August 2007

Test and Measurement Digitization of the Microwave Domain
By Justin Stallings, Senior Product Manager, Rohde & Schwarz

You may find it comforting when faced with making a particularly difficult RF or microwave measurement to look back fondly to when both the measurements and the instruments employed to make them were totally analog and less complex. Twenty years ago, test equipment was indeed “simpler”, reflecting the measurement challenges it was designed to surmount. The principal modulation techniques used in wireless systems were analog -- FM, AM, and single-sideband -- and pulse modulation was (and obviously still is) employed in radar systems. These signal types were comparatively easy to characterize and varied little with time. There were no higher-order digital modulation schemes and their attendant widely-varying RF power levels, peak-to-average ratios, and other attributes to contend with. It was indeed a simpler time.

However, today’s more complex signals have made possible the wireless niceties most of us enjoy, along with digital audio and video terrestrial and satellite broadcasting, among other goodies. The unvarnished truth is that digital technology has grabbed hold of the RF world and almost completely transformed it using digital devices and techniques, moving to digitally implement previously analog functions closer and closer to the antenna port and DUT output every year. Sentimentality aside, that’s a good thing, because today’s measurement challenges could simply not be addressed with the single-purpose buttons and knobs that invoked the analog-implemented functions of “way back then”. Even if they could, there are just so many single-function buttons and dials that can be crammed onto an instrument front panel. Like it or not, “menu, softkey, and display-invoked, digitally-enabled” microwave instruments are here to stay.

However, while instruments of two decades ago and the measurements they performed were simple in comparison to today, many measurements are now actually easier to perform even though they are far more complex. Once again, thanks are due to digital technology (along with some contributions from RF and microwave technology and devices). General-purpose microprocessors, ASICs, DSP, and FPGAs in particular, along with instrument architectures based on Windows and Linux operating systems, have transformed the microwave instrument into a processing powerhouse. Together they enable enormous functionality to be incorporated into today’s instruments that simply could not be accomplished any other way. In constant dollars, today’s general-purpose instruments actually cost less then their predecessors of two decades ago, while arguably providing orders of magnitude improvements in functionality and reliability.

Instruments such as signal generators, vector signal analyzers, spectrum analyzers, and network analyzers perform their core measurements in some cases orders of magnitude faster than before. They provide on-board analysis capability in addition to exporting data for use in PC analysis and presentation software, and can be integrated via Ethernet within an ATE system (or even a complete enterprise) of virtually any size. They can also have their firmware upgraded (and in some cases problems diagnosed) via the Internet, and are arguably easier to use with each generation, thanks to software refinements.

There is no better example than the RF power meter of how digital technology has transformed an instrument that once employed high levels of RF and microwave content. It has essentially become a digital processing and display platform for the information provided by the sensor – the last remaining RF component of the “instrument”. The power meter is today an almost completely digital instrument, and since its functions are also integrated into other instruments, it may in the future simply disappear as a stand-alone instrument class. Finally, since the sensors convert the analog information they capture to digital form, only a PC and software are actually required to complete the package. Thus the physical power meter disappears, a victim of the digitization of RF.

It is highly unlikely that other microwave instruments will be ‘digitized” as totally as the power meter, but even in vector network analyzers, signal generators, and spectrum analyzers, many RF and microwave components and all but the most mundane analog components have been eliminated and their functions performed via a combination of digital components and software.

So while microwave instruments and signal component characterization of 20 years ago were indeed simpler, today’s instruments are remarkably adept at simplifying complex measurement tasks. Most of these measurements could simply not be performed without the formidable power of digital technology. Nostalgia notwithstanding, we’re better off as a result.

ROHDE & SCHWARZ
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