RF Filter Overview – Types & Topologies

RF filters are some of the most important components in receivers. The main functions of the filters are to reject undesirable signals outside the filter pass band and to separate signals according to their frequency. RF filters modify the amplitudes and phases of sinusoidal waveforms that pass through them or, more simply, RF filters remove unwanted frequency components from a signal while preserving desired frequency components–an essential tool for RF engineers. RF filters are designed to operate on signals in the entire radio frequency spectrum, used in broadcast radio, television, wireless communication, scientific research, and military/defense. Filters are also components of other RF/microwave devices, such as duplexers and diplexers, which are used to combine or separate multiple frequency bands. Filter construction varies by application, size, cost, and performance.

Filter Topologies

There are four basic types of filters that accept, attenuating the signal slightly, or reject, heavily attenuate the signal to a specified level, signals in different ways. Lowpass Filters (LPF) allow frequencies below a certain frequency to pass while Highpass Filters (HPF) allow frequencies above a certain frequency to pass. Bandpass Filters (BPF) allow frequencies between two frequencies to pass and Band Reject Filters reject frequencies between two frequencies while passing all others.

The frequency response of a filter can be tunable, if circuit elements within the filter are digitally or analogy adjustable. Tunable filters include diplexers that simultaneously frequency-domain multiplex different frequency bands from two inputs into a combined output and may also have frequency tunable components.

Also, there are triplexers that split a complex signal into three pre-defined frequency bands, and multiplexers that receive multiple signals through its input and outputs the signals separately, according to their frequency. Equalizer filters, on the other hand, are used to render the frequency response flat from end-to-end and are critical to the successful operation of electronic systems. For some multiband/wideband transmissions, radar systems, and scientific applications equalizer filters are necessary to cancel out any group delay and phase delay between different frequency components.

Critical Specifications

Important device parameters in RF filters are passband, impedance, Insertion Loss, and VSWR. Passband is the range of frequencies that can pass through a filter, as in a radio receiver contains a bandpass filter to select the frequency of the desired radio signal out of all the radio waves picked up by its antenna. Impedance is a frequency dependent parameter, and when selecting which type of interconnect to use, the interconnect impedance must match the impedance of the system being used to ensure efficient transmission without substantial reflections. The insertion loss of a filter is determined by the filter bandwidth relative to center frequency, the order of the filter, and the quality factor (Q factor) of the resonators that make up the component. VSWR, voltage standing wave ratio, is a measure of how well impedance is matched from the RF/microwave transmission source to the load.

Common Filter Designs

The Butterworth filter has a flat response within its pass-band and an adequate roll-off and is often considered a good all-round filter adequate for many applications, although it does not provide the sharpest cut-off.

Bessel filter is a type of analog linear filter which preserves the wave shape of filtered signals in the passband. Bessel filters are often used in audio crossover systems.

Chebyshev filters are analog or digital filters and have a steeper roll-off and more passband ripple or stopband ripple than Butterworth filters. They minimize the error between the idealized and the actual filter characteristic over the range of the filter, but with ripples in the passband. Because of the passband ripple inherent in Chebyshev filters, the filters that have a smoother response in the passband but a more irregular response in the stopband are preferred for some applications.

Gaussian filters have the properties of having no overshoot to a step function input while minimizing the rise and fall time as it has the minimum possible group delay. It is considered the ideal time domain filter and are important in areas such as oscilloscopes and digital telecommunication systems.