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The phase and position of the electrodes dictate signal amplitude while the wavelength of the electrodes and the space between dictate the frequency.


The filter selectivity starts to decline above 1.5GHz, and around 3GHz they are generally suitable for use in applications where only modest performance is required.


SAW filters are susceptible to temperature changes so additional compensation is often built-in to counteract this. Common applications for SAW filters include long-term evolution (LTE) bands for telecommunications, 5G systems, Global System for Mobile Communications (GSM), medical systems and radar systems.


Helical Filters – 50MHz to 3GHz


A helical filter can be described as a coil surrounded by a conductive shielding case with a ferrite tuning core in the centre of the coil. One end of the coil is connected to the load the other is open with the length of the wire representing a quarter of the wavelength. Helical filters offer a very environmentally adaptable solution with best performance operating between -30 and 75°C, but also capable of -40 to 85 ˚C.


This type of filter offers a very high Q factor and excellent insertion loss performance.


Helical filters can be connected into two- to six-pole configurations to obtain the correct filter performance with an isolation characteristic of 80 to 90dBc.


Common applications for helical filters are wireless communication systems, broadcast equipment, transmitter/receiver sets, medical systems and signal jamming equipment.


RF cavity filters – 10MHz to 10GHz


These are a basic component of many modern communication systems, and one of the most common filters in the current generation. RF cavity filters can be described as a terminated resonator in a box which is mechanically tuned. Several filters may be joined together to provide a better filter shape. The size of the cavity is dictated by the resonant frequency required – the lower the frequency, the larger the cavity – and they are constructed in a sturdy metal case.


A cavity filter can operate reliably in harsh environments for many years due to its large size and solid structure.


Another significant advantage is that cavity filters can withstand high-power signals and are very suitable for communication applications from 10MHz to 10GHz.


Common applications for cavity filters are base stations, RF high power transmitters, tower communications and satellite communications.


Ceramic Dielectric Filters


A ceramic dielectric filter is made of high-stability piezoelectric ceramics along with a filter comprising several resonators. Ceramic filters are made from specially formulated powders that contain a proprietary formula of metals and chemicals. These are then sintered at a high temperature to form a ceramic resonator and different filter characteristics can be produced by amending the material formulae.


There are usually four types of ceramic filter: band pass, high pass, low pass and band rejection filters and these are commonly used in the following communication product applications: wireless communication equipment including truck communication, train communication, ocean and air communication; broadcasting equipment for TV and cable TV; network communication equipment such as indoor TX and RX devices and two way radio; disaster prevention communication equipment e.g. fire-fighting, emergency broadcast; equipment measurement in laboratory development and test facilities; and signal jamming in municipal buildings such as hospitals and military locations.


Therefore, in summary, when selecting an RF filter start first by defining the centre frequency, this will reduce the types to choose from, then consider the bandwidth the filter needs to support, before considering other characteristics.


If a narrow band filter is required then a high Q factor is necessary while, conversely, a wideband filter requires a lower Q factor. Finally, it cannot be stressed enough that whichever filter type is chosen, the terminating impedance must be matched exactly in the circuit design to achieve the best results. Euroquartz www.computercomponents.com


JULY/AUGUST 2021 | ELECTRONICS TODAY 33


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