Filtering AC signals

How to use this resource

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Below is an interactive which supports the resource. The interactive can be launched by clicking on its image, or through link in the PDF itself.

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Topic areas

Electrical and electronic engineering

  • AC theory
  • Resistance, reactance and impedance
  • Potential divider in an AC circuit
  • Low pass and high pass filters


  • Vector addition
  • Phasor multiplication and division
  • Pythagoras’ theorem
  • Trigonometry


It may be useful to look at the resource ‘AC phasors and fault detection’ to introduce phasor diagrams and the relationship between resistance, reactance and impedance.

Problem statement

An alternating current is not necessarily associated solely with a power supply. Analogue signals from sensors and instruments can also be thought of as being AC signals, although, in contrast to a power supply, the frequency usually changes with time. An example is the recording and playing back of sounds using a microphone and speaker.

However, sometimes such signals are ‘contaminated’ with unwanted components, such as high-frequency microphone ‘hiss’, or low frequency mains ‘hum’, which you want to filter out to improve the sound quality. Another example could be that when playing back sounds, you want to select and direct low frequency sounds to a large bass speaker and higher frequency sounds to smaller ‘tweeter’ speakers to take advantage of the performance characteristics of each.

Yet another example is in the detection and decoding of radio communication signals, where the receiver selects a relatively narrow range of radio frequencies, then, in general, filters out the radio carrier signal to recover the original sound or data signal.

How can you select signals based on their frequency?


Click the image to load Filter 1. 

The above interactive shows the effects on a low and high pass filter based on changes of frequency, resistance and capacitance.

Supported by

  • Royal Academy of Engineering
  • Motorola Solutions Foundation