|RF & microwave attenuator|
|Tuesday, 29 August 2006|
Various attenuator units are used in microwave applications for controlling the frequency response of microwave components. Fixed and variable microwave attenuators provide gain level adjustments for microwave circuits and systems. Variable waveguide attenuators are commonly used to attenuate microwave signals propagating within a waveguide. Waveguide attenuators for use in satellite/microwave communications are generally constructed using lossy dielectric fins positioned to penetrate into a waveguide parallel to the electric fields to reduce the energy level of a signal at the output of the attenuator. Solid state step attenuators are used to control the amplitude of high frequency electrical signals in a variety of microwave systems because of their high reliability and low cost. A solid state step attenuator provides amplitude control at the signal source's output. Step attenuators rely on switches to selectably couple one or more attenuator pads into the circuit. In RF attenuators, the switches are generally activated by control signals which may be toggled by a computer or other control device. Among the switches commonly used in step attenuators are electromechanical RF relays and PIN diode switches. Solid state step attenuators have low manufacturing cost because they are fabricated monolithically, tested in wafer form and packaged inexpensively. The reflection attenuator is a variable attenuator. The reflection attenuator can consist of a 4-port coupler, PIN diodes, and dummy loads. However, for microwave applications, absorptive attenuators (attenuators which absorb some of the signal in the attenuator itselF) are preferred over reflective attenuators which reflect a portion of the input signal back to its source. The important parameters of an absorptive attenuator are its accuracy as a function of frequency, its return loss and its stability over time and temperature.
Microwave attenuators provide attenuation in either discrete step attenuation or continuously variable. Gain control of amplifier cascades generally requires a variable attenuator circuit. In broadband microwave amplifiers, microwave attenuators are indispensable for temperature compensation of gain variation. Voltage-controlled variable attenuators have been widely used for automatic gain control circuits. Various types of voltage variable attenuators have been implemented using monolithic microwave integrated circuit (MMIC) technology. A voltage variable attenuator has an input port, an output port and a control port and provides a predetermined amount of attenuation to a signal propagating between the input port and the output port as determined by a control voltage fed to the control port. Typically, voltage variable attenuators are continuously variable over a predetermined attenuation range while providing a required operational bandwidth and impedance match. One type of voltage controlled variable attenuator is a FET variable absorptive attenuator which utilizes FETs as voltage-controlled resistors to adjust the attenuation. The basic mechanism of the circuit is the change in the low field resistance of a zero-biased FET controlled by gate voltage.