Pulsed radar systems transmit high power signal pulses followed by a pause during which echo signals are received. In many pulsed radar systems, the radio frequency of the emitted pulses remains constant, while the pulse repetition interval (PRI) and the pulse width (PW) vary.
The PRI determines the unambiguous range; the longer the PRI, the higher the unambiguous range. The width of an unmodulated pulse determines the minimum distance to the target and the range resolution. Shorter pulses allow detection at shorter distances and improve the range resolution, i.e., to resolve objects as separate items, but they require more spectral bandwidth. Longer pulses emit moreenergy per pulse and therefore reach higher ranges.
Spectrum analyzers have become the tool of choice for analyzing radar signals. They provide a wider frequency range than oscilloscopes and allow detailed in-pulse measurements of phase andfrequency, which cannot be achieved by simple, power-based pulse analyzers. Spectrum analyzers have made huge leaps in bandwidth analysis over recent years. The R&S FSW signal and spectrum analyzer from Rohde & Schwarz now features up to 2 GHz bandwidth analysis and a frequency range of up to 67 GHz. This makes it possible to analyze even very short pulses.
To analyze radar signals in the modern world the R&S FSW signal and spectrum analyzer needs to offer flexibility. For instance, marine and air surveillance radars regularly change their operation modes. They use different PRI and PW in search mode, acquisition mode or tracking mode where different trade-offs between measurement accuracy, minimum and maximum range and range resolution are required. Further techniques include modulation of phase or frequency during a pulse, which encompasses pulse compression.
For development, optimization and troubleshooting of radar transmitters, pulse trains have to be characterized over long periods. To detect sporadic events or small but continuous effects like temperature drifts, it is desirable to contiguously capture and observe all emitted pulses over a period of up to several minutes.
Furthermore, a common means of influencing radars is range gate pull-off. The radar pulses are recorded and played back delayed, at higher power and probably with a changed pulse shape or frequency than for the naturally scattered pulses from the subject aircraft. The other radar receiver may lock onto the stronger echo return and the resolution cell eventually moves completely away from the subject aircraft. If the play-back is then suddenly stopped, the other radar receiver needs to readjust the leveling and go back through search, acquisition and tracking mode again. Development and optimization of such intelligent influencing techniques and countermeasures also requires the recording and analysis of long radar pulse sequences.
To address such issues, features such as rapid identification of spurious emissions, low phase noise and extensive pulse analysis functions running as software tools on the analyzer provide in-depth signal analysis possibilities, making the R&S FSW an essential tool in the development and production of radar systems.
Figure 1 shows the result of an analysis of radar pulses with the R&S FSW equipped with the R&S FSW-K6 pulse analysis software. Pulses of 1 µs width with a PRI of 100 µs were captured at a 200 MHz sample rate. The table highlights the pulse of interest and displays key parameters for each pulse such as rise time, pulse width, PRI and frequency. The graphs show frequency, magnitude and phase versus time of a single pulse. The analysis software allows further in-depth analysis of pulse parameters such as rise and fall times, dwell time, settling time, overshoot and undershoot.
The required high sample rate in combination with a limited capture buffer, however, reduces the total seamless capture and analysis period. As a solution, the R&S FSW-K6 pulse analysis software has been equipped with efficient memory management for analyzing pulse trends over long periods. It is in the nature of pulsed signals that during pauses only noise can be received. This makes it possible to extend the total capture time by omitting the noise during pauses.
A simple but effective algorithm to increase the total observation period is to store and time-stamp I/Q samples over a user-defined period once a certain power level triggers the capture. In addition, a certain number of pre-trigger samples are also stored. All other samples are omitted until the next trigger event. With typical duty cycles of 1 percent, the maximum observation period can principally be extended by up to a factor of 100.
Practically, with 50 percent pre-trigger capture and a capture time per pulse of twice the pulse period, the maximum recording time is extended by a factor of 50. Lower duty cycles extend the maximum recording time even further. The segmented I/Q capture can be triggered by an external trigger as well as by an IF power trigger.
EVALUATION OF PARAMETER TRENDS
Capturing many consecutive pulses makes it possible to analyze parameter trends and track changes that occur from pulse to pulse. Figure 2 displays the pulse width versus the pulse number over 20s capture time. This reveals that the radar system operates in three different modes (1, 2 and 3 µs pulse width), which appear in a random order. Without segmented capture, the maximum capture time for this signal at 200 MHz sample rate was only 2.3 seconds, not enough to see the pattern of different modes.
Segmented capture increases the total analysis period by omitting the pauses between pulses. Effects that occur over many pulses, like changing modes, become visible, making it easier to analyze complex radar systems with changing parameters.
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