Prototype filters are filters that are electronic and used as templates in order to produce a filter in a modified design for different particular applications. They are the perfect example of a non-dimensionalised design. This allows the filter that is desired to be transformed or scaled to the particular design wanted. They are seen normally in regards to filters that are electronic and this includes the linear analogue filters that are passive. In principle, however this method is or can be applied to different types of linear filters or other processing systems that are signal types. This can include acoustic, optical, or even mechanical. Normally filters need to operate in different frequencies, bandwidths, and impedances. The prototype utility of the filter comes directly from the property that these other filters, every one of them can be derived from the device simply by applying the factor of scaling to the prototype components. This design for a filter need to be only carried out in full once, and this will obtain the other filters by just applying the scaling factor into play.


One of the most useful factors about these prototypes is the ability to transform one band form into another easily. The transform in these prototypes is more than just the simple scale factor, and the band form in here is meant to show the indication of the category of the pass band that the filter has within it and that it possesses. There are different types of band forms, and this includes highpass, lowpass, bandpass, and even bandstop but other types can be possible to make. It is also possible for these filters to have multiple passbands. Actually, in some treatments, the filter for the bandstop can be considered to be a type of passband filters in multiples by being able to have two passbands. More common in these passbands are the prototype filters that are expressed as lowpass filters. Other techniques of this are possible as well.


Low Pass Prototypes

This type of prototype is usually a low pass filter that has a 3dB frequency in the corner of the angular frequency. There are many different formulas that are used to differentiate between the prototypes to tell them apart. The nominal or even characteristic impedance using the filter is set in another type of formula to calculate the Hz. Generally, any type of non zero points of frequency on the response of the filter can be used for a reference for the design of the prototype. One example of this is the filters with the ripples going through the passband for the frequency in the corner are usually said to be the highest frequency when it comes to the highest ripple in the passband rather than calculating it at 3dB. Another example of this is through the image filters in their parameter which is the older design method than this type of more modern design and network of synthesis filters. This will cut off the frequency rather than putting the 3dB point since the cut off of the filter system is well defined in points in this filter type of system. This type of prototype filter is normally only used to product other types of filters of the same type of class and order. One example of this is the fifth order of the Bessel filter type of prototype that can easily be converted into any type of fifth order type of Bessel filter. It cannot be transformed however, into other types of third order Bessel filters or even fifth orders of the Tchebyscheff filters. All of these filters have their own types of formulas and equations to keep them apart.