Variable Attenuator

A variable attenuator is a circuit that decreases the strength of the input signal either continuously or step by step without appreciable signal distortion while substantially maintaining constant impedance match.

Planar Monolithic Industries (PMI) describes attenuators as transmission line components with at least two ports used to reduce input power to a system by a predetermined amount. A variable attenuator operates through its entire dynamic range, in contrast with a switch which is generally restricted to one of two states – ‘on’ or ‘off’.

The attenuation of a circuit inserted into a transmission line is defined as the ratio between decibels of power input to the attenuator, and power transmitted from the circuit to the load. Both fixed and variable attenuators exist; fixed attenuators have a factory preset attenuation value that cannot be changed. Variable attenuators, however, can be adjusted by their users to the attenuation level required, with a number of different methods available.

Mechanically variable attenuators are usually adjusted with a tuning screw or control knob. The necessity for mechanical adjustment means that such variable attenuators are generally unsuited to system requirements. However, electronically controlled variable attenuators are available in both current-controlled and voltage-controlled versions. These are used in many systems, test and lab applications. Most use a PIN diode as the control element.