Q. What is a wave analyzer? Explain any one technique of wave analyzer giving its application also.
Write short note on HETERODYNE Wave analyzer.
Explain the construction and working of heterodyne wave analyes.
Sol. A wave analyzer is an instrument designed to measure relative amplitudes of single frequency components in a complex waveform. Basically, the instrument acts as a frequency selective voltemeter which is tuned to tee frequency of one signal while rejecting 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 a suitable voltmeter or a CRO.
There are two types of wave analyzers, depending upon the frequency ranges used: (i) frequency selective wave analyzer, (ii) Heterodyne wave analyzer.
Frequency Selective Wave Analyzer: This wave analyzer is used for measurements in the audio frequency range.
The analyzer has a filter section with a very narrow passband which can be tuned to the frequency of interest. The block diagram of this wave analyzer is shown.
The waveform to be analyzed in terms of its separate frequency components is applied to an input attenuator that is set buy the meter range switch on the front panel. A driver amplifier feeds the attenuated waveform to a high-Q active filter. The filter consists of a cascaded arrangement of RC resonant sections and filter amplifiers. The passband of the total filet section is covered in decade steps over the entire audio range by switching capacitors in the RC sections. Close tolerance polystyrene capacitors are generally used for selecting the frequency ranges. Precision potentiometers are used to tune the filter to any desired frequency within the selection passband.
A final amplifier stage supplies the selected signal to the mete circuit and to an unturned 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 usually has several voltage ranges as well as a decibel scale.
The bandwidth of the instrument is very narrow, typically about I percent of the selected frequency.
The input signal is fed through an attenuator and amplifier before being mixed with a local oscillator. The frequency of this oscillator is adjusted to give a fixed frequency output which is in the passband of the if amplifier. This signal is then mixed with a second crystal controlled 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.
Good frequency stability in a wave analyzer is obtained by using frequency synthesizers which have high accuracy and resolution or by automatic frequency control in an AFC system, the local oscillator locks to the signal, so eliminates the drift between them.
Applications of Wave Analyzers: wave analyzers have very important applications in the following fields
(i) Electrical measurements
(ii) Sound measurements
(iii) Vibration measurements.
The wave analyzers are applied industrially in the field of reduction of sound and vibrations generated by rotating electrical machines and apparatus. The source of noise and vibration is first identified by wave analyzers before it can be reduced or eliminated. A fine spectrum analysis with a wave analyze shows various discrete frequencies and resonances that can be related to the motion of machines. Once, these sources of sound and vibrations are detected with the help of wave analyzers, ways and means can be found to eliminate them.