Let’s Talk About Real-time Spectrum Analyzers

Traditional swept-tuned measuring receivers such as the spectrum analyzer or EMI receiver (covered in a previous article¹) have served the compliance engineering community well for several decades, and they will likely continue to do so. They are excellent devices for testing the RF emissions of electronic devices, devices that typically emit stable and continuous signals in accordance with national and international standards.

Although a staple of the compliance engineer’s workbench, these receivers still fail to catch spurious signals that occur anywhere away from the instantaneous local oscillator (LO) frequency and thus outside the instrument’s resolution bandwidth (RBW). This failure happens when the signal appears and then almost instantly disappears before the sweep has time to reach it. Additionally, when using these swept-tuned instruments, smaller, narrowband signals can be masked by more powerful, wideband signals. Even the most experienced electromagnetic compatibility (EMC) test operators will most likely not notice these undetectable pulses.

Today’s electronics engineers, more often than not, are working with advanced, non-repetitive, transient types of radio frequency signals. This environment requires full characterization of a suspect signal before a functionality or interference issue can be resolved, making limitations of traditional receivers problematic.

The solution to this measurement dilemma requires an instrument that can uncover obscure effects in RF signals, trigger on those properties, flawlessly capture them into memory, and analyze them in the frequency, time, modulation, statistical and code realms. This very focused diagnostic tool is called the real-time spectrum analyzer (RTSA). The RTSA uses Fast Fourier Transform (FFT) technology to enhance signal analysis so engineers and technicians can capture and understand the most tenuous of RF signals possible in today’s RF rich world.

What does “real-time” mean?

A digital system simulation is said to operate in real-time if its operating speed matches that of the real system which it is simulating.

Real-time operation is a state in which all signal samples are processed continuously and gap-free, for some sort of measurement result or triggering event. In most cases, the measurement results are scalar (power or magnitude) as with traditional spectrum measurements.

Key Attributes of RTSAs

Real-time spectrum analyzers have these key attributes:

• Gap-free analysis
• High-speed measurements (provide unique insights into high-speed signals)
• Consistent measurement speed
• Very wide bandwidth signal capture
• Advanced composite displays
• Frequency-mask triggering (FMT)

Gap-free Analysis

Gap-free analysis means that all computations are performed continuously and fast enough for the output of the analysis to keep up with the changes in the input signal without a loss or gap in the data, while maintaining amplitude accuracy. This is opposite of the operation of a swept-tune receiver where the entire measurement bandwidth is covered by sweeping through the bandwidth one chunk of frequencies at a time, only capturing a signal if it happens to be present at the time the sweep goes by it.

Advanced Signal Capture Specifications

It is possible to capture very short duration signals with an RTSA. One manufacturer specifies that its RTSA can detect and capture signals with durations as short as ~3.6µs with 100% probability of intercept (POI). Meaning there is a 100% probability of detecting signals with durations as short as 3.6 µs while maintaining full amplitude accuracy. This duration is approximately four times shorter than what is possible with a comparable standard swept-tuned spectrum analyzer.

POI is often expressed as the minimum duration of signal that can be observed with 100% probability and still accurately be measured. The POI value will vary depending on manufacturer, model number, and settings applied to the instrument (sampling rate, time-record length or FFT size, windowing function, window size, overlap processing, and noise floor). A higher sample rate helps improve the POI.

Wide Bandwidth Signal Capture

The required instrumentation must have a bandwidth wider than that of the signal-of-interest. A higher bandwidth requires higher sampling and processing rates. Real-time bandwidth (RTBW) is the widest measurement span in which the analyzer can sustain real-time operation. One manufacturer specifies their RTSA with a bandwidth of 160 MHz can detect a signal as short as 5µs 100% of the time.

Advanced Composite Displays

The displays found on RTSA are truly impressive. They provide much more detail than the standard swept-tuned instruments. There are typically two different displays found on RTSAs. One is called the density display, and the other is called the spectrogram display. The density display shows how signals change over time and reveal the presence of transient activity. This display provides a detailed view of ongoing changes in the content of the spectral environment. Typically, the display uses a color scale where warm colors indicate the frequent occurrence of a signal, and cool colors indicate infrequent occurrence. The spectrogram display presents frequency spectra versus time and uses color to indicate magnitude. In conjunction or separately these displays allow you to see signals inside of other signals with time slices as short as 100 µs. The trace has a large buffer of roughly 10,000 result traces. With these displays, the user can quickly see how all signals present interact with each other.

Frequency Mask Triggering

The spectra can be combined into a composite spectrum display or successively compared to a limit mask to implement a frequency-mask trigger (FMT). An FMT can be applied to the calculated spectra, and further measurements and display processing will occur when the trigger criteria are met. FMT logic behavior allows capturing just the signal you’re after and ignoring the rest. An FMT can be easily applied by touching the display area where you want to apply the mask, software manipulation, or via buttons on the front panel of most devices.

Multi-domain triggers allow you to select a specific frequency hop as well as specific durations. These capabilities let you sift through a dense environment and easily find previously undetected intermittent interferers or “signals within signals,” an impossible task for a traditional swept-tuned instrument.

This versatile triggering capability along with a deep signal-capture memory makes the RTSA a handy troubleshooting tool for sophisticated wireless technology, including LTE-Advanced up-link signals and wireless LAN signals.

Closing Remarks

Many RTSA solution providers provide in-one-instrument add-ons which allow users to perform both the traditional swept-mode measurements in addition to real-time measurements. With this solution, you’re able to utilize a best both worlds approach, swept-tuned measurements for compliance testing and RTSA measurements for product development and troubleshooting. This may be a less-expensive option if you already own a swept-tuned instrument and are looking for the capability of an RTSA without incurring the cost of a completely separate instrument. Whichever path you decide to take, it’s worth checking out what an RTSA can do for you and your organization. If solving a problem takes hours instead of days, or days instead of weeks, the cost will pay for itself.

References and Further Reading

1. In Compliance Magazine. (2018, September 4). What Every Electronics Engineer Needs to Know About: Measuring Receivers.
2. Keysight Technologies. Understanding and Applying Probability of Intercept in Real-time Spectrum Analysis, Application Note 5991-4317EN, 2014.
3. Real-Time Spectrum Analysis for EMI Diagnostics, Application Note 37W-22084-1, 2012.
4. Fundamentals of Real-Time Spectrum Analysis Primer, Application Note 37W-17249-5, 2015.