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Spurious Measurements: Making the Best of a Tedious Situation

Blog Post created by benz on Oct 13, 2016

Originally posted Feb 2, 2015

 

Sometimes I need to be reminded to take my own advice

Recently, I’ve been looking into measuring spurious signals and the possibility of using periodic calibration results to improve productivity. I’ll share more about that in a future post, but for now it seemed useful to summarize what I’ve learned—or re-learned—about new and traditional ways to measure spurs.

Spur measurements can be especially time-consuming because they’re usually made over wide frequency ranges and require high sensitivity. Unlike harmonics, spur locations are typically not known beforehand so the only choice is to sweep across wide spans using narrow resolution bandwidths (RBWs) to reduce the analysis noise floor. With spurs near that noise floor, getting the required accuracy and repeatability can be a slow, tedious job.

An engineer experienced in optimizing these measurements reminded me of advice I’ve heard—and shared—before: Don’t measure where you don’t need to, and don’t make measurements that don’t matter.

The first “don’t” is self-explanatory. The frequency spectrum is wide, but the important region is mercifully much narrower—and we should enjoy every possible respite from tedium.

The second “don’t” is less obvious. It’s a reminder to begin with a careful look at the DUT and which measurements are required, and how good those measurements need to be. For example:

  • Do you need to measure specific spur frequencies and amplitudes, or is a limit test sufficient?
  • How much accuracy and variance are acceptable? What noise floor or signal/noise and averaging are needed to achieve this?
  • Are the potential spurs CW? Modulated? Impulsive?

The answers will help you define an efficient test plan and select the features in a signal analyzer that dramatically improve spur measurements.

One especially useful feature is the spurious measurement application. It allows you to build a custom set of frequency ranges, each with optimized settings for RBW, filtering, detectors, etc. You measure only where needed, as shown below.

With the measurement application, you can set up multiple analysis ranges and optimize the settings for each. Measurements are made automatically, with pass/fail limit testing.

With the measurement application, you can set up multiple analysis ranges and optimize the settings for each. Measurements are made automatically, with pass/fail limit testing.

This application is helpful in ATE environments, offloading tasks from the system CPU. It’s also worth considering in R&D because, unfortunately, spur measurements usually have to be repeated… many, many times.

Some recent innovations in digital filters and DSP have dramatically improved sweep rates for narrow RBWs in signal analyzers such as Keysight’s PXA. With sufficient processing, RBW filters can now be swept up to 50 times faster without compromising amplitude or frequency accuracy. The benefit is greatest for RBWs of several to several hundred kilohertz, as is typical for spur measurements (see this recent app note).

One factor that can muck up the works is the presence of non-CW spurs. For example, TDMA schemes often produce time-varying spurs. This violates key assumptions underlying traditional search techniques and makes it much tougher to detect and measure spurs.

Fortunately, signal analyzers have evolved to handle these challenges. In TDMA systems, sync or trigger signals are often available to align gated sweeps that analyze signals only during the desired part of the TDMA frame.

Perhaps the most powerful tool for finding impulsive or transient spurs is the real-time analyzer, which can process all the information in a frequency span without gaps and produce a spectrogram or density display that reveals even the most intermittent signals.

The best tool for precisely measuring time-varying spurs is vector signal analyzer (VSA) software. The software uses RF/microwave signal analyzers, oscilloscopes, etc., to completely capture signals for any type of frequency-, time- and modulation-domain analysis. Signals can be recorded for flexible post-processing as a way to accurately measure all their characteristics from a single acquisition, solving the problem of aligning measurement to the time-varying nature of the signal.

 

It’s no secret that spur detection and measurement are both difficult and essential, but with the right advice and the right equipment you can minimize the tedium.

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