Board mount RF filters are devices mounted to a printed circuit board (PCB) which pass or reject signals by frequency. The filter design determines the amount of insertion loss and phase shift for signals that pass through the filter.

There are several basic types of RF filters: bandpass and band reject. Bandpass filters are active or passive circuits that pass signals from a specific frequency band and reject signals from out-of-band frequencies. Examples of bandpass filters include surface acoustic wave (SAW) filters, crystal filters, and cavity filters. Band reject filters (bandstop filters, notch filters) are tuned circuits that prevent the passage of signals within a specified band of frequencies.

Low pass and high pass filters are sub-types of RF filters. Low pass filters allow low frequencies to pass while rejecting high frequencies. High pass filters pass high frequencies and reject low frequencies.

Performance Specifications

Performance specifications for RF filters include specified frequency, bandwidth, ripple, insertion loss, and voltage standing wave ratio.

Filter type determines the specified frequency. For bandpass and band reject filters, the specified frequency is the center frequency. For low pass filters, the specified frequency is the cut-off frequency. For high pass filters, the specified frequency is the cut-on frequency.

Bandwidth is the range of frequencies that filters pass with minimal attenuation or, in the case of band reject filters, 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).

Design

RF filters and microwave tilters differ in terms of filter design. Choices include ceramic filters, crystal filters, SAW filters, and adjustable filters.

• Ceramic filters are designed using piezoelectric ceramics to pass certain frequencies.
• Crystal filters are designed with a quart crystal resonator.
• SAW filters are designed and manufactured using surface acoustic wave (SAW) technology.
• Adjustable filters are designed to be adjustable in frequency.

Other filter designs for RF filters and microwave filters are also available.

Mounting

There are several mounting styles for RF 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.

Connectors

RF 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 filters and microwave filters include MMCX, Mini-UHF, Type F, Type N, 1.6/5.6, and 7-16 connectors.

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