RF Attenuators Information
RF attenuators are circuits that reduce the power level of a signal by a certain amount (gain) with little or no reflection. They reduce the output signal with respect to the input and measure the power reduction in decibels (dB). There are two basic types of RF attenuators: fixed and variable. Fixed RF attenuators reduce output signal power relative to input signal power by a fixed amount over a specified bandwidth while the input and output impedances are kept close to the nominal level, usually 50 or 75 Ohms. Variable RF attenuators use several different methods to vary attenuation between two values in a specified range. For example, continuously variable devices vary the attenuation in a continuous fashion. By contrast, rf step attenuators vary the attenuation by only a fixed amount at a time. Programmable attenuators and direct-read attenuators that can be set by an external device such as rotary dial are also available. With a digital attenuator, the attenuation can be varied by digital control signals. Because digital signals are discrete in nature, digitally controlled attenuators have a number of finite attenuated states.
RF Attenuator Performance Specs
Performance specifications for RF attenuators include frequency range, maximum attenuation, number of bits, insertion loss, input voltage standing wave ratio, return loss, and reflected power. With digital attenuators, the number of bits is used to change the attenuation. Consequently, if n is the number of bits in the digital control signal, then the number of attenuation states is 2n. Insertion loss is the total RF power transmission loss from the insertion of a device in a transmission line. It is defined as the ratio of signal power at the output of the inserted device to the signal power at the input of the inserted device. Voltage standing wave ratio (VSWR) is a unitless ratio ranging from 1 to infinity that expresses the amount of reflected energy at the input of the device. A value of 1 indicates that all of the energy passes through. Any other value indicates that a portion of the energy is reflected.
Mounting Styles for RF Attenuators
There are several mounting styles for RF attenuators. Connectorized devices attach with coaxial or other types of connectors. Waveguide assemblies consist of a hollow metallic conductor with a rectangular or elliptical cross-section. Some conductors contain solid or gaseous dielectric materials. Surface mount technology (SMT) adds components to a printed circuit board (PCB) by soldering component leads or terminals to the top surface of the board. Through hole technology (THT) mounts components by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board. Flat pack (FPAK) devices have flat leads and are available in a variety of body sizes and pin counts.
Connectors For Use With RF Attenuators
Bayonet Neil-Concelman (BNC) connectors are used in applications to 2 GHz. Threaded Neil-Concelman (TNC) connectors are similar in size to BNC connectors, but feature a threaded coupling nut for applications that require performance to 11 GHz. Miniature coaxial (MCX) connectors provide broadband capability through 6 GHz and are used in applications where weight and physical space are limited. Ultra high frequency (UHF) connectors are designed with non-constant impedance for use in comparatively low voltage and low frequency applications. Subminiature-A or SMA RF attenuator connectors are screw on connectors rated from DC to 18GHz. Subminiature-B (SMB) connectors snap into place and are used for frequencies from DC to 4 GHz. Subminiature-P (SMP) connectors are rated to 40 GHz and, depending on detent type, can withstand from 100 to 100,000 interconnect cycles. Other connectors for RF attenuators include MMCX, Mini-UHF, Type F, Type N, 1.6/5.6, and 7-16 connectors.