Spurious Emissions and Spectrum Emission Mask: Improving Difficult Measurements

Blog Post created by benz on Feb 20, 2017

  Wrestling information from a hostile universe

I can’t make a serious case that the universe is actively hostile, but when you’re making spurious and related measurements, it can seem that way. In terms of information theory, it makes sense: The combination of limited signal-to-noise ratio and the wide spans required mean that you’re gathering lots of information where the effective data rate is low. Spurious and spectrum emission mask (SEM) measurements will be slower and more difficult than you’d like.

In a situation such as this, the opportunity to increase overall productivity is so big that it pays to improve these measurements however we can. In this post I’ll summarize some recent developments that may help, and point to resources that include timeless best practices.

First, let’s recap the importance of spurious and spectrum emission and the reasons why we persist in the face of adversity. Practical spectrum sharing requires tight control of out-of-band signals, so we measure to find problems and comply with standards. Measurements must be made over a wide span, often 10 times our output frequency. However, resolution bandwidths (RBW) are typically narrow to get the required sensitivity. Because sweep time increases inversely with the square of the RBW, sensitivity can come at a painful cost in measurement time.

The result is that these age-old measurements, basic in comparison to things such as digital demodulation, can consume a big chunk of the total time required for RF testing.

Now on to the good news: advances in signal processing and ADCs can make a big difference in measurement time with no performance penalty. Signal analyzers with digital RBW filters can be set to sweep much faster than analog ones, and the analyzers can precisely compensate for the dynamic effects of the faster sweep. Many years ago, when first used at low frequencies, this oversweep processing provided about a 4x improvement. In the more recent history of RF measurements, the enhanced version is called fast sweep, and the speed increase can be 50x.

Two spectrum measurements with equivalent resolution bandwidths and frequency spans are compared.  The fast sweep capability improves sweep speed by a factor of nearly 50 times.

In equivalent measurements of a 26.5 GHz span, the fast sweep feature in an X-Series signal analyzer reduces sweep time from 35.5 seconds to 717 ms, an improvement of nearly 50 times.

I’m happy to report that DSP and filter technologies continue to march forward, and the improvement from oversweep to fast sweep has been extended for some of the most challenging measurements and narrowest RBW settings. For bandwidths of 4.7 kHz and narrower, a newly enhanced fast sweep for most Keysight X-Series signal analyzers provides a further 8x improvement over the original. This speed increase will help with some of the measurements that hurt productivity the most.

Of course, the enduring challenges of spurious measurements can be met by a range of solutions, not all of them new. Keysight’s proven PowerSuite measurement application includes flexible spurious emission testing and has been a standard feature of all X-Series signal analyzers for many years.

A measurement application in a signal analyzer has a number of benefits for spurious measurements, including pass/fail testing and limit lines, automatic identification of spurs, and generation of a results table.

The PowerSuite measurement application includes automatic spurious emission measurements, such as spectrum emission mask, that include tabular results. Multiple frequency ranges can be configured, each with independent resolution and video bandwidths, detectors, and test limits.

PowerSuite allows you to improve spurious measurements by adding information to the tests, measuring only the required frequencies. Another way to add information and save time is to use the customized spurious and spectrum emissions tests included in standard-specific measurement applications.

A new application note, Accelerating Spurious Emission Measurements using Fast-Sweep Techniques, includes more detailed explanations, techniques, and resources. You’ll find it in the growing collection at our signal analysis fundamentals page.