A signal can be analyzed in the time domain, for example, by using a CRO, or in the frequency domain, for example, by using a spectrum analyzer. Time domain analysis has the disadvantage that we cannot see the contribution of individual frequency component of a complex waveform. Signal characteristics that are difficult to see in the time domain frequently become visible in the frequency domainDistortion is a measure of signal impurity. It is usually expressed as a percentage or decibel ratio of the undesired components to the desired components of a signal. Distortion of a device is measured by feeding into it one or more sine waves of various amplitudes and frequencies. In simple terms, frequencies in the output which were not present in the input constitute distortion.HARMONIC DISTORTIONHarmonic distortion results when a voltage at a single frequency f, applied to a nonlinear device or system, creates unwanted voltages at frequencies (harmonics) 2f, 3f, , Nf. The order of the distortion product is given by the frequency multiplier, Individual components of harmonics distortion are defined as follows:

Since it is difficult to find the amplitude of the fundamental alone in the presence of harmonics we use the eqn, The denominator is the amplitude of the signal with all harmonic components.SINAD is a parameter which provides a quantitative measurement of the quality of an audio signal from a communication device. It is the ratio of the total signal power level (desired signal + noise + distortion or SND) to unwanted signal power (noise + distortion or ND). The higher this ratio, the better the quality of the audio signalINTERMODULATION DISTORTIONThis occurs when the nonlinearity of a device or system with multiple input frequencies causes undesired outputs at other frequencies In a communication system, this means that signals in one channel can cause interference with adjacent channels. As the spectrum becomes busier and the channels become more tightly spaced, minimizing intermodulation distortion becomes more important.

DISTORTION ANALYSERThis is the instrument used to measure total harmonic components. It removes the fundamental component of the signal under investigation and measures the remainder. With the switch S1 at Set Level, the band-pass filter is adjusted to the fundamental frequency and attenuator is adjusted to obtain full-scale reading on the meter. The switch position is then changed to Distortion, the notch filter is tuned to the fundamental and attenuator is adjusted again to get full-scale reading on the meter. THD is then calculated from the attenuator position. SINAD can also be measured with this equipment.Any practical signal contains some hum and noise, and the distortion analyzer will include these in the reading. Because of these added components, the correct value measured by the instrument is total harmonic distortion and noise (THD + n).

Additional filters (low pass and high pass) are included on most distortion analyzers to reduce unwanted hum and noiseThe shape of the output waveform from a notch-type analyzer indicates the slope of the nonlinearity. Displaying the residual components on the vertical axis of an oscilloscope and the input signal on the horizontal gives a plot of the transfer characteristics deviation from a best-fit straight line.WAVE ANALYSERThis instrument is used to measure each harmonic component individually. This instrument uses the principle of heterodyne (heterodyne means to mix) where signal of frequency, fsig, is mixed with a local oscillator frequency, fLO, generating frequencies fLO + fsig, fLO fsig and harmonics. A narrow band-pass filter is tuned to pass signal of frequency fLO +fsig to the amplifier which is Then read by the meter. By varying the frequency of the local oscillator, signal amplitudes of fundamental, 2nd harmonic, 3rd harmonic, etc.can be measured individually. Note that each harmonic frequency is converted to a constant frequency, which allows the use of highly selective filters. Since the frequency components are measured one at a time, this is called non-real time analyzer (NRTA). The bandwidth of heterodyne wave analyzer is usually constant. This makes the analysis very difficult if signal frequency does not remain constant during the time required by the instrument for complete analysis

SPECTRUM ANALYSERAn analysis of a complex waveform, in terms of a graphic plot of the amplitude versus frequency, is known as spectrum analysis. Spectrum analysis recognizes the fact that waveforms are composed of the summation of a group of sinusoidal waves, each of an exact frequency and all existing together simultaneously The frequency-domain (amplitude versus frequency) plot is used to observe frequency response

Real-time spectrum analyzers are realized in basic two types:1. Filter bank spectrum analyser 2. Superheterodyne spectrum analyser

FILTER BANK SPECTRUM ANALYSERIt has a bank of fixed tuned filters spread over the frequency range of interest. Each filter passes one frequency. A detector then measures the power level of the signal passing through the filter, producing a dc voltage that drives the vertical plates of the display. An electronic sweep switch selects one output of the detector at a time. As the ramp (sweep) generator sweeps through its frequency range, a trace is drawn across the screen. This trace shows the spectral content of the input signal within a selected range of frequencies.

Superheterodyne spectrum analyserthe input signal is first attenuated, if required. Then it passes through a low-pass (LP) filter. Then it is mixed with a signal from the local oscillator (LO). Because the mixer is a non-linear device, its output includes not only the two original signals but also their harmonics and the sums and differences of the original frequencies and their harmonics. The mixed signal falling within the pass-band of the intermediate-frequency (IF) filter, are amplified (sometimes log amplified to increase the range). The resultant signal is then applied to the vertical plates of a cathode-ray tube (CRT) after it is rectified by the envelope detector. It produces a vertical deflection on the CRT screen (the display). A ramp generator deflects the CRT beam horizontally across the screen from left to right. The ramp also tunes the local oscillator, so that its frequency changes in proportion to the ramp voltage. So we get an X-Y display on the screen.