Distortion Meter: Function, Components & Controls

The Function of a Distortion Meter

A distortion meter is used to measure the total harmonic distortion content in an input waveform. The rms level of the distortion is usually measured as a percentage of the rms level of the complete waveform. Alternatively, a decibel measurement may be employed.

A sine wave input to an electronic circuit may produce an output wave that is distorted, instead of being purely sinusoidal. Irrespective of how severe the distortion may be, it can be demonstrated that all repetitive waveforms consist of a fundamental frequency component and a number of harmonics. The fundamental is a sine wave with the same frequency (f) as the distorted repetitive wave. The harmonics are sine waves with frequency that are multiples of the fundamental frequency, the second harmonic has a frequency 2f, the third harmonic frequency is 3f, and so on.

The distortion in a waveform can be measured in terms of its harmonic content. One typical technique of determining the harmonic content is to suppress the fundamental component and measure the rms value of the combined harmonics. The total harmonic distortions (THD) may then be expressed as a percentage of the rms value of the complete waveform, that is, fundamentally and harmonics. Alternatively, a decibel scale may be used to indicate the distortion content.

Distortion Meter Components and Operation

Rejection Amplifier

The basic component of a fundamental-suppression distortion meter is a notch filter. The filter must heavily attenuate the fundamental frequency component of the input waveform, while passing all the harmonics with no alteration in amplitude or phase. Its suppression frequency must adjustable over a wide frequency range.

The rejection amplifier in figure 1.2 below performs the function of a notch filter. It has two stages of amplification, identified as preamplifier and bridge amplifier, and a Wein bridge circuit between the two. Negative feedback is provided from the output of the bridge amplifier to the input stage of the preamplifier. The two amplification stages produce a high open-loop gain and the negative feedback stabilizes the closed loop gain at approximately 1 dB.

The Wien Bridge balances at only one frequency, giving a minimum null-detector voltage at that frequency. In the rejection amplifier, the bridge output is taken from the null detector terminals, and the input (from the preamplifier) is applied to the bridge supply terminals. The frequency-dependent components of the bridge may be adjusted for any desired fundamental frequency. When the bridge is balanced, the fundamental frequency output to the bridge amplifier is attenuated by approximately 80 dB. All harmonics of the fundamental are passed without any added attenuation or distortion.

Distortion Meter Block Diagram and Controls

The block diagram of a distortion meter is shown below:

In reference to the above figure, the input waveform passes via the 1 MΩ attenuator and the impedance converter before arriving at the rejection amplifier.  The attenuator reduces the signal amplitude to a suitable level for processing and the impedance converter is simply a unity-gain amplifier for interfacing the attenuator and rejection amplifier. Attenuations of up to 50 dB can be selected in 10 dB steps by means of the sensitivity control on the instrument panel. The Vernier knob (sensitivity vernier) provides the adjustment of signal attenuation. The post attenuator following the rejection amplifier is simply a meter range selector. It is controlled by the Meter Range switch on the front panel, which also controls the 1:1 and 1000:1 attenuator.

S₁ is a five-wafer rotary switch: five switches ganged to be controlled by one knob. The knob is identified as Function on the front panel, and it has three positions: Voltmeter, Set Level and Distortion. When S₁ is set at Voltmeter (position 1), the 1:1 and 1000:1 attenuator is switched into the circuit, the rejection amplifier is bypassed, and the complete input waveform (fundamental plus harmonics) is applied to the meter. In this condition, the instrument functions as an AC voltmeter.

When S₁ is in the Set Level position, the input wave passes to the rejection amplifier via the 1 MΩ attenuator and the sensitivity vernier. For this condition of S₁, one of the (null-detector) output terminals of the bridge is grounded (check figure 1.2 above), shorting-out frequency dependent components of the bridge. As a result, only the output from the junction of R₁ and R₂ is passed to the bridge amplifier and the meter circuits. This is the complete input waveform only slightly attenuated by R₁ and R₂. This input sensitivity control can now be adjusted to give a convenient meter reading representing the rms level of the waveform.

Setting S₁ to its third position removes the ground from the bridge (check figure 1.2 above), causing the fundamental frequency to be suppressed.  As a result, only the harmonics passes through to the meter. The rms level of the harmonics can now be measured in relation to the deflection produced by the complete waveform.

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