A new RF tool leveraged two evolving technologies
These days, vector signal analyzers (VSAs) are in broad use, especially in wireless, aerospace, and defense applications. They’re essential for designing the complex modulated and time-varying signals that have become ubiquitous.
However, VSAs haven’t been around nearly as long as spectrum or network analyzers, and I can remember the process—including an informal contest—that yielded the name “vector signal analyzer.” This month marks the 25th anniversary of the first VSA, and I’d like to take a brief look back.
Lots of technical forces were coming together as the 1980s came to a close, both in terms of signals and the equipment to test them. Mobile phones were entering a period of what would become explosive growth, and the transition from analog (1G) to digital modulation (2G) was the way to handle the increase in traffic.
In test equipment, signal processing and digitizers were improving rapidly, and some low-frequency signal analyzers had switched from analog filters to digital ones in their intermediate frequency (IF) stages. Technology and demand were both in place.
These forces converged in the single test and measurement division that had a deep background in both swept and FFT-based products. HP’s Lake Stevens Instrument Division had already produced the first low-frequency swept network and spectrum analyzers (e.g., up to 200 MHz) with digital IF sections. That put the division in a unique position to combine the classic superheterodyne architecture with high performance ADCs and DSP.
Resolution bandwidth filters could be all digital, with better speed, accuracy, and selectivity. Virtually any resolution bandwidth could be produced, from sub-hertz to several megahertz. Perhaps most significant, the entire signal chain could preserve signal phase and therefore vector content.
Processing a signal’s complete vector information was important for obvious and less-obvious reasons. Vector processing allows for accurate, selective analog demodulation, fully separating amplitude from phase or frequency modulation. It also provides complete pulse analysis, and the potential for digital demodulation.
A key decision in this area, driven by a need for accurate pulse analysis, was to perform continuous time-domain vector calibration across the analyzer signal chain. This improvement on frequency-domain calibration was later to be essential to precise digital demodulation of all kinds in the VSA.
Over a span of several years, all of this evolving and improving technology was subjected to extensive discussions and trials with potential customers. Their feedback was crucial to the definition and implementation of the first VSA and, in many ways, they taught us what a VSA should really be. Many changes and refinements were made along the way, and in October 1992 we introduced the first RF vector signal analyzer, the HP 89440A.
This image from the HP test and measurement catalog shows the first RF vector signal analyzer, the 89441A. The bottom section contains the RF receiver and companion source. The two-channel top section was also available as the (baseband) 89410A VSA.
Vector Signal Analyzer—I suppose the name seems obvious in retrospect, but it wasn’t so clear at the time. We were aware that it was a new type of analyzer, one we expected would be an enduring category, and one we wanted to get right. I can’t recall the other candidate names, but remember voting for VSA. After all, it was a signal analyzer—not just a spectrum analyzer—that provided vector results.
Wireless and aerospace/defense test engineers quickly grasped the possibilities. Shortly after introduction, we took the analyzer to its first trade show: engineers were lining up for demos. The signal views and insights provided by the frequency+time+modulation combination was compelling, and we were able to show waterfall and spectrogram displays, along with complete signal capture and playback.
Within the year we added digital demodulation, and the wireless revolution picked up steam. VSAs helped enable the new transmission schemes, from CDMA to high-order QAM, multi-carrier signals, OFDM, and MIMO. Software enhancements allowed the VSA to track the emerging technologies and standards, giving engineers a reliable test solution early in the design process.
Though the name and measurements would continue, the VSA as a separate analyzer type gradually yielded to newer “signal analyzers” with digital vector processing. These analyzers started with swept scalar spectrum analysis, and VSA capability became an option to the base hardware.
Now separate and embedded, Keysight’s 89600 VSA software continues the tradition of supporting the leading edge of wireless technology. The latest example: a new VSA software release supports pre-5G modulation analysis, and will evolve along with the standard.
It’s been a busy quarter century for all of us, and I expect VSAs will be just as useful for the next one.