4 digital phase shifter, with 50-ns switching speed and a phaseshift range specified by the customer, for applications from 2 to 8 GHz.

Although on the outside a multifunction assembly like a beam-forming network can look like a conventional coaxial component, a look inside an open assembly (Fig. 2) reveals the intricate detail needed to switch phase states quickly. This beam-forming assembly, developed by Herley Industries for a phased-array-radar application, includes precision-formed semi-rigid cables for the interface between the 50-O connectors and the PCBs. This particular group at Herley Industries (the former General Microwave) also produces frequency upconverters/ downconverters, source assemblies, and amplitude control modules for electronic-warfare (EW) applications. Amplitude control modules have been developed to cover 0.5 to 40.0 GHz in a total of three assemblies while providing amplitude resolution of 0.1 dB over a 100-dB dynamic range. The assemblies hold harmonics to just -60 dBc while handling 25-ns pulses carrying 90-dB modulation levels.

The integration of multiple component functions within a common housing not only offers the possibility of saving space within a larger system, but can offer benefits in terms of performance, such as control and switching speed. In the model SWF008 four-channel switched filter assembly from TRAK Microwave Corp., four banks of S-to-C-band filters (each with a bandpass and a lowpass filter) are selected by means of single-pole, four-throw (SP4T) switches at input and output ports. The size advantage is that all of these components can be packed within a housing measuring just 2.75 x 1.78 x 0.315 in., at one time the size required for a single filter in that frequency range. The performance advantage is that filter bands can be selected by TTL control in 100 ns or less, while rejecting unwanted signals by 60 dB or more.

MITEQ's expertise in designing small, high-performance components has enabled the firm to also develop extensive lines of IMAs, in the process serving the company well in such applications as avionics and space where size and weight must be minimized. The company's model 127091 miniature downconverter, for example, is ideal for single- and multichannel radar systems. Although housed in a package measuring just 1.870 x 1.510 x 0.131 in. with solder-pin connections and weighing just 30 g, the little subsystem includes five limiters, four multithrow switches, four mixers, multiple lowpass filters, highpass filters, and a pair of quadrature hybrids. It is specified for an input frequency range of 6 to 18 GHz and provides an output range of DC to 600 MHz when fed with a local oscillator (LO) source from 5.4 to 18.6 GHz.

AKON offers a variety of standard and custom IMAs from 0.5 to 20.0 GHz, including its model A20-MH166 five-band amplifier and filter preselector combination that cuts the 1-to-18-GHz range into five subbands of 1 to 2 GHz, 2 to 6 GHz, 6 to 10 GHz, 10 to 14 GHz, and 14 to 18 GHz. The module provides two input ports, for the RF signal of interest and for a test signal. The assembly includes a DC-coupled successive-detection logarithmic amplifier (SDLA) with tangential signal sensitivity (TSS) of -75 dBm that provides detected video outputs as well as a limited RF output signal. This little module is ideal for EW radar warning receivers (RWRs) and electronic-intelligence (ELINT) systems designed as is, but can also be modified to a customer's specific requirements. Companies such as AML Communications have built upon their primary skills in designing and producing microwave amplifiers, and have integrated amplifiers into assemblies with other components, such as switches and filters, to offer subsystem modules largely for military applications. The product lines, which range from 100 kHz to 40 GHz, include receiver front ends, EW multiple-amplifier units, and multichannel amplifiers with limiters.

Although many integrated highfrequency assemblies target military applications, commercial systems integrators can benefit as well from higher levels of integration. Well known for their millimeter-wave products, Endwave Corp. has developed point-to-point millimeterwave transceiver assemblies at 38 and 60 GHz for commercial use, notably for backhaul connections between base stations and switching stations as part of cellular communications networks. For example, a 38-GHz transceiver developed for PDH point-to-point applications measures just 4.6 x 4.3 x 0.79 in. but provides all the transmit and receive functions needed for a quadrature-phase-shift-keying (QPSK) digital point-to-point radio, including an on-board VCO. The firm also offers the transceiver module at other pointto- point frequencies, including 18, 23, and 26 GHz.

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