Digital phosphor technology can compress 1465 spectral measurements 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 technology 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 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.

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