Tips For Making Better Spectrum Measurements

These eight tips will help users of RF/microwave spectrum analyzers improve the accuracy of their measurements.

Spectrum analyzers are workhorse instruments for high-frequency applications, called upon to monitor occupied bandwidths, measure frequency and power, and essentially function as a complete RF/microwave test system in a box. Nevertheless, drawing the best performance and measurement results from an RF/microwave spectrum analyzer requires knowledge and experience. And some of the lessons required can be found in Application Note 1286-1 from Agilent Technologies, “8 Hints for Better Spectrum Analysis.” The 12-page note, which includes a tutorial lesson on setting up a spectrum analyzer for its first series of measurements, is available for free download from the company.

Spectrum analyzers draw from the time-honored superheterodyne receiver architecture fueled by a high-performance local oscillator (LO) to make measurements on signals across wide dynamic ranges and broad frequency ranges. Understanding how to optimize those measurements is often a matter of properly selecting the analyzer’s filters, such as its intermediate-frequency (IF) filters and its resolution-bandwidth (RBW) filters. As the application note points out, the basic receiver block diagram in a modern spectrum analyzer has been augmented in recent years by generous amounts of digital-signal-processing (DSP) components to help simplify, speed, and improve the accuracy of those measurements.

The application note’s advice starts with selecting the best RBW filter for a given measurement, noting the tradeoff between the close-up precision and low-level measurement capabilities possible with narrow RBW filters and the slower measurement speeds required for the narrower filters. The note provides examples of different measurements measuring—for example, a 200-MHz span with a sample detector—and showing the effects of switching between 3- and 10-kHz RBW filters.

Another important bit of advice has to do with improving measurement accuracy, in reference to both the amplitude and the frequency accuracy of a measurement. Because each device under test (DUT) will differ somewhat as viewed by a spectrum analyzer, understanding the interactions of the DUT with the measurement setup are important for establishing calibrated conditions prior to a measurement.

Many spectrum analyzers, for example, provide some form of built-in amplitude correction function that is set up prior to a spectrum-analyzer measurement with the aid of an RF/microwave power meter. The use of a power meter can remove amplitude variations from a test setup and can provide significant differences (especially at the higher frequencies) in a test setup.

Those faced with using a spectrum analyzer for low-level signals will find the application note of particular use when they follow the advice from the section on improving low-level sensitivity. The sensitivity of any analyzer to low-level signals will be limited by the noise of the analyzer itself, and that noise is greatly dependent on the instrument’s settings. Such settings include the way the RBW filter is selected, the amount of input attenuation, and whether or not a preamplifier is used with the measurement. The application note reviews a number of different exercises that can be applied to lower a spectrum analyzer’s displayed average noise level (DANL) and reveal a low-level signal of interest.

In addition to these hints, the 12-page note includes useful advice on finding sources of distortion within the analyzer, optimizing the dynamic range, and even increasing the measurement speed of the instrument for a given test. Copies are available for free download in PDF form from the company’s website.

Agilent Technologies, 5301 Stevens Creek Blvd., Santa Clara, CA 95051; (877) 424-4536, (408) 345-8886,

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