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Some resources have pre-requisites; where this is the case, you'll be able to access any required resources through the Pre-requisites section.

- 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

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?

Resources

**Electrical & Electronic:** AC theory | Resistance, reactance and impedance

**Mathematics:** Phasors | Vector addition | Pythagoras' theorem | Trigonometry

**Prerequisites:** None

Resources

**Electrical & Electronic:** AC true, reactive and apparent power | AC power factor

**Mathematics:** Vector addition | Pythagoras' theorem | Trigonometry

**Prerequisites:** AC phasors and fault detection

Resources

**Electrical & Electronic: **Ohm's law | Electrical power | Kirchhoff's current law (1st law) | Kirchhoff's voltage law (2nd law) | Resistors in series and parallel

**Mathematics:** Simultaneous equations

**Prerequisites:** None

Resources

**Electrical & Electronic:** Voltmeter | Waveforms | Amplitude | Time period | Frequency | Instantaneous values | Peak-to-peak value | Peak value | Combining waveforms | Root mean square (rms) value | Phase angle | Form factor | Average value

**Mathematics: ** Square and square roots | Basic arithmetic

**Prerequisites: ** None

Resources

**Electrical & Electronic:** The solenoid | Magnetic flux | Magnetic flux density | Magnetomotive force | Field strength | Permeability

**Prerequisites:** None