A Tektronix demo board is used to provide digital modulation signals in the RSA306 Spectrum Analyzer.
Hi and welcome to our 77th Test and Measurement Video. Our continuing exploration of the many features and capabilities of the Tektronix RSA306 PC-Based Spectrum Analyzer will focus today on digital modulation.
By way of review, our previous video, number 76, covered analog modulation, not to be confused with amplitude modulation, which is one variety that exists in both analog and digital modulation. The others are frequency and phase modulation. In all of these, sound, video or other information, which cannot propagate through space on its own due to the low frequency, is instead made to modulate an otherwise uniform carrier wave. At the receiver, in a process known as detection or demodulation, the audio, video or other signal is extracted from the carrier wave, subjected to further amplification and fed to speaker, video display device or elsewhere as desired. This is very simple. In contrast, digital modulation is more complex and involves a steeper learning curve.
There are four principle digital modulation techniques, with numerous subspecies, variations and competing models that fall outside these four formats. These techniques are based on keying:
• Phase-shift keying, in which a finite number of phases are employed
• Frequency-shift keying, in which a finite number of frequencies are employed
• Amplitude-shift keying, in which a finite number of amplitudes are employed
• Quadrature amplitude modulation, in which a finite number greater than two each of phases and amplitudes are employed
The operative concept in these methods of digital modulation is that the number of each of the above attributes is finite, not infinite as in the sliding-scale model that we see in analog modulation. The values are discrete and limited in number, in a characteristic digital manner.
Each of the phases, frequencies or amplitudes corresponds to a definite binary bit pattern. These bits make up the symbol that is conveyed by the digitally modulated carrier wave.
In this demonstration of digital modulation, we have once again cabled together an owner-supplied PC with the Tektronix proprietary SignalVu-PC software installed, along with the RSA306 PC-Based Spectrum Analyzer and a Tektronix demo board that provides a broad array of signals for designed to demonstrate the product.
This demonstration makes use of optional modulation analysis capabilities, so you have to ascertain whether your version of SignalVu-PC includes this license. To find out, click on Tools: Manage Licenses. A trial license can be obtained.
In the demo board, use the Row and Column buttons to change the signal type from FM to QPSK1. As for the RSA306, in the Presets Menu, click Application.
Select Modulation Analysis and OK.
Click the DPX display, set the Center Frequency to 2.445315 GHz and Span to 10 MHz.
You can save setups so that they appear in the Applications Presets window. Save your setup in the folder C:\SignalVu-PC files\User Presets. The saved setup will appear in the Application Presets window with the name you give the file.
Click the Constellation Display and then click the Settings button.
Under the Modulation Params, select QPSK from the Modulation Type list, set the Symbol Rate to 3.072 MHz, set the Measurement Filter to Root Raised Cosine, Reference Filter to Raised Cosine and Filter Parameter to 0.33
In most modulations schemes, signals are filtered to reduce the bandwidth prior to transmission. When the signal under test has been passed through a modulation filter, a matching measurement filter should be selected in the instrument in order to interpret symbols properly. The Reference Filter, while not used for demodulation, is important for making the proper error vector measurement calculations. This filter will normally be defined by the modulation/demodulation standard.
Click the Stop button.
Select the Symbol Table, right click the screen and select Add Marker.
Drag the marker to any cell in the Symbol Table. This sets the marker time value to the starting time for the cell containing the marker.
Look for MR in the Constellation Graph. Move the marker to different cells in the Symbol Table.
Each symbol value is assigned a position in the constellation diagram. The Symbol Table will be used to demonstrate the mapping between quadrants in the Constellation Diagram and the Symbol Values.
Not only are all the displays correlated through the shared data set, but the markers, which are inherently correlated, make it very easy to see how a data point in one domain relates to a specific data point in any other. This can greatly reduce troubleshooting time.
Thanks for watching. New videos are added periodically, so check back frequently.