RF limiters are circuits that prevent the amplitude of a waveform from exceeding a specified value. There are three basic types of RF limiters: conventional, feedback, and high power. Conventional PIN diode limiters protect sensitive circuit elements by reducing high power inputs to low levels that can be tolerated by the next component. Feedback limiters have lower threshold and leakage levels than standard limiters in order to protect more sensitive components. High power limiters protect sensitive receiver circuits from close proximity, high power signals. Applications for RF limiters include stabilizing generator outputs, providing constant amplitude signals in phase-sensitive systems, and reducing amplitude variation in frequency modulation (FM) systems.
Performance specifications for RF limiters include frequency band, peak power, average power, insertion loss, spike leakage, flat leakage, pulse width, recovery time, and input voltage standing wave ratio (VSWR). Insertion loss is the total RF power transmission loss resulting from the insertion of a device in a transmission line. Spike leakage is the amount of current that leaves on the phase and does not return on the ground. Flat leakage is the undesirable loss of a signal through an RF limiter under normal usage. Pulse width is the time interval between the leading edge and the trailing edge of a pulse at a point where the amplitude is half of the peak value. Recovery time is the interval required for normalization of the signal. Voltage standing wave ratio (VSWR) is a unitless ratio that ranges from one to infinity and expresses the amount of reflected energy at the input of the device. A value of one indicates that all of the energy passes through. Any other value indicates that a portion of the energy is deflected.
RF limiters use several types of connectors. Bayonet Neil-Concelman (BNC) connectors are used in video and RF 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. Micro-miniature coaxial (MMCX) connectors are smaller than MCX connectors, but feature a more robust interface for greater durability. Ultra high frequency (UHF) connectors are designed with non-constant impedance for use in comparatively low voltage and low frequency applications. Mini-UHF connectors are well suited applications in which size, weight and cost are critical. Subminiature-A (SMA) connectors directly interface the cable dielectric without air gaps. 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 limiters include Type F, Type N, 1.6/5.6, and 7/16 connectors.
There are several package types for RF limiters. 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 on a printed circuit board (PCB) by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board. Connectorized devices attach with coaxial or other types of connectors. Waveguide assemblies are used in microwave waveguide systems. They consist of a hollow metallic conductor with a rectangular, elliptical, or circular cross-section.