RF Filters

In very general terms, RF filters are electrical circuits that are designed to have specific characteristics with respect to the transmission or attenuation of various frequencies that may be applied to it. This allows them to be specific in nature. There are three basic types of RF filters:

High Pass
Low Pass
Band Pass

High pass RF filters have a cut-off frequency. What this means is that above that frequency there is little or no loss in transmission, but below that frequency there is considerable attenuation. For general purposes, it can be thought of as being the opposite of the low pass filter.

A low pass filter will allow all frequencies below a specified one (called the cut-off frequency) to be transmitted with little or no loss, but that will attenuate all frequencies above the cut-off frequency. It should be noted that most high pass and low pass RF filters can be very difficult to construct properly. It is for this reason that many hobbyists and novice users will simply purchase one when possible.

The band pass RF filters are those that will transmit a selected band of frequencies with substantially no loss during the transmission. However, they will attenuate all frequencies either higher or lower than the desired band and thus that is how it gets its name.

When you need more output filtering than what you get with traditional designs you may want to use what are known as Second Harmonic Optimized Low Pass RF Fiters.

If you want to use high speed silicon diodes as RF filters switches you may want to look into building a diode switch band pass filter. This will allow switching between two or more filters and can be very useful for some applications.

Generally speaking, band pass RF filters will cost more than either the high or low pass RF filters. You can, however, find schematics and interesting information about all of these types of RF filters either online or in your local library. A lot of this information can be used to save money and can be used to customize a particular system should that be one of your needs.

A study of RF filters is all but mandatory for anyone wishing to enter into the electronics field as a professional or as a hobbyist. But, as mentioned above, there is a wealth of good information on the subject available to anyone who is willing to look for it.