Measuring with Modern Spectrum Analyzers

July 13, 2016

 

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

 

Spectrum analyzers 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 analyzers. The earliest analyzers 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 analyzers were large and cumbersome. Soon, single detector analyzers 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 analyzer was already available commercially.

 

Today, spectrum analyzers combine the frequency performance of the swept and the speed and transient detection capability of the FFT analyzers. There are two types:  Real-Time analyzers & vector signal analyzers. 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 analyzer often uses a frequency mask trigger to detect intermittent or transient errors in a complex RF signal.

 

Measurement Techniques

 

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

  1. Input power – The maximum input RF power of most spectrum analyzers is +30 dBm,  and  0V DC.   Any DC voltage applied to the input connector will severely damage a spectrum analyzer.
  2. Frequency Range and Span – Typically, a center 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 analyzer 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 analyzer can be set to a much narrower span and resolution bandwidth.  FFT analyzers are generally faster than swept spectrum analyzers for narrow frequency spans. 
  4. Markers – Most engineers are familiar with using markers to aid in measurement on an oscilloscope, and spectrum analyzers 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 analyzers contain automated functions that can make these measurements and more, with a simple touch of a button.

 

Conclusion

 

Modern spectrum analyzers encompass a wide variety of capabilities to tackle the most complicated and simplest of RF signals. Analyzers 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 analyzer 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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