Spectrum Analyzer Use, Types and Operation

A spectrum analyzer or spectral analyzer is a device used to examine the spectral composition of some electrical, acoustic, or optical waveforms. It may also measure the power spectrum. Present-day spectrum analyzers can measure segments of the frequency spectra from 0 hertz to as high as 300 gigahertz when used with waveguide mixers.

Analog Spectrum Analyzers:
An analog spectrum analyzer uses either a variable band-pass filter whose mid-frequency is automatically tuned (shifted, swept) through the range of frequencies of which the spectrum is to be measured or a superheterodyne receiver where the local oscillator is swept through a range of frequencies.

Digital Spectrum Analyzers:
A digital spectrum analyzer computes the discrete Fourier transform (DFT), a mathematical process that transforms a waveform into the components of its frequency spectrum.

Some spectrum analyzers use a hybrid technique where the incoming signal is first down-converted to a lower frequency using superheterodyne techniques and then analyzed using fast fourier transformation (FFT) techniques.

Spectrum Analyzer Operation:
Usually, a spectrum analyzer displays a power spectrum over a given frequency range in real time, changing the display as the properties of the signal change. There is a trade-off between how quickly the display can be updated and the frequency resolution, which is for example relevant for distinguishing frequency components that are close together. With a digital spectrum analyzer, the frequency resolution is Δν = 1 / T, the inverse of the time T over which the waveform is measured and Fourier transformed. With an analog spectrum analyzer, it is dependent on the bandwidth setting of the bandpass filter. However, an analog spectrum analyzer will not produce meaningful results if the filter bandwidth (in Hz) is smaller than the square root of the sweep speed (in Hz/s), which means that an analog spectrum analyzer can never beat a digital one in terms of frequency resolution for a given acquisition time. Choosing a wider bandpass filter will improve the signal-to-noise ratio at the expense of a decreased frequency resolution.

With Fourier transform analysis in a digital spectrum analyzer, it is necessary to sample the input signal with a sampling frequency vs. that is at least twice the highest frequency that is present in the signal, due to the Nyquist limit. A Fourier transform will then produce a spectrum containing all frequencies from zero to νs / 2. This can place considerable demands on the required analog-to-digital converter and processing power for the Fourier transform. Often, one is only interested in a narrow frequency range, for example between 88 and 108 MHz, which would require at least a sampling frequency of 216 MHz, not counting the low-pass anti-aliasing filter. In such cases, it can be more economic to first use a superheterodyne receiver to transform the signal to a lower range, such as 8 to 28 MHz, and then sample the signal at 56 MHz. This is how an analog-digital-hybrid spectrum analyzer works.

For use with very weak signals, a pre-amplifier can be used, although harmonic and intermodulation distortion may lead to the creation of new frequency components that were not present in the original signal.

Acoustic Use:
In acoustics, a spectrograph converts a sound wave into a sound spectrogram. The first acoustic spectrograph was developed during World War II at Bell Telephone Laboratories, and was widely used in speech science, acoustic phonetics and audiology research, before eventually being superseded by digital signal processing techniques. Now spectrum analyzers cover the acoustic range.

RF Use:
In telecommunications, for example, spectrum analyzers are used to determine occupied bandwidth and track interference sources. Cell planners use this equipment to determine interference sources in the GSM/TETRA and UMTS technology. Popular RF bands for spectral analysis: Wi-Fi, Industrial Remotes, Wireless Microphone, Business & Emergency Two-Way, Assisted Listening, Telco / Cellular, Intercom, and Radio / TV Broadcast.

Complex Waveforms:
Complex waveforms are divided into two groups, Periodic Waves and Non-Periodic Waves. Periodic waves contain the fundamental frequency and its related harmonics. Non-periodic waves contain a continuous band of frequencies resulting from the repetition period of the fundamental frequency approaching infinity and thereby creating a continuous frequency spectrum.

Modulation Measurements:
In all types of modulation, the carrier is varied in proportion to the instantaneous variations of the modulating waveform. The two basic properties of the carrier available for modulation are the Amplitude Characteristic and Angular (frequency or phase) Characteristic.

Amplitude Modulation:
The modulation energy in an amplitude-modulated wave is contained entirely within the sidebands. For 100% modulation, the total sideband power would be one-half of the carrier power; therefore, each sideband would be 6 dB less than the carrier, or one-fourth of the power of the carrier. Since the carrier component is not changed with AM transmission, the total power in the 100-percent-modulated wave is 50% higher than in the un-modulated carrier. The primary advantage of the log display that is provided by the spectrum analyzer over the linear display provided by the oscilloscopes for percentage of modulation measurements is that the high dynamic range of the spectrum analyzer (up to 70 dB) allows accurate measurements of values as low as 0.06%. It also allows the measurements of low-level distortion of AM signals.

Real-time Spectrum Analyzer Applications include, Radar and Pulse Testing, Radio Communications, Spectrum Management, RFID Measurements and WLAN Measurements.