A spectrum analyzer continuously sweeps the local signal frequency which is mixed in the measuring frequency band of an input signal to sequentially convert respective frequency components in the measuring frequency band into intermediate-frequency signals of certain frequencies and displays their levels on a display screen with the abscissa representing frequency. A spectrum analyzer is essentially a receiver that is tuned or swept across a band of frequencies, and the amplitude of received signals is displayed on a screen as a function of frequency. Analysis of signals in the frequency domain is widely used to obtain physical and electrical system performance information. The displayed portion of the signal spectrum typically includes a number of peaks corresponding to different signal frequencies. The peaks often move over time and change amplitude. Most spectrum analyzers, especially those used for high frequency RF and microwave signals, are implemented as heterodyne receivers that frequency shift or frequency convert the signal prior to detecting and measuring the power of the signal. A typical spectrum analyzer may employ three or four stages of frequency conversion prior to a signal power measurement. Frequency spectrum analyzers are frequently used for analyzing frequency spectrum in an input signal. Such frequency spectrum is displayed on a frequency domain screen with a power level in a vertical direction while a frequency dispersion in a horizontal direction. Typically in such a frequency spectrum analyzer, levels of frequency spectrum are displayed in a vertical direction with respect to a frequency range in a horizontal direction. A frequency spectrum analyzer includes three or more frequency converters connected in series each of which is formed of a frequency mixer, a local oscillator and a band pass filter to produce intermediate frequency (IF) signals without image responses. Audio spectrum analyzers are used for analysis of audio waveforms or signals over a specified frequency range. Typically, this frequency range is 20 Hz to 20 kHz, which is the range of audible signals.

Optical spectrum analyzers are instruments that measure the optical power as a function of wavelength or frequency. Advantages of optical spectrum analyzers are their dynamic range and performing measurements involving many discrete spectral lines. Generally, the measured optical signal and the calibration optical signal both follow the same optical path through the optical spectrum analyzer and occupy the same general region of the optical spectrum. Optical spectrum analyzers typically comprise a tunable filter that bandpass filters an input signal. A detector is used to measure the filtered signal and thereby determine the optical power within the filter's current pass band. A spectrometer spectrally divides the light under measurement by transmitting the components thereof at different, wavelength-by-wavelength angles using a chromatic dispersion device, and detects the light thus spectrally divided by the chromatic dispersion device, using an optical detector. An optical spectrum analyzer uses an output from the optical detector of the spectrometer to measure the wavelengths of optical signals. Optical spectrum analyzers frequently use a diffraction grating for separating the light beam to be analyzed into its component wavelengths. The input light beam to be analyzed is collimated and is directed at the diffraction grating. The light beam is spatially dispersed by the grating, since different wavelengths are diffracted at different angles. In optical spectrum analyzers, the light intensity of a light beam is displayed as a function of wavelength over a predetermined wavelength range. Parameters of importance include wavelength range, wavelength and amplitude accuracy, sensitivity, resolution, measurement speed, polarization insensitivity and dynamic range.

Spectrum analyzers generally fall into two categories: sweep type spectrum analyzers and fast Fourier transform (FFT) based spectrum analyzers. The sweep type spectrum analyzer means a spectrum analyzer of the type in which a local oscillator continuously performs a frequency sweep operation, a frequency spectrum component included in a signal to be measured is converted, by the frequency sweep operations, into an intermediate frequency signal consisting of a constant frequency component, and the power of the intermediate frequency signal is detected and displayed on a screen. Spectrum analyzers of the swept-frequency type are based on the use of a mixer and a variable frequency local oscillator. The input signal to be analyzed is mixed with the output of a sweeping first local oscillator to produce a first intermediate frequency signal. Any incoming signal that mixes with a harmonic frequency of the sweeping first local oscillator produces an analyzer response. Swept spectrum analyzers utilize one or more band pass filters in combination with a tunable mixer to measure the signal amplitude at a given frequency. By sweeping or changing the center frequency received, one may develop a plot of amplitude versus frequency for the signal. However on the other hand, there is a disadvantage in this type of spectrum analyzer that a time length required for one frequency analysis (time length of frequency sweep) must be longer as frequency resolution is made higher. The sweep type spectrum analyzer is non-real time instruments. If the signal contains a frequency component within the sweep window, the power of that frequency component will be displayed. However, because the sweep window is moving with time, if the signal under analysis has a frequency component occurring outside of the sweep window, the component will be missed and therefore not displayed. Sweep type spectrum analyzers are widely utilized to examine the spectra of various types of signal, by directly displaying the relative amplitudes of the signal frequency components.

A fast Fourier transform analyzer (FFT analyzer) analyzes the response characteristic of an electrical device when supplied with a test signal, or the response characteristic of a mechanical device. FFT analyzers are real time spectrum analyzers that overcome the shortcomings of swept frequency instruments by simultaneously monitoring the entire signal band of the signal, usually by means of a bank of parallel frequency filters. Such real time spectrum analyzers employ digital signal processing techniques, which sample and quantize the signal in the time domain. The resulting digitized signal is then processed by means of either a fast Fourier transform (FFT) filter or a finite impulse response (FIR) filter to obtain a frequency response output. FFT based audio spectrum analyzers utilize Fourier transforms to analyze audio signals. The Fourier transform is based on the principal that any signal or waveform can be represented as a combination of sine waves of various frequencies. The Fourier transform involves splitting or decomposing a signal into these component frequencies or sine waves. In the FFT type spectrum analyzer, the oscillation frequency of a local oscillator is changed stepwise, the oscillation frequency in each step is resolved into a spectrum by the FFT transform means, and the Fourier transform results obtained in all of those steps are stored in a memory and are displayed on a display device.