Lesson Summary 

Define the importance of the filter in the history of electronics.

Filters are an important component for signal processing used in most electronics.  The history of electrical filtering can be traced back to early attempts in the 1880's to filter telegraph signals.  Simple linear filters, not too dissimilar from the RC filter, were the building blocks of telephone technology.   These filters provided a dynamic way to eliminate other telephone calls and only allow one user to speak to another user.  In the modern era most filtering takes place in the digital domain but the core concepts are the same.  Table 1 shows various the frequency bandwidths in which different forms of communication take place. It is through filtering these signals that any electronic communication device is usable. 

 Table 1. Common communication frequency bandwidths


Low Pass Filter - A filter that passes low-frequency signals and attenuates (reduces the amplitude of) signals with frequencies higher than the cutoff frequency.



High Pass Filter - A filter that passes high-frequency signals and attenuates (reduces the amplitude of) signals with frequencies lower than the cutoff frequency.



Band Pass Filter  - A filter that passes frequencies within a certain range and attenuates (reduces the amplitude of) frequencies outside that range.



Bandwidth - The difference between upper and lower frequencies.



Cutoff Frequency - The boundary in a filter at which signal begins to be reduced.



Q Factor - The amplitude and bandwidth of a filter at its cutoff frequency.  

Voltage Controlled Filter (VCF) - An audio filter that allows its cutoff frequency and resonance (Q) to be altered by an external control voltage. A VCF can act as a low-pass, high-pass,  or band-pass filter.