Assemble High-Power Attenuator Systems

Working with off-the-shelf components and connectors, it is still possible to assemble a cost-effective attenuation system for 2 to 1000 MHz capable of levels to 12.5 kW.

High-power signal processing and testing requires the use of highfrequency attenuators designed to handle large signals without damage or performance degradation. Especially when working with signals exceeding 1 kW, as common in commercial broadcast and military radar systems, reliable attenuators are critical to any test system. Most attenuators on the market rely on thick or thin film resistive designs screen printed or deposited on a flat ceramic substrate, typically beryllium oxide (BeO), which requires special handling, processing, and disposal procedures. This approach is well suited for lower-power attenuators, but can be difficult and expensive to adapt to power levels beyond 1 kW. Fortunately, another attenuator option cost-effectively provides high average power and high peak power capabilities utilizing standard, offthe- shelf products. The following will explain how to configure a highpower attenuation system from 1 to 12.5 kW for use in broadcast, electromagnetic- compatibility (EMC), semiconductor, military, medical, scientific, and other high-power applications with available standard components.

The first step in developing an effective high-power attenuator system is to choose the proper RF load for the maximum rated peak and average power levels expected in the application. For example, if an application calls for an attenuator with 10-kW average power-handling capability, then a 10-kW load should be specified. One of the most cost-effective and compact types for this power level is an oil (silicon or mineral) dielectric load (Fig. 1). Without the oil dielectric, forced-air cooling a resistor or resistors can result in a load design that is as large as a rack cabinet.

The second step in assembling the high-power attenuator system involves using coaxial connectors that are constructed to handle high power levels, by specifying the load's connector as a 1-5/8-in. EIA flange. This connection joins the load to the rigid line section in the system and typically is not disconnected during a measurement procedure. The rigid line is supplied with a 1-5/8-in. connector, which is typically rated for power-handling capability of about 30 kW. Therefore, the connection to this rigid line section should mate EIA flanges. A coupling assembly between the rigid line and the load connector includes all bolts and hardware needed to mate the two connectors.

The third step in assembling the high-power attenuator system is to specify a 1-5/8-in. flanged rigid line section for attachment to the highpower load (Fig. 2). Once the load's connector has been specified, the coupler/sensor module must be assembled. It consists of a coaxial rigid line section designed to mate to the load. This line section can be supplied in a variety of configurations, such as single, dual, or triple socket. For simplicity, a single socket rigid line section with 1-5/8-in. EIA flange connectors serves many applications.

The fourth step in assembling a high-power attenuator system involves choosing an RF sampling el nondirectional device that has a broadband coupling loop or plate that parallels the center conductor of the rigid line. To be broadband and have negligible impact on insertion loss and VSWR, its mechanical dimensions are held to a very tight tolerance. Once inserted into the line section, this portion of the assembly becomes a 2-to-1000-MHz nondirectional coupler, capable of operating to the power rating of the load. Sampling elements called X-Tractors have a variable set screw that enables minor adjustments of 8 dB from a preset coupling value of 49 dB (Fig. 3). The low signal output is via a BNC female connector on the top of the element. In theory, it is possible to further reduce the signal level by attaching a standard BNC fixed attenuator to the element. The attenuator should be rated for 2 W CW.

An optional fifth step in developing the high-power attenuator system is to choose an input connector for the rigid line section. For truly high-power handling capability, the rigid line section can accept another 1-5/ 8-in. coupler and line section. For convenience, however, it may be preferable to have the input to the rigid line section be more of a standard coaxial type connector, such as a Type N, HN, LC, or 7/16-in. EIA female connector. To accomplish this, standard 1-5/8-in. coaxial adapter kits accept a wide variety of different coaxial connectors in support of many different types of applications. It should be noted that an additional coupler will be needed for this portion of the attenuator assembly (Fig. 4). The final system, assembled with commercial off-the-shelf components and industry-standard coaxial connectors, covers a frequency range from 2 to 1000 MHz with excellent amplitude flatness above about 50 MHz (Fig.5). Below that frequency, fixed-frequency attenuators and more narrowband solutions can be applied for applications requiring higher accuracy.

Specifying a cost-effective attenuator system for high-power testing and other high-power applications does not require custom products, long lead times, multiple vendors, and high non-recurring-engineering (NRE) costs. Time, cost, and space can be saved by using standard off-the-shelf products that are properly matched together. The table offers an example of how standard models can be typically used in a 1-5/8-in. attenuator system that is geared for very high power levels through 1 GHz.

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