Multifunction Subsystems Employ Advanced MIC Techniques
By Narda Microwave-East

The Microwave Integrated Circuit (MIC), once the principal microwave fabrication technique employed by designers everywhere, is sometimes overshadowed by its more modern monolithic (MMIC) counterparts. However, increasing functional complexity, higher degrees of specialization, aggressive deployment schedules with limited production volume and lower NRE costs are examples where implementation as an MIC is the best (or only) choice. Although many MICs are still fabricated the same way they were 20 years ago, much more can be achieved through design innovation and exploitation of new material technology. In its latest MIC products, Narda Microwave-East has redefined what can be achieved by the MIC, the result of which is a growing family of multifunction devices that are smaller, lighter, less expensive, more reliable, and better performing than “traditional” MICs. They also lend themselves far better to customization, which significantly reduces development time and cost.

Early MMICs were often compared against the same circuit fabricated as an MIC, which illustrated the obvious benefits of chip-scale design, and today these advantages are even greater. So why haven’t MICs simply vanished? The answer is that the MIC is still a viable approach because in many cases it provides the most cost-effective way to achieve a high level of functional integration and performance, especially at millimeter-wave frequencies. In addition, while MMICs are orders of magnitude smaller than MICs, they are orders of magnitude more costly to produce in small quantities and redesigns are prohibitively expensive. They are also ill suited to highly customized subsystems whose production volume is not measured in the tens of thousands or more.

That said, traditional MICs are nevertheless large, heavy, and encumbered by the mechanical attachment methods employed to fix carriers and circuit boards within the housing. For example, the common method of screw-mounting carriers into the housing increases size and weight and introduces mechanical considerations that complicate fabrication. This also inherently creates discontinuities that reduce performance and require significant effort to reduce or eliminate.

To mitigate the problems associated with traditional MICs, Narda has adopted fundamentally different approaches that are employed either singly or together depending on the module’s intended functions. These MIC assemblies can deliver high levels of analog, digital, and microwave functional integration, use FPGAs for signal processing, control and temperature compensation, and employ high frequency PTFE based multilayer circuit boards with via interconnects that solve many of the problems posed by standard MIC design practices. One of the company’s most significant achievements is the Model 10512 signal source (see “A dual-channel, DSP-based FM arbitrary waveform generator”, p 82.) The DSP-based module is based on dual highly-linearized voltage-controlled oscillators (VCOs). While many of Narda’s MIC based assemblies utilize unpackaged die and are hermetically sealed for military and/or high reliability commercial applications, the company also extensively uses surface-mount technology for fabrication of modules that operate up to about 26 GHz. They offer a very cost-effective solution, especially for higher-volume applications.

Reinventing the MIC
MICs typically employ copper-molybdenum (copper-moly) carriers that have a linear coefficient of expansion similar to that of GaAs as well as high thermal conductivity. This ensures that the device-to-carrier interface will remain stable over wide temperature extremes while insuring lowest device junction temperatures. Traditional MICs can suffer size, weight, and performance penalties if careful consideration is not given to accommodating these carriers and the screws used to attach them. The housing-to-carrier or carrier-to-carrier discontinuities also limit electrical performance, especially as operating frequencies increase. One of the techniques employed by Narda to circumvent these shortcomings is to eliminate mounting screws and carrier-to-housing interfaces entirely by attaching carriers into housings using flexible, electrically-and thermally-conductive silver epoxy and PTFE-based, 50-ohm transmission line boards to bridge the housing-to-carrier gap.

For many of its millimeter-wave MIC applications, Narda eliminates carriers by employing specialized housing materials that have thermal conductivity similar to copper-moly and a linear coefficient of expansion very near that of GaAs. The more efficient space utilization afforded by this technique allows more functions to be incorporated in a given footprint and effectively removes the possibility of discontinuities, which are the bane of traditional MIC designs. In applications where its modules are intended for high-volume applications, the company places both the RF and control circuits on a single, double-sided-circuit board/plate assembly, which further reduces size and weight. Taken together, the techniques developed by Narda allow the company to address applications well into the millimeter-wave region.

Digitally Controlled Attenuators (DCAs)
One of Narda’s most recent products is the DCA Series of digitally controlled attenuators (Figure 1) that combines the company’s new MIC fabrication techniques with GaAs SP2T pHEMT devices to deliver a combination of fast switching speed, full monotonicity, and low power consumption that was previously unavailable in a FET-based, switched-bit DCA. Conventional FET switches offer fast switching time from 20% to 80% of final value, but can take tens to hundreds of microseconds to settle to 95% of their final value. On the other hand, conventional “fast-settling” PIN diode switches tend to be power hungry.

After evaluating a large number of devices from several vendors, Narda identified a GaAs SP2T pHEMT device that offers fast settling time and low power consumption. Operating over a temperature range of –55º C to +95º C, the attenuators are available in 2-bit, 3-bit, and 6-bit models in frequency ranges of DC to 6 GHz or DC to 18 GHz. Each model provides 1 dB resolution with +/-0.25 dB accuracy and a switching time of less than 90 ns between any two settings. Unlike conventional “fast settling” PIN diode 6-bit DCAs that typically consume 3 W of DC power, the DCA Series attenuators consume only 50 mW from a +/- 5V DC power supply.

A typical MIC attenuator of this type would be constructed using a housing in which cavities are machined into each side, creating a “floor” that separates RF and control functions. The RF portion of the module would be located on one side and the DC power, switching, and control portion on the other side. In the Narda DCA Series, the housing is machined all the way though and the entire circuit is fabricated as a multi-layer circuit board with a metal plate (core) separating RF components on one side and the control, switching, and power components on the other. Non-contacting “through-plate” vias are used to make the connections between the two, which eliminates the need for wire bonds to facilitate feedthroughs.

Ka-band 10-W “Manpack” Transmitter
The Model 10513 is a compact module that forms the transmitter section of a direct conversion manpack transceiver operating at Ka band (Figure 2). It combines a proprietary even-harmonic I/Q vector modulator with a GaAs MMIC implementation of a 50-dB variable gain block, a 1-W driver, and a 10-W power amplifier, and operates from 28 VDC battery power.

The Model 10513 employs direct conversion in which I and Q baseband signals are directly upconverted to the desired output frequency within the 30 to 31 GHz band without the need for an intermediate frequency conversion step. This greatly simplifies the design and reduces the size and cost of the module. Using even-harmonic mixers that provide superior carrier suppression in the modulator further enhances performance. Narda employs a patented technique for increasing the input compression properties of these even-harmonic mixers that eliminates problems associated with other approaches such as increased conversion loss and reduced bandwidth. To mitigate modulator-induced imbalance over frequency and temperature, I and Q channel amplitude, phase, and DC offset data undergo digital pre-distortion before being applied to the module. This data is collected during factory alignment using an ATE-based characterization routine that also uploads the data into on-board memory. The transmitter delivers a minimum of 10 W from 30 to 31 GHz, has built-in output level control with a monitoring coupler, and utilizes digital-compensation techniques to maintain essentially constant output power over temperature.

Other examples of Narda’s MIC products include:

• Ka-band block upconverter/5-W power amplifier: The Model 10514 is a compact unit that upconverts a 1-to-2 GHz L-band input signal to 30 to 31 GHz and produces minimum RF output power of 5 W. Output level control and monitoring coupler are integrated into the unit and digital temperature compensation is provided from -30º C to +80º C.

• 40 to 43 GHz amplifier with variable phase and gain: Designed for use as a clock amplifier for OC-768 40-Gb/s lightwave communication systems, the Model 10205 delivers 8 Vpp output with a -8 to +4 dBm RF input. It provides phase control of 90 to 180 deg., 10 dB of automatic gain control, and includes RF power detectors and harmonics filters. The unit offers excellent amplitude and phase tracking over temperature and input power.

Modules fabricated using surface-mount technology include:

• 39 to 44 GHz voltage-tuned dielectric resonator clock oscillator: This unit, mechanically tunable from 39 to 44 GHz, can be used as a phase-locked clock source for OC-768 40-Gb/s lightwave communication systems. It is exceptionally stable and provides 51 MHz of linear electronic tuning range. It has very low jitter, typical 44 GHz phase noise of -90 dBc/Hz at a 10 kHz offset, and delivers RF output power of +5 dBm.

• Complex frequency generator: With RF inputs of 4.9 and 6 GHz, the Model 10515 (Figure 3) provides selectable outputs of 4.9, 6.95, 12.8, and 14.5 GHz. It is designed to be used as an LO generator in satellite communication terminals.

• L-band transceiver: The Model 15172 transceiver provides a compact solution for military satcom systems that employ a 70-MHz IF, allowing them to interface with single-band, tri-band, or quad-band block converters. It upconverts or downconverts signals between the 70-MHz IF and the entire band from 950 MHz to 2 GHz, and can set the output frequency at any point within the band with step sizes as small as 20 kHz and as broad as 1 MHz. Phase noise performance exceeds MIL-STD-188-165A. The unit measures 6 x 8 x 1.5 in. and weighs only 4 lb. Operating temperature range is -20º C to +70º C.

• Ka band/L band transceiver: The Model 10516 transceiver provides RF output power of +24 dBm at frequencies from 27.99 to 28.35 GHz and has 37 dBc of conversion gain. The receiver section has a noise figure of 6 dB and gain is adjustable from 5 to 50 dB. It is designed for service in hostile outdoor environments and can maintain phase lock under shock conditions of up to 1-Joule.

While all of the MIC and SMT products discussed in this article were designed initially for specific applications, Narda’s fabrication techniques allow a broad array of variables to be quickly tailored to meet varied customer requirements. More information about Narda’s MIC and SMT subsystems is available at our website.

NARDA Microwave-EAST
www.nardamicrowave.com/east
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