‘Refresh rate’ or ‘scan time’ are properties of an RF spectrum analyzer related to performance that are obvious and visible. Less visible is the property ‘resolution bandwidth’ or RBW. Though a high ‘refresh rate’ and low RBW are desirable properties, achieving both may not be possible. During the course of product development many choices and trade-offs are made — based on cost, resources, time, and marketing considerations. When designing the Touchstone and Clear Waves software, when a trade-off arose between ‘refresh rate’ and RBW, we chose in favor of RBW. In this post we will explain why — and we’ll use Clear Waves as an example.
When comparing the refresh rate of the RF Explorer device with that of Clear Waves the device appears much faster. This is partly machine dependent — that is, Clear Waves is very graphical and several diagnostic charts are repeatedly re-calculated and re-drawn, which takes time. The faster and more powerful your PC machine, then the faster will be the graphical updates. However, the primary reason why Clear Waves appears slower than the device is the software breaks each spectrum trace into many subscans — in order to achieve finer resolution and better accuracy. For example, each Clear Waves trace may actually include data from 390 subscans.
When you think about it, fast scanning provides little diagnostic value. It looks good, but it doesn’t change the results. The only instance where fast scanning is valuable is when you are trying to detect transient signals — but then a low-cost, RF Explorer spectrum analyzer is probably not the right tool for that type of application. Clear Waves is designed for engineers who are tasked with assigning channels and coordinating frequencies — so, the quality of the data is more important than how fast the data can be repeatedly displayed. Clear Waves incorporates a proprietary algorithm that performs intermodulation analysis and generates a resultant frequency set. In order to truly take advantage of the intermodulation analysis you need good data.
In order to find all the available open space we perform spectrum scans at a higher resolution (lower RBW). Let’s say we settle on an RBW of 50 KHz (which is the default for Clear Waves). RF Explorer collects 224 data points per scan — for the sake of argument let’s round this down to 200 data points per scan. At a scan rate of 200 data points per scan, this means our frequency span can only be 10 MHz if we want an RBW of 50 KHz (since 200 times 50 KHz is 1 MHz). So, what happens when a user specifies a frequency of 470 – 600 MHz — i.e. 130 MHz? This means Clear Waves breaks the work down into 13 subscans, each one 10 MHz in width. In this way, Clear Waves can achieve a very high resolution. Clear Waves also repeats each subscan 3 times — for purposes of accuracy. When Clear Waves completes a scan from 470 – 600 MHz it has actually performed 390 subscans and now has a lot of data to work with — and it is accurate. But all that data comes at a price — refresh rate.
To summarize, if we were not using the data for intermodulation analysis, then the extra data and accuracy are less important. But since the RF spectrum is often very crowded and open real estate comes at such a premium, then we want to give the intermodulation analysis as much good data as possible so the resultant frequency set it computes contains as many intermod-compatible channels as possible.