Illustrated Dictionary of Engineering Terms
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Distortion. Any unwanted change between an input signal and output signal. An un-desired change between a transmitted signal and the received signal. A common example shown below is frequency distortion. A handy trouble-shooting technique to show the distortion is to input a square-wave to the circuit and observe the resulting output side-by-side on an oscilloscope.

High Frequency Distortion

High frequency distortion is easily shown using a square wave. The leading or rising edge of a square wave is a fast rising step signal increasing from 0 to 100 volts [in this example]. The falling edge normally changes at the same rate. In digital electronics the rise time is in the order of a few nano-seconds which translates to a very high frequency.
High frequency distortion is shown by a reduction of slowing of the rise [or fall] time. In this case a rounding of the rising edge of the square wave.

High Frequency Distortion of a Square Wave
High Frequency Distortion

The pulse times or duration shown in the graph is not relevant to the high frequency distortion. However the greater the distortion the smaller the pulse width becomes, as the rise and fall times increase. The high frequency distortion could be such that the rise and fall times become so long as to take up the entire time of the duration of the pulse. Also the rise time may appear different because the circuit uses different components, so it may be normal to have a rise time different than a pulses fall time.

Low Frequency Distortion

Low frequency distortion is also easily shown using a square wave as an input. The pulse should be flat during the transition between rising and falling edges. However in the presence of low frequency distortion the pulse is attenuated.
Although not necessarily clear this is low frequency distortion because the top of the pulse is steady or flat, therefore a low frequency. The pulse does not change in voltage level or frequency.

Low Frequency Distortion of a Square Wave
Low Frequency Distortion

Note that Low frequency distortion is not the same thing as Pulse Droop, although pulse droop could be a form of distortion.

Related Distortion Terms.

Amplitude Distortion: The difference between the signal applied to the input of a system and the resultant signal at the output of the system. The distortion created in a non-linear system, or caused by a network with a non-linear transfer function.

Attenuation Distortion: Unwanted changes in the gain or attenuation of a signal which occurs with changes in frequency.

Delay Distortion: Frequency depended signal distortion caused by the varying delay of individual components of a waveform.

Harmonic Distortion: A signal having an un-desired harmonic added to signal that was never present in the original signal.

Phase Distortion: A change in phase of a signal that does not correspond to a change in frequency.

Notes on the square wave diagrams;
In the case of High Frequency distorted square wave that output would appear the same if the circuit were driving a capacitive load, a load having a high capacitance component. The slow rise time being caused by the capacitor charging up to the maximum voltage. However, there does not have to be an actual capacitor in the circuit to cause a slow rise time. The capacitor could be the capacitance of a long trace on a printed wiring board, the the capacitance of a cable run being used to carry the signal.

The Low High Frequency distorted square wave could be caused by a capacitor slowly discharging, leakage current out of the driver or the load demand changing.

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