Agilent - Achieving Amplitude Accuracy in Modern Spectrum Analyzers

Our thanks to Agilent Technologies for allowing us to reprint the following article.
By Joe Gorin, Master Engineer Signal Analysis Division R&D, Agilent Technologies

Spectrum analyzers are among the most versatile of RF/microwave measurement tools, with signal power among the most common measurement made with the instruments. Traditionally, the combination of a power meter and power sensor has been the measurement toolof choice for its well-characterized traceability path back to reference standards at national standards laboratories. But modern spectrum analyzers have made dramatic improvements in amplitude accuracy, with levels approaching (but not exceeding) the power meter and sensor. Understanding the error terms associated with a spectrum analyzer's relative and absolute amplitude accuracies can help an engineer interpret the analyzer's specifications when selecting a measurement tool and balancing price/performance tradeoffs.


The earliest spectrum analyzers were fully analog, and even an operator's skill in reading and recording the measurement results impacted amplitude accuracy. The first digital display spectrum analyzers were introduced in the 1970s, with the HP 8566A and HP 8568A models from Hewlett-Packard Co. (now Agilent Technologies) among the most accurate and popular instruments of that time. They featured digital displays and digital marker readouts for improved accuracy.


The signal at the end of the analog chain was the filtered (and unipolar) result of detecting the intermediate frequency (IF) signal level and therefore directly proportional to the amplitude of the signal in the selected resolution bandwidth (Fig. 1). It was called the "video" signal because it had driven the Y-axis video deflection plates in previous all-analog spectrum analyzers. As the block diagram shows, the analog signal was processed by an analog-to-digital converter (ADC) for storage and display without using cathode-ray tube (CRT) persistence, allowing for communication with remote users. Because of this architecture, and the fact that a user no longer needed to interpret the results on the display, improved accuracy was possible compared to the all-analog spectrum analyzer architecture...



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