October 10, 2013
Radar designers and test engineers have quite a challenge with today’s advanced radar systems. Greater precision is necessary to measure narrower pulse widths and/or to examine intra-pulse behavior with finer resolution, including rise/fall edge effects or the profile within a pulse compression signal. That is true of all radar systems, which are becoming more common in both military and civilian applications, such as:
- Surveillance (threat identification, motion detection, or proximity fuses)
- Detection and tracking (target identification and pursuit or maritime rescue )
- Navigation (automotive collision avoidance or air traffic control)
- High-resolution imaging (terrain mapping or landing guidance)
- Weather tracking (storm avoidance or wind profiling)
Fortunately for these engineers, modern vector network analyzer (VNA) technology is up to the challenge of today’s radar systems. A VNA such as the Anritsu VectorStar™ MS4640B with PulseView™ eliminate the common tradeoffs and limitations associated with many previous generations of VNAs. This advanced capability gives engineers confidence in their cutting-edge designs.
Using this new generation of VNA, engineers can employ a number of S-parameter measurements and pulse profiling techniques when testing radar systems such as the one shown in figure 1. Let’s discuss the three most common measurement modes.
Figure 1
Point-in-Pulse
Point-in-pulse quantifies S-parameter data at any region of time within a pulse. Measurements are made with swept frequency or power and plotted accordingly. This measurement mode is useful when trying to avoid possible edge effects of the pulse. Point-in-pulse measurements are useful when you need to measure the pulse as a whole, but the structure within the pulse is not of great interest nor is the variation from pulse to pulse.
Pulse Profiling
A VNA can also be used to make pulse profiling measurements, which focus on the structure within the pulse. The measurements are made in the time domain, while the frequency and power are kept constant. This measurement mode is useful for determining pulse characteristics, such as overshoot/undershoot, droop, and edge response (e.g., rise/fall time).
In order to characterize the portion of the pulse being profiled, start and stop times are specified relative to the synch pulse, along with a number of time points/steps and measurement width. The measurement may begin before the physical pulse arrives at the DUT and end after the pulse is gone to ensure potential delays and all DUT transients are captured. The measurement window width determines the measurement resolution. Averaging may be performed across multiple pulses. Figure 2 shows an example of a pulse profile measurement.
Figure 2
Variation between pulses may be difficult to observe with pulse profiling measurements. To capture variation, a pulse-to-pulse measurement is preferred.
Pulse-to-Pulse
The pulse-to-pulse measurement quantifies variations between pulses in a pulse stream. This measurement mode is useful when trying to determine if the pulse characteristics vary during a given period. For example, high-power amplifiers may have thermal effects that can cause variances in the gain or phase over time.
With pulse-to-pulse, wide ranges of measurement window widths and pulse widths are available, as in the other two modes. The upper limit is only restricted by the record length. For example, the VectorStar MS4640B VNA, with a record length of 0.5 seconds, allows for very wide pulses or low repetition rates. Again, an engineer can orchestrate measurements that cycle through a variety of frequencies and/or powers using multiple channels or setups.
There is much more that can be discussed on this topic, especially given the complexity of radar systems currently in the design stage. A full application note entitled VNA’s High-Speed Architecture Advances Radar Pulse Measurement Timing Resolution and Accuracy provides much greater detail.
