Digital phosphor technology reveals transient and weak signals that otherwise would be difficult or impossible to detect.
Greetings once more and welcome to our 74th Test and Measurement Video. Today we will look again at the Tektronix RSA306 PC-Based Spectrum Analyzer. Being a real-time instrument, it can see how traces evolve over time. Also, it is capable of displaying signals that cannot be seen in a swept-spectrum analyzer.
DPX is a Tektronix trademark that refers to their digital phosphor technology, which reveals signal details not otherwise available to the user. It is analogous to high-persistence coatings that were applied to the inner surface of CRT screens so that the image would persist. In DPX, this persistence is digitally realized, permitting the user to see signal details that are missed in conventional spectrum analyzers and vector signal analyzers.
First, by way of background, we’ll consider the Tektronix RSA306 PC-Based Spectrum Analyzer. Like the oscilloscope, this instrument, rather than having a built-in flat screen display, is cabled to a user-supplied PC. This is advantageous because given the fact that in today’s world nearly everyone on the planet owns or has access to a PC, either laptop or desktop. The other essential part of the equation is the software, SignalVu-PC, which is available as a free download from the Tektronix website.
A bench-type spectrum analyzer is fairly expensive, more costly than an oscilloscope with comparable specifications. The price is much less for the PC-based instrument and, as we shall see, the amount of functionality, ease of operation and output quality are substantial.
The advantage in DPX is that it enables capture of brief transients. It is a tool for isolating, viewing and analyzing very short duration and low-amplitude signals that may lurk below segments of a high-definition FM signal that is being investigated.
To see how DPX works, we’ve cabled together a PC laptop, RSA306 Spectrum Analyzer and the proprietary demoboard that was built to demonstrate the use of the instrument.
DPX shows how traces change in two ways. First, it uses color shading to indicate the consistency of a trace. Also, persistence is used to hold signals on the screen longer so that they may be seen. In these ways, DPX displays transient signal behavior so as to enable the user to isolate gitches, interference and instability.
To begin, it is necessary to configure the demo board. Keep the same settings as in Video 73, which demonstrated the RSA306 Save function. Use the Row and Column buttons to change the signal from OFDM to Infrequent Hop, which denotes infrequent hopping signals.
That’s it for configuring the board.
To configure the RSA306 Spectrum Analyzer, keep the same settings as in Video 73. Click on Run.
In General Signal Viewing, select DPX from the Available Displays box.
Click the Add button or double click the DPX icon in the Selected Displays block. Then click the OK button.
In the DP display, set Center Frequency at 2.4453 GHz and set Span to 40 MHz.
Select the Split View.
You can now see the hopping signals displayed in the DPX spectrum, and compare it to the standard spectrum display on the right side. Note that the standard spectrum
display frequently or always misses the transient signal.
Select the DPX display by clicking anywhere on it and click the Settings button. This permits you to change the color scheme.
In the temperature palette, the highest color, maximum, is deep red and the lowest color, minimum, is dark blue. Values between maximum and minimum are represented by
other colors on the palette.
Under the Bitmaps Scale tab, set the Max scale to 3 percent for the DPX Spectrum display.
Under the Amplitude Scale tab, set the minimum color scale to -80 dBm for the DPXogram display.
In the Prefs tab, enable Show Parameter readouts.
Click Stop to pause the measurement. Right click the DPXogram screen, select Add Marker and drag the marker to one hopping signal. Then add another marker and drag it to the closest hopping signal. The Delta Marker will show the time between two hopping signals is about 1.25 seconds.
You can set the size of the DPX spectrogram by selecting the divider bar between the displays and moving it up or down to add more area to the desired display.
The minimum event duration to ensure 100 percent probability of intercept or event capture depends upon the interaction of Span, Resolution Bandwidth and FFT length. You can use this time value to see if you are at risk of either missing a narrow transient or of its captured amplitude being less than the actual signal. The minimum 100 percent probability of intercept for RSA306 is 100 microseconds.
These are the fundamentals, contained in DPX, for becoming really adept at transient capture, which is a prerequisite for successful troubleshooting and product development in this context.
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