Passive electronic filters perform similar function as other types of filters such as water filters or oil filters. Like a water filter that removes unwanted particles from water, a passive electronic filter removes unwanted frequencies from electronic signals.
Unwanted electronic signals can enter into a circuit from any number of places. The power line that connects different electronic devices can be one source. Also, an electronic signal sent through the air, such as a cellular phone signal, is another source.
Frequency is an electronic signal often exhibits a pattern that repeats itself over a specific time interval. This repetition rate is known as the frequency of the electronic signal. Electronic signals often exhibit a pattern that is defined mathematically as a sine wave. If you examine the sine wave, it has a peak positive voltage level and a peak negative voltage level at specific and repeating intervals in time. The higher the frequency of the sine wave, the more positive peaks and negative peaks will occur over a given time frame.
Frequency is measured in Hertz. A Hertz is equal to one cycle per second. A sine wave with a frequency of one Hertz will have one positive peak and negative peak every second. A sine wave with a frequency of 10 Hertz will have 10 positive peaks and 10 negative peaks every second.
A sound can be described as a sine wave at a specific frequency. The higher the frequency of the sound the higher the pitch of the sound. A deep voice has a lower frequency than a shrill voice, also known as a high pitched voice. A whistle a has higher frequency than a car muffler.
When a filter receives an electronic signal it will either let the signal through or block it, just like a water filter or light filter. The factor that determines if the signal will be passed or blocked is its bandwidth. The bandwidth is the frequency range of the electronic signal that the filter will pass through. Any electronic signal that has a frequency outside of the filter's frequency range will not pass through the filter.
All filters are designed to have one or two cut-off frequencies. The cut-off frequencies represent the minimum and maximum frequencies that the filter will pass through.
There are three basic types of filters: the low-pass filter, the band-pass filter and the high-pass filter. A low-pass filter will only let pass electronic signals that have a frequency below its cut-off. A band-pass filter will only let pass electronic signals that have a frequency between the lower cut-off frequency and the upper cut-off frequency. A high-pass filter will only let pass electronic signals that a frequency above its cut-off.