Originally posted Mar 18, 2013
Related concepts, though they don’t mean exactly the same thing
The previous post discussed real time analysis mostly in general terms, focusing on the core concept of processing every signal sample for some type of result. Before future posts get into detail on example measurements it’s worth taking a minute to look at real time analysis specifically as performed by the tools specifically named real-time analyzers.
Ancient real-time history, non-RF
Real time analyzers have been around at audio frequencies since the 1970s. The earliest ones were implemented with analog technologies, applying signals to parallel filter banks for audio frequency analysis. Frequency resolution was often limited to octave or 1/3 octave steps.
Digital technologies matured enough in the late 1970s and early 1980s to make bench top real time audio analyzers practical through fast Fourier transform (FFT) processing. Resolution was modest compared to today’s standards, generally power-of-two values such as 128 or 256 points (FFT “bins”). Nonetheless this was much better than earlier parallel-filter analyzers.
RF signal monitoring/surveillance tools
Real-time capability at RF was especially valuable for signal monitoring and surveillance applications, leading to early, specialized (and expensive!) solutions in this area. Some solutions were eventually paired with real-time demodulators as well. These high performance solutions were capable for general RF analysis but remained niche solutions, not focused at general spectrum analysis and thus not a part of the tool kit of most RF engineers.
Real-time analyzers come to general RF analysis
The first real-time analyzer focused at general-purpose RF measurements to gain traction was the RSA6000 from Tektronix, introduced a few years ago. The analyzer popularized a density display, which Tektronix calls DPX, that provides a way to make good use of the enormous number of spectra produced by real-time hardware calculations. Follow-on models from Tektronix have improved performance and widened the range of solutions in this area. These real-time analyzers use heterodyne downconversion to an IF and are thus similar to traditional spectrum analyzers but are non-swept and exclusively FFT-based. Wide spans are implemented through the use of stepped or stitched FFTs.
Agilent has recently introduced its own real time spectrum analyzer as part of the N9030A PXA signal analyzers (www.keysight.com/find/rtsa). Agilent’s real-time spectrum analyzer is an upgrade option for a mainstream signal analyzer, and thus supports swept spectrum analysis along with full vector signal analysis and digital demodulation. Because it can be added to an existing signal analyzer platform it eliminates the need for a dedicated real-time analysis tool. It also offers the widest bandwidth, best dynamic range, and best probability-of-intercept of available real-time spectrum analyzers. Isn’t competition wonderful!
Defining these RF real-time analyzers
Today’s real time analyzers are implemented in slightly different ways and have a different balance of features but share common characteristics and benefits:
- Measurements are gap free. There is no dead time between acquisitions and all sampled data is processed. The process can continue for an arbitrary duration
- Measurements and display updates are fast and continuous. Real-time calculations demand that FFTs be performed in hardware using dedicated ASICs or FPGAs or both. Thus the update rate is not subject to an instrument’s Windows task interruptions.
- High speed and/or data-dense displays make sense of all the measurement data. Specialized displays are needed to make constructive use of the hundreds of thousands of spectra calculated every second. Look for density or histogram displays, spectrograms, and fast power vs. time (PVT) traces.
- Spectral or frequency mask triggers (FMTs) provide an alternative way to use the fast spectrum calculations and thus a trigger type with a new set of benefits for looking at agile signals or complex signal environments.
Future posts will look at real applications for all this measurement horsepower.