Rotary step attenuators are invaluable when performing measurements where signal levels, such as the output of a signal generator, must be controlled in precisely known increments. Although such components are often associated with high cost and limited reliability, a patented design developed by Trilithic (Indianapolis, IN) delivers high attenuation accuracy at competitive costs. The firm's RSA series of rotary step attenuators includes 16 members with as much as 110 dB attenuation from DC to 3 GHz and models with 10 dB attenuation through 4.5 GHz.

The RSA rotary step attenuators benefit from a unique patented design that leverages modern manufacturing processes and materials science. The attenuators incorporate a detailed rotor assembly to achieve the large number of attenuation steps with precise control. This design approach significantly reduces the machining cost and assembly time of the rotor assembly. Attenuation for all models is controlled by a rotary knob that makes adjustments in 1-dB, detented steps.

In spite of the cost savings, the attenuators achieve high attenuation accuracy (see table). The highest-frequency unit, model RSA-45510, operates from DC to 4.5 GHz. It offers an attenuation range of 0 to 10 dB in 1-dB steps with attenuation accuracy of ±0.2 dB from DC to 1 GHz, ±0.3 dB from 1 to 2 GHz, ±0.4 dB from 2 to 3 GHz, and ±0.5 dB from 3 to 4.5 GHz. The attenuator exhibits insertion loss of only 0.3 dB from DC to 1 GHz, 0.5 dB from 1 to 2 GHz, 0.7 dB from 2 to 3 GHz, and 0.8 dB from 3.0 to 4.0 GHz and 1.0 dB from 4.0 to 4.5 GHz. The maximum VSWR is 1.25:1 from DC to 1 GHz (maximum return loss of 19.1 dB), 1.30:1 from 1 to 2 GHz (maximum return loss of 17.7 dB), 1.40:1 from 2 to 3 GHz (maximum return loss of 15.5 dB), and 1.50:1 from 3 to 4.5 GHz (maximum return loss of 14 dB).

One of the more impressive members of the RSA series in terms of total attenuation is the model RSA-35110D, with a 0-to-110-dB attenuation range from DC to 3 GHz (see figure). The rotary step attenuator, which is adjustable in 1-dB steps, could even provide the basis for amplitude control in a low-cost RF signal generator. The 110-dB model RSA-35110D is characteristic of the new attenuator line, delivering extremely high attenuation accuracy at lower attenuation settings and lower frequencies, but maintaining respectable attenuation accuracy at even the highest attenuation settings and frequencies. The model RSA-35110D achieves attenuation accuracy of ±0.25 dB or less than 1.5 percent of the attenuation setting from DC to 1 GHz. From 1 to 2 GHz, the attenuation accuracy is ±0.35 dB or less than 2 percent of the attenuation setting. From 2 to 3 GHz, the attenuation accuracy is ±0.50 dB or less than 3 percent for attenuation settings from 1 to 99 dB, and is less than 6 percent for the highest attenuation settings of 100 to 110 dB. Insertion loss is less than 0.60 dB from DC to 1 GHz, less than 1.0 dB from 1 to 2 GHz, and less than 1.5 dB from 2 to 3 GHz. Maximum VSWR is 1.30:1 from DC to 1 GHz, 1.40:1 from 1 to 2 GHz, and 1.50:1 from 2 to 3 GHz.

In between those two attenuation ranges, model RSA-3550D features a 0-to-50-dB attenuation range that can be set in 1-dB increments at frequencies from DC to 3 GHz. It features attenuation accuracy of ±0.25 dB or less than 1 percent of the attenuation setting from DC to 1 GHz. From 1 to 2 GHz, the attenuation accuracy is ±0.35 dB or less than 2 percent of the attenuation setting. From 2 to 3 GHz, the attenuation accuracy is ±0.50 dB or less than 3 percent of the attenuation setting. Insertion loss is 0.60 dB or less from DC to 1 GHz, 1.0 dB or less from 1 to 2 GHz, and 1.5 dB or less from 2 to 3 GHz. Maximum VSWR is 1.30:1 from DC to 1 GHz, 1.40:1 from 1 to 2 GHz, and 1.50:1 from 2 to 3 GHz.

All of the RSA series rotary step attenuators are supplied in metal housings with a choice of SMA, BNC, TNC, or Type N connectors, although BNC connectors are not recommended for applications above 1 GHz. The rugged rotary step attenuators are built to withstand at least 200,000 attenuation steps and are rated for average input-power levels as high as 2 W (at +25° C). Trilithic, Inc., 9710 Park Davis Dr., Indianapolis, IN 46235; (800) 344-2412, (317) 895-3600, FAX: (317) 895-3612, e-mail: sales@trilithic.com, Internet: www.trilithic.com.