RF Filters

Radio frequency filters are electrical devices that are used to filter out unwanted signals or allow selected frequencies and signals to pass. There are several basic types of RF filters including: band reject, bandpass, high pass and low pass.

In the band reject type of filter, a specified frequency range is not allowed to pass through while other frequencies are allowed to pass through. In the bandpass type of filter a selected range of frequencies are allowed to pass while other frequencies are filtered. Some examples of bandpass filters include surface acoustic wave (SAW) filters, crystal filters, and cavity filters. In the high pass filter only the higher range of frequencies are allowed to pass through. With the low pass style of filter only the low frequencies are allowed to pass through.

While selecting RFfilters, you have to select the specified frequency for the filter, the ripple, the insertion loss, the voltage standing wave and the package type.

The type of filter will determine the specified frequency. The specified frequency for bandpass and band reject filters is the center frequency. The specified frequency is the cut-off frequency for low pass filters and the specified frequency is the cut-on frequency for high pass filters. The bandwidth is the range of frequencies that filters are able to pass with minimal attenuation or with band reject filters, the maximum attenuation. Ripple is the peak-to-peak variation of the passband response. Insertion loss is the total radio frequency power transmission loss which results from the insertion of a device in a transmission line. Insertion loss is the signal power at the output of the inserted device divided by the signal power at its input. Voltage standing wave ratio (VSWR) is a unit-less ratio that ranges from one to infinity that expresses the amount of reflected energy. A value of one means that all of the energy passes through. Any other value means that some of the energy is deflected. VSWR can also be expressed in terms of decibels (dB).

There are numerous types of mounting devices for these filters including surface mount technology (SMT), through hole technology (THT), Flat pack devices (FPAK), Connectorized devices, and waveguide assemblies. There are also several types of connectors for RF filters such as Bayonet Neil-Concelman (BNC), Threaded Neil-Concelman (TNC), Miniature coaxial (MCX), Ultra high frequency (UHF), Subminiature-A (SMA), Subminiature-B (SMB), Subminiature-P (SMP), MMCX, Mini-UHF, Type F, Type N, 1.6/5.6, and 7-16 connectors.

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