The following example outlines the discovery and analysis of a brief, intermittent RF signal anomaly using a traditional spectrum analyzer and a modern RTSA with digital phosphor technology. The signal in question is a continuous-wave (CW) sinusoid at 2.4453 GHz. Every 1.28 seconds, its frequency changes for about 100 µs, then returns to normal. The duty factor of this transient is less than 0.01%.

The traditional swept-tuned spectrum analyzer is set up for a five-second sweep of its MaxHold trace. It shows that there is something occurring around the signal, as can be seen in Figure 3. This sweep rate was empirically determined to be the optimum rate for reliable capture of this signal in the shortest time. Faster sweep times can reduce the probability of intercept and result in fewer intersections of the sweep with the signal transient.

Using a modern RTSA with digital phosphor technology, however, the instrument's display — with both the bitmap and a +PeakHold trace — shows much more information about the transient after the same five-second period, as can be seen in Figure 4.

After 120 seconds (four sweeps of 30 seconds), additional clues are visible in the swept analyzer's display, as shown in Figure 5. In contrast, after only 20 seconds, the digital phosphor display shows a much more informative picture, which can be seen in Figure 6. Looking at the digital phosphor display, it is obvious at first glance that the CW signal is hopping up to a frequency approximately 3 MHz higher than its starting point, but overshooting by 2.5 MHz, then undershooting a little, and finally settling. Then it hops back to 2.4453 GHz, again with some frequency overshoot and settling.

In addition to the level of detail on a spectrum display, the probability of intercept (POI) varies for different analyzer classes.

Swept-tuned and step-tuned spectrum analyzers cannot provide 100% POI for a signal that isn't continuously present because they spend only a short period of time tuned to each small portion of their frequency span during a sweep. If something happens in any part of the span other than where it is tuned at that instant, the event will not be detected or displayed. There is also a period of time between sweeps during which the analyzer is not paying attention to the input signal. VSAs and other FFT-based analyzers also miss signals during the time between acquisitions. Their POI is typically better than a swept analyzer's, albeit not appreciably, depending on a combination of factors including span, resolution bandwidth (RBW) and processing time.

RTSAs, on the other hand, capture data across all frequencies within their real-time span (up to 110 MHz for select RTSAs) during every acquisition. With unique, advanced features such as frequency mask trigger, the POI with these instruments increases to 100%, ensuring capture of any spectral event matching the trigger definition. When operating in free run mode as a simple spectrum analyzer, the RTSA has a POI similar to other FFT-based analyzers, with gaps between each acquisition. Adding digital phosphor technology to the RTSA, however, brings 100% POI to free run mode for any signal at least 24 µs long and wit in the real-time bandwidth of the RTSA.

In addition to guaranteeing detection of short, infrequent signal events, digital phosphor technology provides a true representation of multiple RF signals occupying the same frequency range. More dramatic than any technical specification is how quickly RF designers and network operators can discover and resolve problems with a clear view of fleeting signals on the digital phosphor display.

Kathy Engholm is the user experience architect for the real-time spectrum analyzers product line at Tektronix. A principal engineer, she has worked at Tektronix since receiving her BSEE from Iowa State University in 1980. Her roles have included electrical hardware design, marketing and market research, sales, management, human interface design and product planning. In addition, she has designed for oscilloscopes, logic analyzers, video test equipment and telecommunications test sets.

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