Yesterday’s Resolution Bandwidth is not the Best One Today

Blog Post created by benz on Mar 16, 2017

  Good news about RBW, noise floor, measurement speed, and finding hidden signals

Some things never change, and others evolve while you’re busy with something else. In recent years, RF measurements provide good examples of both, especially in signal analysis fundamentals such as resolution bandwidth filtering, measurement noise floor, and speed or throughput. It can be a challenge to keep up, so I hope this blog and resources such as the one I’ll mention below will help.

One eternal truth: tradeoffs define your performance envelope. Optimizing those tradeoffs is an important part of your engineering contribution. Fortunately, on the measurement side, that envelope is getting substantially larger, due to the combined effect of improved hardware performance plus signal processing that is getting both faster and more sophisticated.

Perhaps the most common tradeoffs made in signal analysis—often done instinctively and automatically by RF engineers—are the settings for resolution bandwidth and attenuation, affecting noise floor and dynamic range due to analyzer-generated distortion. Looking at the figure below, the endpoints of the lines vary according to analyzer hardware performance, but the slopes and the implications for optimizing measurements are timeless.

Graphic shows how signal analyzer noise floor varies with RBW, along with how second and third order distortion vary with mixer level. Intersection of these lines shows mixer level and resolution bandwidth settings that optimize dynamic range

Resolution-bandwidth settings determine noise floor, while attenuation settings determine mixer level and resulting analyzer-produced distortion. Noise floor and distortion establish the basis for the analyzer’s dynamic range.

This diagram has been around for decades, helping engineers understand how to optimize attenuation and resolution bandwidth. For decades, too, RF engineers have come up against a principal limit of the performance envelope, where the obvious benefit of reducing resolution bandwidth collides with the slower sweep speeds resulting from those smaller bandwidths.

That resolution bandwidth limit has been pushed back substantially, with dramatic improvements in ADCs, DACs, and DSP. Digital RBW filters can be hundreds of times faster than analog ones, opening up the use of much narrower resolution bandwidths than had been practical, and giving RF engineers new choices in optimization. As with preamps, the improvements in noise floor or signal-to-noise ratio can be exchanged for benefits ranging from faster throughput, to better margins, to the ability to use less-expensive test equipment.

Improvements in DSP and signal converters have also enabled new types of analysis such as digital demodulation, signal capture and playback, and real-time spectrum analysis. These capabilities are essential to the design, optimization and troubleshooting of new wireless and radar systems.

If you’d like to know more, and take advantage of some of these envelope-expanding capabilities, check out the new application note Signal Analysis Measurement Fundamentals. It provides a deeper dive into techniques and resources, and you’ll find it in the growing collection at our signal analysis fundamentals page.