As applications ranging from communications to defense place new demands on tight amplitude control, microwave engineers increasingly rely on digital attenuators for precision and linearity.
Attenuators are part of any design's amplitude control. Digital attenuators help to simplify that control in microwave and RF systems. In a variety of marketsincluding commercial communications, military, and test areasamplitude is used for modulation, detection, linearity improvement, and a number of other functions in a system. Suppliers of digital step attenuators (DSAs) help to provide the means of controlling power levels swiftly and with precision, such as in cellular base stations. The latest DSAs exhibit enhanced accuracy and least significant bits (LSBs) in the tenths of decibels for precise control of amplitude and minimal insertion loss.
In a concise application note called simply "Digital Step Attenuators," Mini-Circuits defines a digital or electronic step attenuator as a component that varies attenuation by digital control signals.1 Just as digital signals have finite states, a digitally controlled attenuator has a corresponding number of finite attenuation states. The note includes the most-asked questions about DSAs and provides useful advice for engineers who are not familiar with them. For example, if the engineer exceeds the specified compression point for a DSAtypically given at 0-dBm input harmonics will be generated that could affect system operation. In addition, errors will occur in measured output levels because some fundamental signal power has been transferred to the harmonic products.
According to the firm, most step attenuators suffer relatively wide attenuation variation over the specified frequency band. For example, a 3-dB step could be off by as much as 1 dB, which translates into attenuation of 2 to 4 dB. With its TTL-controlled step attenuators, Mini-Circuits promises that this difference will be 0.3 dB or less. In cases of DSAs with poor return-loss performance, a change in attenuation state from 3 dB to a new setting at 6 dB may result in attenuation being off by as much as 2 dB. Mini-Circuits' DSAs exhibit 24 dB return loss and can maintain a 6-dB attenuation control setting within 0.6 dB of the nominal.
The company spotlights its DAT family of DSAs in application note AN-70-004, titled "Digital Step Attenuators Offer Precision and Linearity."2 The note explains that the LSB value of a DSA is dictated by both temperaturedependent attenuation variations and accuracy (often limited by the manufacturing process). If the temperature variation is low and accuracy is high, a minimum attenuation step can be extremely small. For example, Mini- Circuits' Super RF CMOS devices offer attenuation steps as small as 0.5 dB. The most-significant-bit (MSB) attenuation value is dictated by temperature- and frequency-dependent attenuation variations and the amount of isolation possible through the attenuator's switching devices. It also is decided by the semiconductor process variation. Mini-Circuits' Super RF CMOS attenuators provide an MSB to 16 dB.
The 75-O DAT-15575-PN(+), for example, delivers 15.5 dB total attenuation in 0.5-dB steps. The 5-b device offers a parallel control interface and dual supply voltage. It boasts typical accuracy of 0.1 dB. The attenuator exhibits return loss of 20 dB with an input third-order intercept point of +52 dBm. From DC to 1.2 GHz, typical insertion loss is 1.2 dB with a maximum of 1.8 dB. From 1.2 to 2.0 GHz, typical insertion loss is 1.6 dB with a maximum of 2.1 dB. At a 2-dB attenuation setting, the attenuator is accurate within 0.7 dB from DC to 1.2 GHz and 0.15 dB from 1.2 to 2.0 GHz. It targets applications including base-station infrastructure, CATV and direct broadcast satellite (DBS), multichannel multipoint distribution service (MMDS) and wireless localarea networks (WLANs), and poweramplifier- distortion canceling loops.
To serve fourth-generation (4G) Long Term Evolution (LTE) communications needs, Peregrine Semiconductor developed the 50-O PE43204 2-b UltraCMOS DSA. It provides as much as 18-dB attenuation in 6-, 12-, or 18-dB steps from 50 to 3000 MHz. This component is based on the firm's UltraCMOS silicon-on-insulator (SOI) technology. Typically, the PE43204 DSA features an attenuation error of +0.1 dB from 50 MHz to roughly 2000 MHz. Maximum attenuation error is -0.25/+0.40 dB over the same frequency range. From 2000 to 3000 MHz, attenuation error is typically +0.2 dB with a maximum of -0.10/+0.50 dB. The DSA varies 11 deg. in relative phase for all attenuation states. It exhibits insertion loss that is typically 0.6 dB with a maximum of 0.7 dB. The PE43204 DSA provides typical return loss of 15 dB across its operating frequency range.
This DSA is well suited for use with transmitters. Yet the PE43204's fast switching also makes it a fit for diversity receive applications, in which it can be used to protect the receive path and prevents overdriving the receive-channel analog-to-digital converter (ADC). By leveraging the firm's HaRP technology, the PE43204 suffers no gate lag or phase drift. This translates into very fast settling time and an input third-order intercept point above +61 dBm at 3 GHz. The DSA typically switches in 26 ns. By comparison, gallium-arsenide (GaAs)based alternatives offer switching speeds to 130 ns.
The TEA4000-7 DSA from Telemakus LLC underscores the trend toward making a personal computer into a test and measurement system (see "USB Devices Simplify RF/ Microwave Testing," February, p. 94). Using switches in addition to multiple attenuators, engineers can utilize this attenuator to create complex automatictest- equipment (ATE) systems (Fig. 1). The 7-b digital attenuator covers 31.75 dB in 0.25-dB steps. It exhibits better than 20 dB return loss from 1 MHz to 4 GHz. The TEA4000-7, which is fully terminated at all ports, includes SMA RF connectors with male at port 1 and female at port 2 to allow for easy interconnection. The DC/control connector is USB type A, which permits direct connection to a PC or via a Universal Serial Bus (USB) extender cable. The device also has 0.5 GB of Flash memory containing installation files, a datasheet, and test results. The Windows-based user interface allows simple control of the attenuator including a ramp function. The attenuator can be used with common ATE software as well.
Like all components, attenuator frequency must match the frequency of the application that it intends to serve. Due to the growth in communications and other applications that fall into broadband fre-quencies, it should come as no surprise that many attenuator developments target this arena. To serve broadband applications like electronic-warfare (EW) systems and instrumentation, for instance, Mimix Asia has introduced a gallium-arsenide (GaAs) monolithic-microwave-integratedcircuit (MMIC)-packaged digital attenuator. Over DC to 18 GHz, the XA1000-QH delivers an attenuation range of 28 dB and handles +24 dBm output power at 1-dB compression. Housed in a 4-x-4-mm QFN package, the attenuator is digitally controlled with 5-b operation and an LSB of 0.9 dB. The XA1000-QH is well suited for instrumentation, microwave point-to-point radio, satellite communications, and military radar.
A broadband digital attenuator from Pulsar Microwave spans 6 to 16 GHz (Fig. 2). Dubbed model DAT-23-480/2S, the 8-b, 0.25-dB LSB, 0-to-64-dB digital attenuator exhibits insertion loss to 6 dB with a maximum VSWR of 2.0:1. It typically switches in 500 ns. The attenuator offers 1.2 dB flatness at 64 dB. The attenuator's control logic is TTL. Its sibling, model DAT-25-480/2S, also is an 8 b, 0-to-64-dB digital attenuator with an LSB of 0.25 dB. Yet it covers 6 to 18 GHz while exhibiting insertion loss to 6.5 dB with maximum VSWR of 2.0:1. Control logic is TTL and switching time is 500 ns typical. Flatness is 3.0 dB at 64 dB.
Broadband digital attenuators also are among the numerous attenuators available from Hittite Microwave. The HMC306MS10(E), for example, is a 5-b positive-control, GaAs IC digital attenuator that comes in a 10-lead MSOP surface-mount plastic package. From 0.7 to 3.8 GHz, it typically exhibits insertion loss below 1.5 to 2.3 dB. The attenuator's bit values are 0.5 (LSB), 1, 2, 4, and 8 dB for a total attenuation of 15.5 dB. The HMC306MS10(E) boasts typical attenuation accuracy of 0.2 dB with an input third-order intercept point to +52 dBm. Each attenuation state can be selected via 5-b control voltage inputs, which are toggled between 0 and +3 to +5 V. Among this attenuator's target applications are UMTS/3G infrastructure; ISM, MMDS, WLAN, and WiMAX; microwave radio and VSAT, and test equipment and sensors.
Model DVAT-0518-60-8-SK-196 is one programmableattenuator option from American Microwave Corp. This variable attenuator/ modulator spans 0.5 to 18.0 GHz with as much as 50 dB attenuation. It exhibits insertion loss to 4.5 dB. At 40 dB, attenuation flatness is 3.0 dB. The attenuator provides accuracy of 1.3 dB from above 30 through 50 dB. It provides 8-b binary TTL control.
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The PA series binary-programmable step attenuators from RLC Electronics cover DC to 20 GHz (Fig. 3). Two basic models offer attenuation ranges of 15 and 70 dB. Attenuation is determined by the number of bits per model with bits controlling the amounts of attenuation in ascending order, such as 1, 2, 4, and 8 dB for a 4-b, 15-dB unit. The attenuators are equipped with failsafe or latching operation and either 12- or 28-V coils. TTL drivers are available as an option. The attenuators span a choice of frequency ranges. From 5.0 to 12.4 GHz, for example, the PA-124 offers a range of 0 to 15 dB in 1-dB steps. It exhibits insertion loss to 1.6 dB with a maximum VSWR of 1.7:1. The 50-O attenuator offers maximum accuracy of 0.5 dB per cell. Attenuation cells are 1, 2, 4, and 8 dB.
Covering 10 to 2000 MHz, the 5-b model TAD2104 from Spectrum Microwave offers a 31-dB attenuation range in 1-, 2-, 4-, 8-, and 16-dB steps. The GaAs device typically switches in 15 ns. Usually, it exhibits insertion loss below 6.5 dB with VSWR of 1.5:1. It exhibits maximum insertion loss of 8.5 dB with VSWR of 1.75:1. The attenuator generally offers accuracy of 0.5 dB (2 percent).
Although the market for broadband digital attenuators is generating a great number of products, attenuator development continues to be strong in the lower-frequency ranges. At the center of the range of digital attenuators available from Skyworks, Inc. is the AA106-86 or AA106-86LF. It targets applications like cellular radio, wireless data, and wireless-local-loop gain-level-control circuits. This 5-b GaAs integrated-circuit (IC) FET component attenuates in 0.5-dB steps to 15.5 dB. It offers 0.5-dB LSB positive control from 0.5 to 2.0 GHz. From 0.5 to 1 GHz, the digital attenuator typically exhibits insertion loss to 2.0 dB with a maximum of 2.4 dB and accuracy of 0.2 + 3 percent of dB attenuation setting in dB. Over a span of 1.0 to 2.0 GHz, it usually exhibits insertion loss of 3.0 dB with a limit of 3.4 dB and accuracy of 0.3 + 5 percent of dB attenuation setting in dB. The 50-O attenuator typically offers a VSWR of 1.5:1 from 0.5 to 2.0 GHz with a maximum of 2.0:1.
In addition to GaAs FETs, the extensive line of digital attenuators from DAICO Industries leverages PIN and Schottky diode technologies. The 1- through 8-b attenuators offer 0.1 dB LSB with as much as 127-dB attenuation. The foursection DAT0984-1 GaAs attenuator, for example, covers 10 to 1000 MHz with a 15-dB range and 1-dB LSB. It typically exhibits insertion loss of 1.9 dB with a maximum of 2.5 dB and a VSWR of 1.5:1. It switches in 0.03 s/ maximum under TTL control.
This sampling of attenuators represents only a small number of the suppliers who provide digital attenuators. An array of attenuator choices are available from firms like JFW, Alan Industries, Micronetics, Trilithic, Aerowave, Quasar, Herley, GT Microwave, Merrimac, Waveline, Richardson, and RF-Lambda, among others. Although most engineers focus on the frequency range and then on key aspects like insertion loss and switching speed, it also is crucial to remember that attenuators are fabricated using many different technologies. The process technology will impact performance. In addition, attenuators come in a range of package sizes and types. Such elements should be considered early in the design process.
REFERENCES
1. "Digital Step Attenuators," Mini-Circuits application note, www.minicircuits.com/pages/pdfs/dsa5-2.pdf.
2. "Digital Step Attenuators Offer Precision and Linearity," Mini-Circuits application note AN-70-004, www.minicircuits.com/pages/pdfs/an70004.pdf.