Distortion Meter


The distortion menter measures the distortion generated within a circuit, usually an amplifier.

First specify an input and an output node. An input and output node must be chosen, as the distortion meter needs an input to inject a pure signal, which will compare the output’s signal against in calculating the distortion in the output signal.
Second, a test frequency needs to be entered (usually 1kHz for audio circuits). You can also set the Amplitude and offset of the test signal.
The next option is what will the distortion meter measure? A pure signal has no harmonics, as it contains only one frequency, whereas an impure signal can hold many frequencies, which are usually even multiples of the original single frequency. Thus a pure 1kHz signal can gives rise to 2kHz, 3kHz, 4kHz harmonic frequencies. (Non-harmonic frequencies are called “beats,” which are measured with “intermodulation distortion,” IM, meter. Usually, all the added frequencies are higher than the fundamental, pure input signal, but not always. Certain digital circuits, for example, can give rise to sub-harmonic signals.)

The Total Harmonic Distortion (THD) figure is the sum of all the harmonically related signal’s contribution to the signal’s magnitude. Thus, if 0.9 volts of a 1-volt output signal is made up of fundamental frequency and 0.1 volts is made up of the fundamental’s second harmonic frequency, the THD equals 10%. The distortion meter can display THD of a circuit or just one of the first five harmonics.

The option to display the results as a percentage or in dBs is quite straight forward. One percent distortion equals the distortion’s contribution to the output signal being –40dB down in amplitude relative to the output signal. The formula is:
DB = 20Log (distortion signal / output signal)

The distortion meter’s display range should be set to the maximum amount of distortion the user is willing to accept. The meter displays its measurements in two ways: text and a moving bar graph. The bar’s length expands and contracts with the distortion being measured, which makes it an analog readout. When the measured current exceeds the distortion meter’s range, the bar changes color from its normal blue to red. Pressing the “Auto” button overrides the fixed range and auto adjusts the range to twice the highest distortion reading it sees. Pressing the “Setup” button toggles the length of the Distortion Meter, so that when its setup is complete, the Distortion Meter becomes only as tall as its display band.