Fixed vs. Step Attenuators: Match Improvement vs. Level Control

Fixed attenuators, often called attenuator pads, are placed inline to match impedances between mismatched components. By absorbing reflected power in both directions, a fixed attenuator significantly improves the voltage standing wave ratio (VSWR) seen by the signal source.

For example, the 8493C fixed attenuator maintains an exceptionally low maximum SWR of 1.1 from DC to 8 GHz, rising to only 1.25 at 26.5 GHz. This performance makes it ideal for stabilizing port matches. In addition to SWR reduction, fixed attenuators protect sensitive receiver front-ends from overload. Units such as the 8491A handle up to 2 W of average power and feature robust Type-N connectors, making them ideal for placement on the input channels of test instruments.

When a test procedure requires varying signal amplitudes to characterize receivers or active components, step attenuators are necessary. These devices allow operators to change attenuation in precise increments. The 8494B manual step attenuator provides an attenuation range of 0 to 11 dB in 1 dB steps across a frequency range of DC to 18 GHz.

Because step attenuators rely on internal mechanical switches to redirect signals through different resistive paths, their design is inherently more complex than a single fixed pad. The 8494B uses internal contacts made of precision gold-plated leaf springs to ensure a long operating life of over 5 million cycles per section. It achieves a maximum attenuation repeatability of 0.03 dB, ensuring consistent measurements over long test cycles.

Integrating attenuators into a coaxial network requires weighing the low-SWR response of fixed components against the versatility of step components. A fixed attenuator like the 8491A offers tight attenuation accuracy (such as ±0.3 dB for 3 dB and 6 dB values) and a very low maximum VSWR of 1.2 from DC to 4 GHz.

Conversely, the internal switches within step attenuators introduce parasitics that degrade VSWR. The 8494B, for example, has a maximum VSWR of 1.5 from DC to 8 GHz, which increases to 1.9 from 12.4 to 18 GHz. Step attenuators also typically feature lower average power limits (1 W for the 8494B) compared to fixed units (2 W for the 8491A and 8493C). In many high-performance setups, engineers place a low-value fixed attenuator in series with a step attenuator to improve the overall match while preserving adjustable level control.