Selecting an attenuator for an application requires a great deal of filtering through specifications. Often, it’s more than just a simple matter of sorting through performance variables and picking a type of package. That’s because some performance limit, such as power-handling capability, will greatly influence the type and size of package.
Tremendous progress has been made in circuit materials and attenuator designs during the last few decades. However, smaller attenuators, such as surface-mount-technology (SMT) components, are still limited in power-handling capabilities compared to larger attenuators in metal packages with coaxial connectors or waveguide signal ports.
Eyes on PIM Distortion
Many key performance parameters, such as insertion loss, attenuation accuracy, and power-handling capability, were reviewed in an earlier article on attenuators. The article also provided a brief review of the mechanical options for RF/microwave attenuators, including fixed, continuously variable, and programmable attenuators that change attenuation in discrete steps.
One parameter that’s growing in importance for various types of attenuators is the passive-intermodulation (PIM) distortion level. Such distortion, found in passive components like cables, connectors, and attenuators, can be caused by the unwanted mixing of multiple signals. Low PIM is critical to achieving dependable performance in cellular basestations and other communications systems using complex modulation schemes.
More companies than ever now offer fixed and variable attenuators with low PIM levels, such as the LPAR50-dB-1WWP series of low PIM fixed attenuators from MECA. Multiple-attenuator configurations are available, including 15, 20, 30, and 40 dB. Although supplied in somewhat large enclosures (Fig. 1), they are designed to handle high power levels and are capable of 50 W average power and 2.5 kW peak power from 0.689 to 2.7 GHz. The devices are supplied with 7/16 and Type N connectors insulated with virgin-grade polytetrafluoroethylene material (PTFE) to minimize the generation of PIM at high power levels, even during attenuation of the signal amplitude.
Temp-Variable Attenuators on the Rise
Another trend in RF/microwave attenuators is the growing popularity of temperature-variable attenuators, as circuit and system designers discover the practical applications for these novel devices. These fixed and variable attenuators are fabricated with various thermistor-type materials that exhibit characteristics of changing attenuation with temperature. Firms that offer this type of attenuator typically publish a set of attenuation-versus-temperature responses, which allows designers to select the attenuation with a temperature curve that’s most suited to their application.
The technology isn’t new. One form of it was actually patented in the early 1990s by EMC Technology, a pioneer in the technology. The firm’s Thermopad temperature-variable attenuator line is winning over more designers as a solution for RF/microwave amplitude variations caused by changes in temperature.
For example, these attenuators can be used in series with power amplifiers that exhibit variations in gain due to temperature, either from environmental effects or from their own heating effects at high power levels. By selecting a temperature-variable attenuator with an attenuation/temperature response that’s close to the inverse of the amplifier’s gain/temperature response, the amplifier’s output power can be held fairly stable with temperature and without the need of more complex automatic-level-control (ALC) circuits.
The patented technology combines level setting and temperature compensation in a single device, helping to reduce component counts and costs. The totally passive Thermopad temperature-variable attenuators add little or no distortion to a system, and they can be supplied with attenuation/temperature responses tailored to the needs of a system or circuit, with negative and positive coefficients of attenuation possible.
The Thermopad attenuators have minimal effect on signal phase or time-domain response, which means minimal distortion on complex I/Q signal modulation formats. Models in the company’s MTVA line of absorptive RF/microwave attenuators (Fig. 2), for example, come in compact surface-mount housings that are smaller at higher frequencies, and feature attenuation values from 1 to 9 dB at frequencies from dc to 18 GHz.
To introduce designers to the concept of temperature-variable attenuation, the company offers a wideband temperature-variable design kit usable from dc to 20 GHz. With the TVA-DKIT, engineers can explore broadband circuits using attenuators with different temperature coefficients of attenuation (TCA) in components measuring only 0.075 × 0.060 in. (1.90 × 1.52 mm). The kit contains attenuators with attenuation values from 2 to 6 dB, as well as a variety of different attenuation-versus-temperature responses for experimentation.
Other Temp-Variable Attenuator Options
The firm isn’t the only one offering temperature-variable attenuators. API Technologies–Inmet has developed the TCAF N05 Series of temperature-variable attenuators for applications from dc to 12 GHz. These chip-sized, surface-mountable temperature-variable attenuators come in fixed attenuation values of 2, 3, 4, 5, and 6 dB with 0.5-dB attenuation accuracy for each attenuation value. They’re designed with a negative attenuation-versus-temperature response of –0.005 dB/°C to compensate for amplitude variations, such as changes in amplifier gain. These attenuators operate with bias or control voltage and can handle power levels to 2 W.
Other players in the temperature-variable attenuator market include Thin Film Technology Corp. and International Manufacturing Services (IMS). Thin Film Technology offers the ATV0805J Series of temperature-variable attenuators in a compact 0805 package style (0.079 × 0.049 × 0.035 in.). These RoHS-compliant components are fabricated with thermally sensitive thermistor material on alumina substrates. The product line, usable from dc to 10 GHz, is available with attenuation values from 1 to 6 dB and an attenuation tolerance of ±0.5 dB. They are rated for power levels to 63 mW.
IMS enables circuit designers to tackle effective dissipation of heat through its Therma-Bridge and Therma-Plane series of resistive components—the AV-0805 line of thin-film temperature-variable chip attenuators. Fabricated on alumina substrate material, these surface-mount devices are characterized to 5 GHz and available in attenuation values from 1 to 10 dB in 1-dB increments. Eight or nine attenuation-versus-temperature response curves are available for the attenuators, depending on attenuation value. The compact chips are operate in the –40 to +100°C temperature range, and are rated for power levels to 63 mW.
What’s been covered here represents just a few examples of how attenuators are evolving to handle amplitude-control issues in RF/microwave systems. As demonstrated by MECA’s attenuator, high-power handling capabilities still require considerable physical size to dissipate heat, which will limit the power-handling capabilities of chip-sized attenuators.
Resistive materials continue to advance, though. As a result, more circuit and system designers are discovering the effectiveness of adding attenuation (in particular, temperature-variable attenuation) in series with power amplifiers to help level the amplitude gain response in systems such as cellular-communications base stations, cable-television (CATV) infrastructure equipment, and satellite-communications (satcom) systems equipment.