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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addition to the fundamental tester. Such a converter may introduce phase and flatness impairments or other distortion. Corrections for these must be an integral part of the measure- ment system. Pulse bandwidth is the next consideration. Modern radars are using wider bandwidth pulses, such as faster rise times and wider modulation bandwidths. Many measurements can be properly recorded only if the entire bandwidth is captured at once. The third consideration is modulation. What varied modu- lations need to be measured, and what properties of the modulation are critical? Some types of chirped pulses require only that the carrier frequency sweep over the specified range. But many others require that the carrier sweep meets a linearity specification. These pulse parameters impact the linearity and dynamic range requirements placed on the test equipment, as well as the phase and frequency flatness of the instrument measurement bandwidth. Measurements of small signals in the presence of high- power ones, or high-accuracy phase measurements over long time intervals, may require a high dynamic range or bit depth of digitization. Additionally, complex modulation schemes may require built-in specialized demodulation processes. Traditional Oscilloscope Measurements The oscilloscope is the fundamental tool for examining vary- ing voltage versus time. It is well suited to display the shape of baseband pulses. The origin of oscilloscope performance parameters traces back to characterizations of early radar pulses. Today's ultra-high-performance real-time oscillo- scopes have bandwidth up to 70 GHz and are designed to capture and display either repetitive or one-shot signals. Real- time oscilloscopes work well for displaying baseband pulses. Pulses with very fast transition times or very short duration (sub-nanosecond or shorter) can be accurately seen on a high-bandwidth oscilloscope. Modern oscilloscopes operate on live time-domain data, while frequency domain measurements are made on the time-sampled acquisitions of stored data. This architecture allows oscilloscopes to discover baseband pulse time-domain errors, such as a single pulse that has a narrower pulse width than even hundreds of thousands of correct pulses. The oscilloscope provides a time-domain display with a high waveform capture rate, while an acquisition technology processor operates directly on the digital samples live from the A/D converter, allowing it to discover rapid variations or one-shot events in the time-domain display. For wideband measurements, this allows the oscilloscope to see even momentary transient events using the voltage versus time dis- play. In Figure 1, a one-time transient has been captured and is shown in blue. The blue represents very low-occurrence transients, while red represents parts of the waveform that are constantly recurring. One of the most highly developed capabilities of the oscil- loscope is triggering. Recent advances have enabled meth- ods of triggering an acquisition or measurement based on the voltages and voltage changes in one or more channels. These range in complexity from simple edge or voltage- level triggering to complex logic and timing comparisons for combinations of all of the input channels available. Pattern recognition, both parallel and serial, triggering on "runt" or "glitch" signals all are available in oscilloscopes. One useful trigger for radar applications is the ability to specify two discrete trigger events as a condition for acqui- sition. This is known as a trigger sequence. The main or "A"-trigger responds to a set of qualifications that may range from a simple edge transition to a complex logic combina- tion on multiple inputs. Then, an edge-driven "B-Delayed" trigger can be specified to occur after a delay expressed in time or events. The B-trigger is not limited to edge triggering. Instead, the oscilloscope allows the B-trigger to look, after its delay period, for a condition chosen from the same broad list of trigger types used in the A-trigger. For example, a designer now can use the B-trigger to look for a suspected transient occurring hundreds of nanoseconds after an A-trigger has defined the beginning of an operational cycle. Advanced trigger types, such as pulse width trigger, can be used to capture and examine specific RF pulses in a series of pulses that vary in time or in amplitude. For baseband pulses, the triggers based on edges, levels, pulse width, and transition times are of the most interest. If triggering based on events related to different frequencies is needed, then a spectrum analyzer will be required. Timing Measurements Traditional pulse measurements once were made by visual examination of the display on an oscilloscope. This was accomplished by viewing the shape of a baseband pulse; available measurements were limited to timing and voltage Techniques 34 Figure 1: Discovery of a single transient glitch in a train of pulses Electronic Military & Defense Annual Resource, 5th Edition