The digitally-controlled attenuator (DCA) is a fundamental element of any system that combines microwave components and embedded digital control in its architecture. It consists of a bank of fixed attenuator pads, each of which can be switched in or out to adaptively select the optimum signal level for a given situation. Typical applications for DCAs include receiver automatic gain control (AGC), sensitivity time control (STC) in radar receivers, transmitter output power control, and waveform generation for communications and electronic warfare (EW) systems. Choosing a DCA can be a daunting process because of conflicting requirements such as frequency range, accuracy, switching speed, power consumption and size.

For example, a DCA that meets a designer’s requirement for fast switching speed may be too large or consume too much power. Narda developed its DCA Series of 63 dB attenuators to reduce or eliminate some of the compromises that users of DCAs typically experience. The attenuators combine fast switching speed, full monotonicity and low power consumption previously unavailable in FET-based or PIN diode-based DCAs. The DCA series includes 2-b, 3-b and 6-b models in frequency ranges of DC to 6 GHz or DC to 18 GHz. They consume only 50 mW from their ±5 VDC power supplies, far less than what is typical of DCAs that employ PIN diode switches.

Why Digital?
DCAs have a number of advantages over their analog counterparts. They lend themselves better to microcontroller-based applications because they are controlled by digital words and do not require an analog-to-digital converter to develop a control voltage. Like all digital components, DCAs are immune to noise on the digital control lines in contrast with an analog control voltage that can experience noise on the control line, affecting attenuation. In addition, their switching speeds are limited only by the speed of the microwave switches and their associated driver circuits, so DCAs are the best choice when fast response time is an important consideration. DCAs also have higher and more consistent output compression points because the individual switching elements are linear (resistive pads are either in or out), in contrast with continuously-variable, FET-based attenuators whose compression points vary significantly with attenuation level.

Switching Speed Versus Power Consumption
Traditional high-speed DCAs utilize switches based on PIN diodes, FETs, or PHEMTs. PIN diodes offer the fastest switching times, but consume the most power. FET- and PHEMT-based switches use much less power and are advertised with very fast settling times (some as fast as 3 ns), but these settling times are specified for 20 to 80 percent. The last 20 percent is reached after tens or even hundreds of microseconds in some devices. In a six-stage DCA application, 12 switches (two per stage) are cascaded. For a change of attenuation from 31 to 32 dB, for instance, all six stages are switched at once.

The DCA series attenuators settle to within 1 dB of final value when each of the 12 switches has reached its 98 percent point. To achieve the best combination of fast switching speed and low power consumption, Narda evaluated a variety of FET and PHEMT switches and identified a family of PHEMT types that achieve 1 dB, 100 ns settling time in a six-stage DCA design. The DCA series attenuators have a resolution of 1 dB, guaranteed monotonicity over their entire attenuation range and typical step accuracy of ±0.25 dB.

Hermetic sealing is offered as an option, which together with an operating temperature range of –55° to +95°C make the devices well suited for aerospace and defense applications. More detailed specifications are shown in Table 1.

Reinventing the MIC
Narda pioneered the development and manufacture of microwave integrated circuits (MIC), delivering its first production quantities over 25 years ago for EW receiver programs. These products used carrier-based technology to integrate a variety of microwave components into multifunctional assemblies that were housed in hermetic machined packages. Many of these designs are still in production. Over the past few years, Narda has rapidly enhanced its ability to produce these products using advanced microwave and digital technology, resulting in its Ultimate Microwave Integrated Circuit technology. It combines microwave hybrid manufacturing with multilayer printed circuit boards that incorporate advanced digital devices. These MICs surpass their predecessors by achieving very dense integration of microwave, analog and digital functions, improving reliability and enabling high-volume production at a much lower cost.

The DCA Series attenuators measure 2.2 x 0.84 x 0.38 in. and weigh 1.1 oz. In addition to the standard 2-b, 3-b and 6-b models that have operating frequencies up to 18 GHz, custom versions can be created that optimize specific performance parameters. Additional components such as filters and other active and passive components can be integrated as well to form a multi-function module that will typically occupy less space than a design fabricated with conventional MIC techniques.

Narda Microwave-East
Hauppauge, NY (631) 231-1700
RS No. 301