In the old days, the only place you’d ever find an RF spectrum analyzer was on a bench in the microwave lab. This instrument could display the “power versus frequency” of an RF signal through a fluorescent-green trace on the black CRT display. The spectrum analyzer was—and still is—seen as a highly sensitive and very expensive instrument that you didn’t dare move without extreme care.
Back then, taking the spectrum analyzer “into the field” was simply unimaginable. But the instrument has fundamentally changed, enabling its use far from the laboratory. Today, you’ll find spectrum analyzers driven around in minivans recording GPS signals. Or, you can find them on a ship prototyping new radar designs.
Even in the lab, spectrum analyzers are taking on a broader purpose for two reasons. First, the transition from primarily analog to primarily digital architectures has made many “problems” solvable with an RF signal analyzer. Second, the reduction in size and new form factors such as PXI has made these RF instruments portable enough to be practical in field applications.
Yesterday And Today
In the old days, a swept-tuned analyzer produced a spectrum entirely through analog components. A local oscillator was swept through the band of interest, and the resulting intermediate frequency (IF) signal was filtered with a resolution bandwidth filter and then “measured” with a power detector. The measured power at each frequency step produced the display of the spectrum analyzer.
For a wide range of reasons, the architecture of the modern “signal analyzer” more closely resembles that of a software-defined radio than the traditional spectrum analyzer. Modern RF signal analyzers mix the RF signal to higher intermediate frequencies and employ a wideband intermediate frequency filter, which is digitized with an analog-to-digital converter (ADC). In software, the IF signal is downconverted further to baseband, where it can be analyzed as in-phase (I) and quadrature-phase (Q) signals. As a result, the modern signal analyzer relies heavily on signal processing to produce the appropriate spectrum display.
Today, nearly every spectrum analyzer can deliver vector data (IQ samples) to a PC from an instrument control bus. Specifically in PXI, the combination of a high-speed PC-based data bus, an ability to get wideband IQ data from an RF signal analyzer, and high-performance computing power (think multicore CPUs) has enabled a wide range of new “embedded” applications for PC-based RF signal analyzers.
For example, in RF record and playback applications, an engineer will take a PXI system configured with an RF signal analyzer and a high-speed hard drive (or an array of hard drives) into the field. By connecting an antenna to the RF input of the signal analyzer and setting the RF signal analyzer into a continuous capture mode, the engineer can capture long continuous records of “off-the-air” samples (Fig. 1). Once the system is returned to the lab, the same engineer can play back the recording to reconstruct the signal that was observed in the field.
A Second example where engineers are using PXI RF signal analyzers in embedded applications is channel emulation. Channel emulators are highly specialized and very expensive instruments that recreate the impairments a signal experiences at it propagates through a wireless channel. They can introduce multi-path reflections, Doppler shift, time, and frequency selective fading.
Today, modern RF signal analyzers can, in some instances, replace the traditional channel emulator when combined with a signal generator and a real-time processing module such as an FPGA. An RF signal analyzer sends IQ samples directly the FPGA processing module (Fig. 2). Inside the FPGA, specialized signal processing is applied to these IQ samples to emulate the behavior of the wireless channel. Finally, these processed IQ samples are generated with an RF signal generator and introduced to a device under test (DUT).
PXI RF signal analyzers are often used as embedded devices in software defined radio applications as well. Today’s latest wireless standards such as 802.11ac and Long-Term Evolution (LTE) began as conceptual ideas that had to be proven out with a prototype transmitter and receiver. Often, once the first prototype baseband algorithms of a next-generation standard are designed in software, they’re “tested” as over-the-air transmissions.
Because modern RF signal analyzers readily provide access to sampled IQ data, these instruments are often an idea choice as a prototype receiver. Of course, the use of signal analyzers as software-defined radios isn’t limited to communications systems. For example, engineers today are also using the instruments to prototype radar systems and medical devices.
More Than A Large Boat
The spectrum analyzer is no longer just the “large boat” in the lab used solely to make spur sweeps and harmonic measurements. Instead, it’s the modular, PC-based radio that helps engineers execute a wide range of unique applications.
Of course, the software-defined nature of the modern RF signal analyzer is only part of the magic. Especially with PXI, the shrinking size of instrumentation makes the idea of an “embedded” signal analyzer far more practical than it ever was before. As a result, we’ve seen PXI used at the forefront of many applications once served by completely custom hardware.