This is Part 4 in a series of articles which review the basics of conventional swept versus real-time spectrum analyzers and highlight some of the recent advances and instrument form-factors.
Read Part 1, Guide to Real-Time Spectrum Analyzers: Types, here.
Read Part 2, Guide to Real-Time Spectrum Analyzers: Definitions, here.
Read Part 3, Guide to Real-Time Spectrum Analyzers: Advantages of Swept Versus Real-Time Spectrum Analyzers, here.
INTRODUCTION
There are a number of interesting applications for real-time spectrum analyzers, primarily when measuring brief events or digital modulations. These include (1) pulse measurements, (2) evaluating radar, (3) VCO and PLL analysis, (4) RFID transponders, (5) spectrum management and surveillance, (6) radio communication and wireless, and (7) EMI analysis. We’ll be discussing these applications in the section below.
1. PULSE MEASUREMENTS
Pulse information for carrier frequency, rise and fall times, occupied spectrum, and pulse width are some of the many measurements that may be performed. Examples might include pulse repetition frequency (PRF), duty cycle, and pulse-to-pulse phase information.
2. RADAR
Analyzing pulse measurements in both the time and frequency domain simplifies radar testing and characterization. Things like spurs, noise figure, and spectrum occupancy are easily measured. The persistence mode can also reveal interference issues with the radar system.
3. VCO/PLL ANALYSIS
Voltage-controlled oscillators or phase-locked loop ICs can often produce random, sweeping, or glitch behavior that would not be detected by normal swept analyzers. Real-time analyzers can capture this frequency versus time (and even temperature) behavior in order to optimize circuit designs.
4. RFID
Things like radio frequency identification (RFID), near-field communication (NFC), and tire pressure-monitoring systems (TPMS) may all be monitored and analyzed using real-time analyzers. These systems operate from 135 kHz to 2.4 GHz and in both active and passive modes. Many of these systems operate in the Industrial, Scientific, and Medical (ISM) bands and are thus susceptible to other interfering signals, which may easily be identified using real-time analysis.
5. SPECTRUM MANAGEMENT AND SURVEILLANCE
Detection of intermittent signals in the presence of other signals is near impossible with swept analyzers. For example, real-time analyzers can easily differentiate different Wi-Fi access points using the same channel or a short impulsive signal amidst several others. This is one area where real-time analyzers really shine.
Keep in mind this assumes the spectrum being monitored is no wider than the real-time bandwidth. Using frequency mask triggers helps capture any intermittent signals with near-100% probability. Recording of spectrum usage for later playback and analysis is important for both surveillance and spectrum management (Figure 1).

6. RADIO COMMUNICATIONS AND WIRELESS
Most of today’s radio communications systems are now digitally controlled with remote or automatically adjustable frequency and power. In addition, with the proliferation of mobile products and the “Internet of Things” (IoT) expanding exponentially, the probability of interference—especially in the license-free ISM bands—will only increase.
Real-time spectrum analyzers will more likely become the tool of choice in resolving these potential interference or coexistence issues (Figure 2). In addition, real-time analyzers would be ideal as a basis for mobile direction-finding (DFing) interference sources to commercial broadcast or communications systems (Reference 1).

7. ELECTROMAGNETIC INTERFERENCE (EMI)
For the same reasons above, as well as the growing complexity within both military and commercial products, the chances of one device interfering with another device or communications system is also on the increase (Figure 3).
In addition, clock rates and edge speeds are ever increasing, and test failures are on the rise. Troubleshooting with a real-time analyzer makes short work of intermittent or complex EMI problems. Fortunately, EMI standards bodies are gradually incorporating the newer-technology FFT-based spectrum analyzers as options for compliance measurements.
One distinct advantage of real-time analyzers, when used for compliance testing, is that they can capture harmonic signals within the required spectrum range very quickly, saving test time, as well as capture complex frequency versus time analysis.

SUMMARY
While real-time spectrum analyzers are more expensive, they are generally the better choice for capturing fast-moving, dynamic, or very narrow impulsive signals, as well as analyzing and displaying digital modulations clearly.
Just be aware of their limitations when looking across a span greater than their real-time bandwidth. Thanks to low-cost components, there are real-time solutions to fit almost any budget and performance level.
REFERENCES
[1] Wyatt & Gruber, Radio Frequency Interference (RFI) Pocket Guide, 2015, SciTech Publishing, http://www.amazon.com/Radio-Frequency-Interference-Pocket-Electromagnetics/dp/1613532199/ref=asap_bc?ie=UTF8