Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.
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about 500 MHz, the greater bandwidth of an oscilloscope is required to make the measurement. A simple view of the modulated pulse waveform is seen in Figure 5. With multiple pulses included in one waveform, it is not easy to see differences between the individual pulses. If there are differences, they are more subtle than can be seen from a voltage waveform. The signal analysis software uses the digitized voltage waveform stored in the oscilloscope to make the RF measure- ments. The frequency spectrum is shown in Figure 6. This pulse chirps from 250 MHz to 750 MHz. Initially, a pulse table was selected to analyze timing and amplitude measurements. The average ON power of the pulse numbers did have very small variations showing, but they did not appear significant at first glance. In order to see the small variations, the vertical scale was "zoomed" to a sen- sitivity of 0.1 dB per division. At this setting, there was very clearly a periodic variation of the pulse amplitude. The total peak-to-peak amplitude variation was about 0.07 dB. The horizontal scale is simply the number of the pulse whose amplitude is plotted vertically. The effective way to fully analyze the variation is to use the FFT of the trend data from this plot. As shown in Figure 7, the spectrum plot distinctly shows a peak disturbance at 4 kHz, which is 53 dB below the average value of the amplitude. Something is modulating the pulse power at a 4 kHz rate. The frequency of the disturbance can assist in troubleshooting components or subsystems within the radar causing this problem. To convert this dB reading to the amplitude variation, work backwards through the decibel formula: V = antilog (dB/20). Minus 53 dB equates to an RMS voltage variation of 0.22 percent. This is 0.63 percent peak-to-peak. Thus, a very subtle disturbance of this radar transmitter proved simple to analyze. Summary Improvements in radar technology have brought with them new ways to manipulate pulse transmission. This makes testing and measuring signals, as well as troubleshooting designs, a much more complex process than ever before, requiring more advanced test equipment and a variety of new methods. Designers are helped in this task by a power- ful new generation of signal generators, spectrum analyzers, and ultra high-performance 70 GHz oscilloscopes, along with VSA software that offers capabilities like real-time processing and analysis tools, as well as advanced triggering. Techniques 38 Figure 5: The voltage waveform of a 500 MHz wide chirped pulse Electronic Military & Defense Annual Resource, 5th Edition Figure 6: Spectrum display of the 500 MHz chirp using Tektronix SignalVu vector signal analysis software running on a DPO70000SX oscilloscope Figure 7: FFT of the trend of the pulse amplitudes Chris Lo berg is a senior technical marketing manager at Tektronix, responsible for oscilloscopes in the Americas Region. In more than 13 years with Tektronix, Loberg has served in various positions, including marketing manager for Tektronix' Optical Business Unit. His extensive background in technology marketing also includes positions with Grass Valley Group and IBM. Loberg holds an MBA in marketing from San Jose State University.