Wave Analyzers: Function, Types & Applications

The Function of a Wave Analyzer

Generally, a periodic waveform can be represented as a sum of a dc component and a series of sinusoidal harmonics. The analysis of a waveform entails the determination of the values of amplitude, frequency and at times the phase angle of the harmonic components. A wave analyzer is an instrument built to measure relative amplitude of single frequency components in a complex waveform.
Principally, the wave analyzer acts as a frequency selective voltmeter which is tuned to the frequency of one signal while discarding all other signal components. The desired frequency is selected by a frequency calibrated dial to the point of maximum amplitude. The amplitude is indicated either by an appropriate voltmeter or a CRO.

Types of Wave Analyzers

There are two types of wave analyzers, depending on the frequency ranges employed:

• Frequency selective wave analyzer.
• Heterodyne wave analyzer.

Frequency Selective Wave Analyzer

This type is used in audio-frequency (20 Hz to 20 kHz) measurement. It comprises of a narrow band pass filter which can be tuned to the frequency of choice. The block diagram below shows the components that make up this wave analyzer.

The waveform to be analyzed in terms of its separate frequency components is applied to an input attenuator that is set by the meter range switch on the front panel. The driver amplifier feeds the attenuated waveform to a high-Q active filter. The filter comprises of a cascaded arrangement of RC resonant sections and filter amplifiers.

The pass band of the whole filter section is covered in decade steps over the entire audio range by switch capacitors in the RC sections. Close-tolerance polystyrene capacitors are typically used for selecting the frequency ranges. Precision potentiometers are used to tune the filter to any desired frequency within the selection pass band.

The final amplifier stage supplies the selected signal to the meter circuit and to the un-tuned buffer amplifier. The buffer amplifier can be used to drive a recorder or an electronic counter. The meter is driven by an average type detector and normally has several voltage ranges in addition to decibels scale.

Heterodyne Wave Analyzer

This wave analyzer is used to measure the frequency in megahertz range. The components that make up this wave analyzer are illustrated by the block diagram below:

The signal input is fed through an attenuator and amplifier, before being mixed with a local oscillator signal. The frequency of this oscillator is adjusted to give a fixed frequency output which is in the pass band of the amplifier. In the following stage, this signal is mixed with a second crystal oscillator, whose frequency is such that the output from the mixer is centered on zero frequency. The subsequent active filter has a controllable bandwidth, and passes the selected component of the frequency to the indicating meter.

Reliable frequency stability in a wave analyzer is achieved by using frequency synthesizers, which have high accuracy and resolution, or by automatic frequency control where the local oscillator locks to the signal and thus eliminates the drift between them.

Applications of Wave Analyzers

Wave analyzers play an important role in electrical, sound, and vibration measurements. They are used in industries to detect and minimize sound and vibrations generated by rotating machines and equipment. An effective spectrum analysis with a wave analyzer shows the various discrete frequencies and resonances that can be linked to the motion of machines.

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