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Filter Facts

A Short Course on Filter Characteristics

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    Over the years Allen Avionics has used its experience and knowledge to supply customers with a comprehensive catalog that is easy to use and filled with valuable information. By reading FILTER FACTS, you will gain a good general understanding of LC Filters and some of their applications. This should also help when you have a need for special or custom filter designs. Allen Avionics Engineering and Sales Staff will be glad to answer any questions that you may have regarding our filter products. 

     An LC Filter may be defined as a passive device consisting of capacitors and inductors in a particular array such that a group of specified frequencies pass with very little attenuation while the undesired frequencies are attenuated. The four most common types of LC Filters (Lowpass, Highpass, Bandpass, Band Reject) are described on the following illustrations. The ideal filter would be a device that exhibits no attenuation in the passband and infinite attenuation to all other frequencies. However, a filter with these characteristics cannot be manufactured. Allen Avionics uses state-of-the-art computer aided design and filter modeling to produce the highest quality LC Filters available. The amplitude response of a filter from its passband to its reject band is defined by its shape factor. In the catalog, the Lowpass and Highpass Filters have their shape factors expressed as the ratio of the 20dB and 40dB frequencies to that of the 3dB frequency. For example: a Lowpass filter with its cutoff at 1MHz and its 20dB point at 1.6MHz has a ratio of 1.6 at 20dB. This can be found by dividing the 20dB frequency by the 3dB frequency i.e. (1.6MHz/1.0MHz) = 1.6 When a Highpass Filter is used, the ratio is expressed as a number less than one. The cutoff ratios of Lowpass and Highpass filters that Allen Avionics can manufacture are listed in the tabulated listings for Lowpass and Highpass filters.

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