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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amplitude. These measurements were sufficient, though, as pulses generally were very simple, as shown in Figure 2. As pulse complexity has increased, fully automated base- band pulse timing measurements are required. Modern oscilloscopes provide single-button selection of rise time, fall time, pulse width, and other parameters. However, most of these measurements do not focus on the measurement enve- lopes of modulated radar signals. When used on pulse-modulated carriers, these measure- ments are of limited utility, because they are presented with the carrier of the signal instead of the detected pulse. This results in pulse width measurements that are made on a single carrier cycle and rise times of the carrier instead of the modulated pulse. This barrier is overcome easily by placing detectors or trigger settings on the input of the oscilloscope to remove the carrier. An example is shown in Figure 3. Adding to the value of the oscilloscope for radar measure- ments is the availability of vector signal analysis software, which combines the wide bandwidth of the oscilloscope (up to 70 GHz) with the fully automated pulse-measurement capabilities of a high-end spectrum analyzer. The vector sig- nal analysis software can use any of the input channels of the oscilloscope. It also can use the oscilloscope's "math trace" capability to combine data from multiple channels and then use math functions on the data. The measurements will have the same dynamic range as the oscilloscope while maintain- ing its full bandwidth. Spectrum Analyzer Measurements A traditional swept spectrum analyzer is a simple RF detector that is effectively swept across a selected span of RF frequen- cies. This produces a display of the combined RF spectrum of all signals within the selected span of frequencies. Measurements of carrier frequency, pulse width, and pulse duration can be made by observing the lines within the spectrum display, aided by on-screen marker readouts. The spectrum analyzer does particularly well at displaying the spectrum of a pulse-modulated RF carrier, provided the signal is repetitive and stable and the resolution bandwidth (RBW) and video bandwidth (VBW) controls are correctly set. Spectrum analyzers usually are optimized for the high dynamic range needed to see very small signals in the pres- ence of very large ones. Fast Fourier transform (FFT) based vector signal analyzers (VSAs) that use internal digitizers to sample an acquisition bandwidth at a fixed frequency may have as much as 75 to 85 dB SFDR (spurious-free dynamic range). Because of the inverse relationship between frequency and time, it is possible to determine basic pulse timing parameters using the spectrum analyzer frequency domain display. The pulse repetition time (pulse period) is the inverse of the frequency spacing between the finely spaced lines within the larger spectrum envelope. The pulse width is the inverse of the frequency spacing between the nulls in the spectrum envelope. Using a swept spectrum analyzer, there can be an alias between the sweep time and the pulse rate. The analyzer will provide a vertical deflection only at the exact time the pulse is ON and will produce no deflection during the pulse off-time. This may appear to be the pulse repetition frequency (PRF) lines, but the apparent frequency spacing will change as the sweep rate of the analyzer is varied. This manual change of sweep time is necessary to determine whether the lines seen are PRF or the sweep-time alias. FFT-based VSAs do not exhibit this alias. Swept spectrum analyzers also have a zero-span mode, Techniques 36 Figure 2: Measuring RF envelope timing with an oscilloscope Figure 3: Pulse width trigger capturing the RF envelope of the signal instead of the carrier data Electronic Military & Defense Annual Resource, 5th Edition Figure 4: Multiple chirps in one band