Spectrum Analyzer

A spectrum analyzer or spectral analyzer is a device, which is used to calculate the magnitude of an input signal within the full frequency range. The main purpose of the spectrum analyzer is to measure the power of the spectrum of “known” and “unknown” signals. The Electrical signal is used as an input signal in a spectrum analyzer for measurements. However, it can measure other signals such as acoustic pressure waves or optical light waves converted to their electrical signals using transducers.

The spectrum analyzer displays the frequency on the horizontal axis and amplitude on the vertical axis. A spectrum analyzer might look like an oscilloscope to a casual observer. But in reality, it is different except for those laboratory instruments which can operate as an oscilloscope or a spectrum analyzer.

Basic Concept

• The study of the energy distribution of a signal as a function of frequency is called spectrum analysis.
• The Electronic instrument which shows an energy distribution of the signal graphically as a function of frequency on the CRO is called a spectrum analyzer.
• This study gives information about the

1. Bandwidth
2. The type of modulation
3. Spurious signal generation
4. Modulation index
5. Attenuation
6. The component levels

• In the case of the normal oscilloscope, the signal is plotted as a function of time i.e. amplitudes versus time plot is obtained. Such an analysis is called time-domain analysis, whereas, in the case of the spectrum analyzer, the signal is plotted as a function of frequency. This analysis is called frequency domain analysis.
• The spectrum analysis is divided into two groups:
• Audio frequency (AF) analysis
• Radiofrequency (RF) analysis
• The RF spectrum analysis covers a frequency range of 10 MHz to 40 GHz and hence is more important because it includes the majority of communication, navigation, radar, and industrial instrumentation frequency bands.

Block Diagram of Spectrum Analyzer

The spectrum analyzer is an instrument that brings together a superheterodyne radio receiver with a swept frequency local oscillator and an oscilloscope to present a display of amplitude versus frequency. The block diagram consists of:

1. Input Mixer
2. Sweep Oscillator
3. Filter
4. Detector
5. Display (CRT)

• The above Figure shows a simplified block diagram of a spectrum analyzer.
• Normally, for RF or microwave signal analysis, the swept technique is preferred.
• The main function of the spectrum analyzer is to obtain the amplitude versus frequency plot from the frequency spectrum under the test.
• They can be classified as a scanning type and non-scanning type. The scanning type spectrum analyzers use swept technique while the non-scanning type is called real-time spectrum analyzers.
• The saw-tooth generator generates the saw-tooth waveform. This saw-tooth waveform is applied to the horizontal plates of the CRO.
• The saw-tooth signal is also applied to the voltage tuned local oscillator. This acts as a frequency-controlled element of the local oscillator.
• When the saw-tooth signal is applied to the voltage tuned local oscillator, its frequency changes from f_min to f_max.
• The RF input signal is applied to the mixer. The output of the voltage tuned oscillator is used to beat with the input signal in order to produce intermediate frequency. This IF component is produced when the corresponding component is present in the input signal.
• The resulting IF signal is applied to the detector and video amplifier. The IF corresponding to the component is amplified and detected and then it is applied to the vertical deflecting plates of the CRO, producing a plot of amplitude versus frequency.

Real-Time Spectrum Analyzer

• The real-time analyzer is of non-scanning type.
• Its function is to present the effect of change in all input frequencies on its spectrum display instantly.
• The real-time spectrum analyzer is not used at RF or microwave frequencies.
• The real-time analyzer is used at audio frequencies.
• It consists of 24 channels. It converts the range from 50 Hz to 10kHz.
• Each channel consists of a bandpass filter and an RMS detector as a number of bandpass filters are used; it is called a multifilter real-time spectrum analyzer.

• The electronic switch is used to scan the output of each filter sequentially. The output from the electronic switch output scanner is given to the vertical deflecting plates of the CRO.
• The scan generator provides a saw-tooth output. This output is synchronized with the electronic switch. The output from the scan generator is given to the horizontal deflecting plates of the CRO.
• In this analyzer, the composite amplitude of the signal within the bandwidth of each filter is displayed as a function of the frequency range of the filter.

Characteristics of Spectrum Analyzer

Frequency resolution and bandwidth

• Frequency resolution is defined as the ability of the spectrum analyzer to separate the signals which are closely spaced in frequency.
• The resolution of the spectrum analyzer is limited by its narrowest IF bandwidth e.g. if the narrowest IF bandwidth is 10 kHz then the nearest any two signals can be resolved is 10 kHz.

Sweep desensitization

• This effect caused in the spectrum analyzer is due to too much scanning of a signal. As a result of desensitization, there is a loss in amplitude, calibration, sensitivity, and resolution. This effect can be detected easily and it can be corrected.
• In order to avoid this problem, care should be taken so that the scan velocity does not exceed 3 dB bandwidth of IF filter in Hz.

Sensitivity

• Sensitivity is the ability of the spectrum analyzer to measure small signals such as those which are determined by their own internally generated noise.
• The noise figure varies from 25 dB at low frequency to 40 dB at 12 GHz.
• From the noise figures and thermal noise levels in various bandwidths, the sensitivity of the spectrum analyzer can be determined.

Dynamic Range

• It is the ability of the spectrum analyzer to display the true spectra of small and large signals simultaneously.
• It is also defined as the ratio of signal level to noise level, at the signal levels where spurious distortion products start to become visible above the noise level of display.
• The dynamic range of spectrum analyzer generally various from 60 dB to 70 dB.

Application

1. It is used to check the spectral purity of signal sources.
2. For evaluation of local electromagnetic interference (EMI) problems.
3. Do site surveys prior to installing radio receiving or transmitting equipment.
4. Test transmitters.
5. Analyze signatures.
6. For tuning a parametric amplifier.
7. The measurement of the antenna pattern.
8. RF interface testing.

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