The RTG pulse detection algorithm operates as a threshold detector, monitoring signal power at the RTG input and triggering a detection event whenever the power crosses a configured threshold.

Reference Level and Pulse Detection Calibration

Although the RTG generates simulated targets at varying power levels, the system assumes the incoming radar pulse maintains a constant power level. This fixed input allows for the configuration of a reference level that optimally maximizes the system's dynamic range.

NI recommends adjusting the radar signal at the RTG input to a range of ±10 dBm. Operating within this range protects the hardware from damage while ensuring optimal signal quality during acquisition. The reference level should be configured approximately 3 dB above the peak expected radar power.

Because the RTG applies a calibrated amount of attenuation, any fluctuations in the input radar power translate directly to variations in the target output power. Furthermore, the chosen reference level directly impacts the performance of the pulse detection algorithm. Because the algorithm operates as a threshold detector, the reference level does not adjust automatically; it must be manually tuned to the incoming signal.

Pulse Detection Modes

The pulse detection algorithm supports two different modes of operation: Auto and Manual.

Pulse Detection Auto Mode

In Auto Mode (default), the algorithm monitors the input signal power and updates the minimum and maximum threshold levels used for detecting the rising and falling edges of a pulse. The pulse detection block continuously monitors the input signal to maintain a history of the peak power. Based on this peak power, the threshold levels used to detect pulse edges are as follows:

  • Maximum threshold (rising edge): 50% of the peak power
  • Minimum threshold (falling edge): 25% of the peak power

When the input signal power increases, Auto Pulse Detection automatically recalibrates the maximum threshold using the most recently calculated peak power. However, when the input signal power decreases, the maximum threshold does not automatically reduce. In this case, you can only lower the threshold using either of the following methods:

  • Performing a manual reset using the Reset Pulse Detection VI
  • Enabling periodic auto-calibration by configuring the Auto Threshold Time parameter
  • Auto Threshold Time parameter—Defines the time interval, in seconds, after which the RTG automatically resets the existing threshold levels and recalibrates the pulse detection algorithm. During recalibration, new threshold levels are established based on the current input signal power. The default value is 0. When set to 0, the threshold levels are not recalibrated automatically. Configuring a non-zero value enables periodic automatic recalibration. NI recommends setting Auto Threshold Time to a value more than the Pulse Repetition Interval (PRI) to ensure that at least one pulse is captured during each calibration interval. To manually reset the threshold levels in Auto Mode, use the Reset Pulse Detection VI.
    Note At least one valid pulse must occur within the Auto Threshold Time window for the pulse detection algorithm to correctly calibrate the threshold levels based on the signal power. If no pulse is present, the calibration might be influenced by the noise floor instead of the actual signal.

    • Pulse Detection Manual Mode

    In Manual Mode, you must explicitly run the Pulse Detection Calibration to establish the threshold levels based on the input signal level present at the time of calibration.

    When Pulse Detection Calibration is initiated, the RTG resets the signal monitor and calculates the peak power of the input signal. Using this peak power, the threshold levels for pulse edge detection are as follows:

    • Maximum threshold (rising edge): 60% of the peak power
    • Minimum threshold (falling edge): 20% of the peak power

    Because the thresholds are derived from the signal present during calibration, ensure that a known, representative input signal is applied when running the Manual Pulse Detection Calibration.

    Clocking and Timing Adjustments

    You can adjust pulse detection timing to prevent pulse-width truncation on input signals with slow rise or fall times.

    Pre-Trigger Clocks and Post-Trigger Clocks

    Figure 11. RTG Pre-Trigger and Post-Trigger Clocks

    The pulse detection algorithm determines the beginning and ending of a pulse. In the Pulse Aware personality, these points define when a target configuration is applied to an incoming pulse.

    If the input pulse has slow rise or fall times, the default detection points might cause partial pulse width clipping. In such cases, timing adjustments can help ensure the target configuration fully encompasses the pulse duration. The RTG provides configurable timing offsets that allow you to shift when the pulse detection block triggers the pulse rising and falling edges relative to the edges detected by the algorithm. Set these parameters in the Configuration (INI) file.

    Timing Adjustment Parameters

  • pre_trigger_clocks—Specifies the number of FPGA clock cycles by which the pulse rising edge trigger occurs before the rising edge detected by the pulse detection algorithm.
  • post_trigger_clocks—Specifies the number of FPGA clock cycles by which the pulse falling edge trigger occurs after the falling edge detected by the pulse detection algorithm.
    • Figure 12. RTG Timing Adjustment Parameters