RF Radio Frequency Filter Review

Electronics, Instrumentation & Electrical Database

RF Radio Frequency Filter Review

An RF (radio frequency) filter is a device that is utilized to allow or stop selected signals or frequencies, or used to eliminate (filter out) any unwanted signals. In other words, an RF filter is designed to allow for the attenuation or transmission of a range of frequencies that would be applied. For instance, an RF filter helps to cut out RF interference that could occur if a hairdryer, lamp, or other "noisy" device is activated.

Generally, there are four types of RF filters:

• A high pass filter has a cut-off frequency, thereby allowing minimal or no loss in transmission for high frequencies, but considerably attenuating any low frequencies.
• A low pass filter is the opposite that of a high pass filter - that is, allowing for the transmission frequencies below the cut-off frequency, but attenuating any frequencies above the cut-off frequency.
• A band pass filter will allow for the transmission of a selected range or band of frequencies with no attenuation, but will attenuate frequencies below or above (lower or higher than) the desired or allowed band. Examples of band pass filters include cavity filters, surface acoustic wave (SAW) filters and crystal filters.
• A band reject filter will attenuate a frequency range or band while allowing all other frequencies to pass unattenuated. Examples of band reject filters include notch filters and bandstop filters.

There are several parameters that should be considered when selecting an RF filter:

• Package Type - through hole mount, surface mount, etc.
• Insertion Loss - the signal strength at the output of the filter divided by the input signal power
• Specified Frequency (which applies to band reject and band pass filters) - the center frequency for high and low pass, specifying the cut-off frequency
• Voltage Standing Ratio - this is the quantity of "reflected" energy
• Ripple - peak to peak difference of the filter response

Various types of RF filters can be found in air traffic control and communications systems, medical alert systems, telemetry applications, two-way pagers, and satellite communications. Other uses include garage door openers, fire control radars, keyless locks, and radar and missile guidance systems.