Measuring with Modern Spectrum Analysers

August 5, 2016

In today's world of cellular communication and high-speed wireless internet, measuring signal behaviour in these devices is more important than ever. Modern spectrum analysers use sophisticated digital signal processing to test devices that rely on complex modulations.

 

Spectrum analysers were developed in the late 1950s and early 1960s. This was before digital electronics became commonplace, so it's no surprise they took the form of swept frequency analysers. The earliest analysers used an array of detectors to capture signals in real time, but also used multiple detection circuits. Using the electronics of the day, that meant analysers were large and cumbersome. Soon, single detector analysers were developed by using heterodyning techniques.

 

Although Fast Fourier transform-like techniques were developed as early as 1805 by Carl Gauss. The technique lay hidden until mathematicians James Cooley and John Tukey developed a similar method we now know as the Fast Fourier Transform, which they published in 1965. The power inherent in the Cooley-Tukey FFT algorithm was quickly recognized, and by 1967 the first FFT frequency analyser was already available commercially.

 

Today, spectrum analysers combine the frequency performance of the swept and the speed and transient detection capability of the FFT analysers. There are two types:  Real-Time analysers & vector signal analysers. The main difference between the two is a real-time capability in the digital signal processing. Both store incoming waveforms before passing the signal through the FFT algorithm. The Real-Time analyser often uses a frequency mask trigger to detect intermittent or transient errors in a complex RF signal.

 

Measurement Techniques

 

When choosing a spectrum analyser for your application, consider the following:

  1. Input power – The maximum input RF power of most spectrum analysers is +30 dBm, and 0V DC.   Any DC voltage applied to the input connector will severely damage a spectrum analyser.
  2. Frequency Range and Span – Typically, a centre Frequency is selected and the Span setting around that frequency will determine the Start and Stop frequencies.  Your signal should always be within the calibrated scale of the display screen for maximum accuracy.
  3. Sweep Time – If a swept spectrum analyser is in use, sweep time is another important variable. Too fast a sweep rate can miss important details, whereas too slow a scan rate may result in unacceptable measurement times.  Sometimes, it’s necessary to compromise. As a work-around, often a quick sweep over a wide frequency span can be used to isolate an area of interest. Once the frequency band of the area of interest is isolated, the spectrum analyser can be set to a much narrower span and resolution bandwidth.  FFT analysers are generally faster than swept spectrum analysers for narrow frequency spans. 
  4. Markers – Most engineers are familiar with using markers to aid in measurement on an oscilloscope, and spectrum analysers are also equipped with markers for the same reason.  Markers typically display the frequency and RF power of the signal being measured and can perform functions such as Peak value, delta values, etc.  Make use of them to ensure the best possible measurement accuracy.
  5. Expanded measurement features – Phase Noise and Noise Figure (or noise factor) are key issues that can affect the quality of the final end product, if not detected, measured and addressed in the design stage. Fortunately, modern spectrum analysers contain automated functions that can make these measurements and more, with a simple touch of a button.

 

Conclusion

 

Modern spectrum analysers encompass a wide variety of capabilities to tackle the most complicated and simplest of RF signals. Analysers exist in all price ranges, too. Newer models will cost low four figures for basic models, and the most sophisticated, leading-edge technology will run well into five figures.  Older models often can be an attractive alternative and provide similar specifications and functionality. There are even handheld models that can fit inside of a pocket and costs less than benchtop instruments.

Electro Rent carries the right kind of spectrum analyser for your application: various types, models and brands. Our measurement experts will help you decide the best fit instrument and the best method to get that instrument in your hands, utilizing rent options, finance alternatives and options such as selling equipment from our internal X-inventory pool.

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