Three Key Technologies That Power the RF Recorder
RF recorders are still the new kid on the block compared to the ubiquitous spectrum analyzer. The foundational technologies behind the spectrum analyzer have existed for over 50 years. That’s not the case for RF recording instrumentation. So why did it take us so long to get here?
Three key technologies have converged over the last 10 to 15 years to enable the explosive growth of the RF recorder. Each of these are equally critical ingredients for the creation of this new class of instrumentation. It’s important to explore the three key technologies that power the RF recorder in order to fully appreciate how these amazing instruments work.
1: Wide Bandwidth A/D Converters
When we think of an RF recorder, the first thing that comes to mind is its ability to digitize an RF signal. While this seems like a relatively straightforward task to do today, A/D converters struggled to digitize even a few MHz 20 years ago.
Most spectrum analyzers from that era were analog. The analyzers that claimed to be digital were actually hybrids of both analog and digital technology with a swept architecture. An RF recorder on the other hand, needs to be completely digital in order to capture and digitize a wide bandwidth all in one moment of time.
In the late 90’s wideband A/D converters came to the rescue. It was finally possible to digitize an RF signal without resorting to swept frequency techniques. Every year thereafter, bandwidths and sample rates continued to improve. Nowadays A/D converters can digitize GHz not just MHz.
2: High Throughput Data Buses
As important as these new wideband A/D converters were, they introduced a new challenge. They produced a high speed stream of data that often exceeded the capacity of parallel computer buses at the time.
PCI, for example, maxed out at 133 MB/sec which was wholly inadequate for a high speed A/D converter. This was the case especially when there were multiple devices on the bus all competing for shared bandwidth. It was no small victory to successfully transfer all that data across the bus without dropping anything.
The introduction of PCI Express (PCIe) was a significant milestone that expanded the realm of possibilities. PCIe finally delivered not only high bandwidth, but also dedicated paths between devices. The little bit of latency and jitter that remained was easily smoothed out with buffers. Every couple of years, a new PCIe is introduced which is increasingly faster with each new version. PCIe ultimately proved itself to be more than capable of faithfully delivering every byte produced by these wideband data A/D converters.
3: High Speed, High Capacity Permanent Storage Media
Two of the three problems were solved with the A/D converters capturing MHz of data and the PCIe bus transporting the data error-free. The final challenge was storage. While it’s tempting to believe that the problem is easily solved with an off-the-shelf hard drive, it’s simply not the reality.
In the early 2000’s an individual hard drive could store a few hundred GBs and maybe 40 MB of data/sec. The bandwidth and capacity requirements of RF recording far exceed the typical application. Even today, a single hard drive is typically inadequate for RF recording applications.
RAID technology came to the rescue with three significant advantages: speed, capacity, and fault tolerance. While the IT industry was mostly interested in the fault tolerance, the world of RF recording greatly benefited from RAID speed and capacity. A capable disk array could finally keep up with the fire hose of data streaming within the RF recorders.
It’s easy to forget that RF recording was a struggle in the past. It was an exotic technology reserved for those with vast budgets and a healthy dose of patience. Breakthroughs in the key technologies of A/Ds, buses, and storage have brought RF recording to even the smallest labs and the most demanding applications.
They’ve revolutionized the way we develop and validate complex systems. Those of us who worked with RF recorders a decade ago are typically surprised at how easy it has become to create high-fidelity RF captures with the latest instruments.