Q. Explain with the help of a block diagram the working of harmonic distortion analyzer.
Write short note on Harmonic distortion analyzer.
Sol. Several methods have been devised to measure the harmonic distortion caused either by a single harmonic or by the sum of all the harmonics.
Tuned-circuit Harmonic Analyzer: One of the oldest methods of determining the harmonic content of a waveform uses a tuned circuit as I a series resonant circuit, consisting of inductor and capacitor which is tuned to a specific harmonic component is transformer coupled to the input of an amplifier. The output of the amplifier is rectified and applied to a meter circuit. After reding is obtained on the meter, the resonant circuit is retuned to another harmonic frequency and the next reading is taken and so on. The parallel resonant circuit consisting of and provides compensation for the variation in the ac resistance of the series resonant circuit and also for the variations in the amplifier gain over the frequency range of the instrument.
Although numerous modifications of this basic circuit have been developed, tuned-circuit analyzers generally have two major drawbacks: At low frequencies very large values for L and C are required and their physical size becomes rather impractical. Harmonics of the signals frequency are often very close in frequency so that it becomes extremely difficult to distinguish between them. Some circuit refinements can lessen this problem and the analyzer does find useful application, whenever it is important to measure each harmonic component individually rather than to take a single reading for the total harmonic distortion.
Heterodyne Harmonic Analyzer or Wave meter: The difficulties of the tuned circuit are overcome in the heterodyne analyzer by using a highly selective, fixed frequency filter.
The simplified block diagram shows the basic functional sections of the heterodyne harmonic analyzer. The output of a variable frequency oscillator is mixed (heterodyned) successfully with each harmonic of the input single and either the sum or the difference frequency is made equal to the frequency of the filter. Since now each harmonic frequency is converted to a a constant frequency, it is possible to use highly selective filters of the quartz-crystal type. With this technique, only the constant-frequency signal, corresponding to the particular harmonic being measured, is passed and delivered to a metering circuit. The mixer usually consists of a balanced modulator since it offers a simple means of eliminating the original frequency of the harmonic. The low harmonic distortion generated by the balanced modulator is another advantage over different types of mixers. Excellent selectivity is obtained by using quartz-crystal filters or inverse feedback filters.
On some heterodyne analyzers the meter reading is calibrated directly in terms of voltage, other analyzers compare the harmonics of the impressed signal with a reference voltage, usually by making the reference voltage equal to the amplitude of the fundamental. Direct reading instruments of the heterodyne type are sometimes known as frequency-selective voltmeters. In these instruments the frequency of the input signal is read off by a calibrated dial. A low-pass filter in the input circuit excludes the sum of the mixed frequencies and passes only the difference frequency. This voltage is compared to the input signal and read off on a calibrated voltmeter I dBm and volts. The level range for most of these meters is from -90 dBm to +32 dBm.