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Time & Frequency

Digital phosphor technology boosts RF signal discovery and analysis With emerging wireless applications, the RF spectrum is more chaotic than ever before. Consequently, increasingly complex signals are crowding a limited frequency spectrum. This article shows how digital phosphor technology, traditionally used in advanced oscilloscopes, applied to the real-time spectrum analyzers (RTSAs) is now enabling users to view “live RF” signals for the fi rst time. Thus, providing unmatched insight into RF signal behavior, thereby greatly accelerating the discovery and diagnosis of problems relating to time-variant RF signals.

By Kathy Engholm

The radio frequency (RF) spectrum is more chaotic than ever, with more channels and increasingly complex signals crowd-

ing a limited frequency spectrum. As new applications use wireless transmission and digital RF systems proliferate, engineers need better tools to help them fi nd and interpret intricate RF signal behaviors and interactions.

Fortunately, digital phosphor technology, traditionally used in advanced oscilloscopes, has been applied to the RF domain and can now be found in pre-eminent real-time spectrum analyzers (RTSAs). In enabling users to view “live RF” signals for the fi rst time, digital phosphor technology provides unmatched insight into RF signal behavior. In fact, full-motion digital phosphor displays show signals and details that are completely missed by conven-tional spectrum analyzers and vector signal analyzers (VSAs), greatly accelerating the discovery and diagnosis of problems relating to time-variant RF signals.

The name “digital phosphor” derives from the phosphor coating on the inside of cathode ray tubes (CRTs) used as displays in televi-sions, computer monitors and older test equipment. When an electron beam excites the phosphor, it fl uoresces, lighting up the path drawn by the stream of electrons. Although, raster-scan technologies, such as liquid crystal displays (LCDs), eventually replaced CRTs in many applications due to depth and power advantages, the combination of phosphor coatings and vector drawing in CRTs provided several benefi ts

that are useful for modern test and measurement applications.First, this combination offers persistence. Phosphor continues

to glow even after the electron beam has passed. Generally, the fl uorescence fades quickly enough that viewers don’t perceive it lingering, but even a small amount of persistence will allow the human eye to detect events that would otherwise be too short to see.

Second, phosphor coatings and vector drawing deliver pro-portionality. The slower the electron beam passes through a point on the phosphor-coated screen, the brighter the resulting light. Brightness of a spot also increases as the beam hits it more frequently. Users intuitively know how to interpret this z-axis information: a bright section of the trace indicates a frequent event or slow beam motion, and a dim trace results from infrequent events or fast-moving beams.

Persistence and proportionality do not come naturally to instru-ments with LCDs (or even raster CRTs) and a digital signal path. Digital phosphor technology was developed so the analog benefi ts of a vector CRT could be achieved, and even improved upon, with digital oscilloscopes and now RTSAs. Digital enhancements such as intensity grading, selectable color schemes and statistical traces communicate more information in less time.

Digital phosphor technology basicsDigital phosphor technology can compress 1465 spectral mea-

Figure 1. Color-coded low-resolution example (left), and a real DPX display (right).

Frequency

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surements into one screen update every 33 milliseconds, yet this is an oversimplified description of the role it performs in top RTSAs. Every second, 48,828 acquisitions are taken and transformed into spectrums. This high transform rate is the key to detecting infrequent events, but it is far too fast for the LCD to keep pace and well beyond what human eyes can perceive. Therefore, the incoming spectrums are written into a bitmap database at full speed then transferred to the screen at a viewable 30 Hz rate.

The bitmap database can be envisioned as a dense grid created by dividing a spectrum graph into rows representing trace amplitude values and columns for points on the frequency axis. Each cell in this grid contains the count of how many times it was hit by an incoming spectrum. Tracking these counts is how digital phosphor technology implements proportionality, enabling the user to visually distinguish rare transients from normal signals and background noise.

Figure 1 offers a simplified view of the bitmap database as well as the actual digital phosphor display. The grid on the left shows the “number of occurrences” values after nine spectral transforms have been performed. Blank cells contain the value zero, meaning that no points from a spectrum have fallen into them yet. One of the nine spectrums happened to be computed at a time during which the signal was absent, as can be seen by the string of “1” values at the noise floor.

When these values are mapped to a color scale, data turns into information. In this example, warmer colors (red, orange, yellow) indicate more occurrences. The RTSA user can define other intensity-grading schemes. Displaying these colored cells, one per pixel on the screen, creates the spectacular digital phosphor display. The actual three-dimensional bitmap database of leading RTSAs with digital phosphor technology contains 501 columns and 201 rows to accumulate data and produce the spectrum display.

As previously mentioned, 48,828 spectrums enter the database each second. At the end of each frame of more than 1400 input spectrums (roughly 30 times per second), the bitmap database is transferred out for additional processing before being displayed, and data from a new frame starts filling the bitmap.

To implement persistence, the digital phosphor engine can keep the existing counts and add to them as new spectrums arrive, rather than clearing the bitmap database counts to zero at the start of each new frame. Maintaining the full count values across all frames is “infinite persistence.” If only a fraction of each count is carried over to the next frame, it is called “variable persistence.” Adjusting the fraction changes the length of time it takes for a signal event to decay from the database and fade from the display.

Imagine a signal that popped up only once during the time the digital phosphor engine was running. Furthermore, assume that it

was present for all 1465 spectrum updates in a frame and that the variable persistence factor causes 25% attenuation after each frame. The cells it affected would start out with a value of 1465 and be displayed at full force. One frame later, the number of occurrences values become 1099. After the next frame, they are 824, then smaller and smaller until they are invisible. On the RTSA screen, the user would initially see a bright trace with a spike at the signal frequency. The part of the trace where the signal occurred eventually fades away. During this time, the pixels start to brighten at the noise level below the fading signal. In the end, there is only a baseline trace in the display, as can be seen in Figure 2.

Persistence capabilities of RTSAs with digital phosphor technol-ogy are an extremely valuable troubleshooting aid, delivering all the benefits of MaxHold and more. To find out if there is an intermittent signal or occasional shift in frequency or amplitude, the user can turn on infinite persistence and let the RTSA run continuously. When the user returns, not only will the highest level for each frequency point be visible, but also the lowest levels and any points in between. Once the presence of transient behavior or intruding signals has been revealed, the user can characterize the problem in detail with variable persistence.

A colorful bitmap is digital phosphor technology’s signature trace, but it also produces statistical line traces. The database contents are queried for the highest, lowest and average amplitude values recorded in each frequency column. The three resulting trace detections are +Peak, -Peak and Average. The +Peak and -Peak traces instantly and clearly show signal maxima and minima. Average detection finds the mean level for the signal at each frequency point. All these traces

Figure 4. DPX Spectrum display after five seconds bitmap color mapping is “spectral” to emphasize infrequent signals with hot colors. MaxHold trace is yellow.

Figure 3. Swept analyzer after five seconds MaxHold trace.

Figure 2. With variables persistence, a brief CW signal captured by DPX remains in the display for an adjustable period of time before fading away.

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can be saved and restored for use as reference traces.Just like regular spectrum traces, digital phosphor line traces can

be accumulated over ongoing acquisitions to yield MaxHold, MinHold and average trace functions. Using hold on the +Peak trace is almost exactly the same as the MaxHold trace on a typical spectrum analyzer, with the important difference that the digital phosphor trace’s update rate (48 k/s, just like the digital phosphor bitmap) is three orders of magnitude faster.

Detecting short, infrequent signalsThe 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 sig-nals 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 s long and wit in the real-time bandwidth of the RTSA.

In addition to guaranteeing detection of short, infrequent signal events, digital phosphor tech-nology 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. RFD

ABOUT THE AUTHOR

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 hard-ware design, marketing and market research, sales, management, human interface design and product planning. In addition, she has designed for oscilloscopes, logic analyzers, video test equip-ment and telecommunications test sets.

Figure 5. Swept analyzer MaxHold trace after 120 seconds.

Figure 6. DPX bitmap and MaxHold trace after 20 seconds.

Just like regular spectrum traces, digital phosphor line traces can be accumulated over ongoing acquisitions to yield MaxHold, MinHold and average trace functions.