RF band reject filters (bandstop filters, notch filters) are tuned circuits that prevent the passage of signals within a specified band of frequencies. Performance specifications for band reject filters include specified frequency, bandwidth, ripple, insertion loss, and voltage standing wave ratio. The specified frequency is the center frequency. Bandwidth is the range of frequencies that RF band reject filters pass with maximum attenuation. Ripple is the peak-to-peak variation of the passband response. Insertion loss is the total RF power transmission loss resulting from the insertion of a device in a transmission line. Insertion loss is the ratio of signal power at the output of the inserted device to the signal power at its input. Voltage standing wave ratio (VSWR) is a unit-less ratio ranging from 1 to infinity that expresses the amount of reflected energy. A value of 1 indicates that all of the energy passes. Any other value indicates that a portion of the energy is deflected. VSWR can also be expressed in terms of decibels (dB).
There are several mounting styles for RF band reject filters. 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. Connectorized devices attach with coaxial or other types of connectors. They provide exceptional thermal and electrical performance and are available in cavity-up and cavity-down configurations. Waveguide assemblies consist of a hollow metallic conductor with a rectangular or elliptical cross-section. Some conductors contain solid or gaseous dielectric materials.
RF band reject filters use several types of connectors. 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 (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 band reject filters include MMCX, Mini-UHF, Type F, Type N, 1.6/5.6, and 7-16 connectors.
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