Fundamental Measurement Techniques: Spectrum Analysis

Fundamental Measurement Techniques: Spectrum Analysis

Published: 20th October 2014

Part 2 of our new occasional series around ‘Fundamental measurement techniques’ takes a look at the basics of spectrum analysis and two types of analyser.

Here we are looking at frequency domain measurements with an analyser which in its most basic form sweeps a filter across a frequency range so providing a graphical display of the spectral content. The spectrum analyser provides a measure of the magnitude of an input signal versus frequency within the full frequency range of the instrument. The primary use is to measure the power of the spectrum of known and unknown signals.

The display of a spectrum analyser has frequency on the horizontal axis and amplitude displayed on the vertical axis.

Choice

When choosing a spectrum analyser the following specifications are important when making a selection:

  • Frequency Range
  • Frequency and Amplitude Accuracy
  • Frequency Resolution
  • Sensitivity- the smallest signal that can be measured. Getting the Best Sensitivity Requires balancing three settings -Narrowest resolution bandwidth; Minimum RF attenuation; sufficient video filter to smooth noise
  • Distortion- Remember that a spectrum analyser generates and amplifies noise just like any active circuit and that mixers also generate distortion
  • Dynamic Range is the optimum amplitude difference between large and small signals. Your spectrum analyser’s dynamic range is dependent upon: Internal second and/or third order distortion; Displayed noise level; Noise sidebands when close to large signals

To be fully confident in your measurement remember to adjust the measurement procedure for your specific application; test for internal distortion and take sideband noise into account.

Tuned Analysers measure the spectra of electrical signals; dominant frequency; power; distortion; harmonics; bandwidth and other spectral components of a signal in the frequency domain. These parameters are useful in the characterisation of electronic devices, such as wireless transmitters.

Vector signal analysers (VSAs) are also available. VSAs combine sweep methodology with high speed ADCs and other DSP technologies to offer fast, high-resolution spectrum measurements, demodulation, and advanced time-domain analysis. Their primary use is in-channel measurements, such as error vector magnitude and so characterising complex signals such as burst, transient, or modulated signals used in communications, video, broadcast, sonar, and ultrasound imaging applications. VSAs measure the magnitude and phase of the input signal at a single frequency.

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