Originally posted Aug 9, 2016
Different ways to get your signal bits
There’s a long history of synergy and a kind of mutual bootstrapping in the technology of test equipment and the devices it’s used to develop and manufacture. Constant technology improvements lead to welcome—and sometimes crucial—improvements in RF performance. It’s a virtuous cycle that powers our field, but it also presents us with some challenging choices as the landscape evolves.
Signal analyzers and digital oscilloscopes have exemplified these improvements and illustrate the complex choices facing RF engineers. The latest signal analyzer, for example, covers bandwidths as wide as 1 GHz at frequencies up to 50 GHz. New oscilloscopes offer bandwidths as wide as 63 GHz, solidly in the millimeter range. Other oscilloscopes and digitizers, at more modest prices, cover the cellular and WLAN RF bands.
The established solution for spectrum analysis and demodulation of RF/microwave signals is the signal analyzer, and it’s logical to wonder if the technology advances in digital oscilloscopes and signal analysis software have changed your choices. If both hardware platforms can sample the bandwidths and operating frequencies used, how do you get your bits and, ultimately, the results you need?
The answer begins with an understanding of the two different approaches to sampling signals, summarized in these dramatically simplified block diagrams. First, a look at IF sampling:
In this architecture, the signal is downconverted and band-limited before being digitized. Sampling is performed on the intermediate frequency (IF) stage output.
In signal analyzers, the sampling frequency is related to the maximum bandwidth required to represent the signal under test. That frequency is usually low compared to the center frequency of the signal under test, and there is no need to change it with changes in signal center frequency.
The alternative, called baseband sampling, involves direct sampling of the entire signal under test, from DC to at least its highest occupied frequency: CF + ½ OccBW.
Here, the signal undergoes minimal processing before being digitized. The lowpass filter ensures that frequencies above the ADC’s Nyquist sampling criterion do not produce false or alias products in the processed results.
The signal under test is completely represented by baseband sampling and any type of analysis can be performed. Narrowband analysis as performed with a spectrum/signal analyzer—in the time, frequency, and modulation domains—is achieved by implementing filters, mixers, resamplers, and demodulators in DSP. Keysight’s 89600 VSA software is the primary tool for these tasks and many others, and it runs on a variety of sampling platforms.
We thus have two paths to the signal analysis we need, and we’re back to the earlier question about the best sampling choice among evolving technologies. The answer is primarily driven by performance requirements, the operating frequencies and bandwidths involved, and the resulting demands on sample rate.
The architecture of IF sampling allows for analog downconversion and filtering to dramatically reduce the required sample rate. This process has been thoroughly optimized in performance and cost, and focuses ADC performance on the essential signal. Other frequencies are excluded, and the limited bandwidth allows for ADCs with the best resolution, accuracy, and dynamic range.
With baseband sampling, frequency conversion and filtering are done in DSP, requiring a vast amount of digital data reduction to focus analysis on the band in question. This must precede processing for signal-analysis results such as spectrum or demodulation.
The tradeoffs explain why spectrum analysis and demodulation are generally performed using IF sampling. However, the technological evolution mentioned above explains the increasing use of baseband sampling for RF and microwave signal analysis. ADCs and DSPs are improving in cost and quality, and are frequently available on the RF engineer’s bench in the form of high-resolution oscilloscopes. RF and modulation quality performance may be adequate for many measurements, and the extremely wide analysis bandwidths available may be an excellent solution to the demands of radar, EW, and the latest wideband or aggregated-carrier wireless schemes.
Ultimately, personal preference is a factor that can’t be ignored. Do you look for your first insights in the time or frequency domain before delving into measurements such as demodulation? The software and hardware available these days may give you just the choice you want.