Hawkeye: Custom FFT Processing

The custom FFT is displayed by the FFT plot. The custom FFT is computed from the safety service FFT. The custom FFT allows the user to have simultaneously different FFT bandwidth on the same channel, which is not possible with the regular FFT provided by the FFT service.

  1. Activate the custom FFT
  2. Choose the window (Hanning or flat top). Note that for the regular FFT, we always use Hanning window.
  3. The purple dot indicates that the custom FFT is ON.

  1. Both instrument display the same channel
  2. But shows different FFT bandwidth. Changing Zoom / bandshift on one instrument does not affect the other instrument.
  3. Same for the peak-hold trace, which is maintained by each instrument independently. Clearing the peak-hold on one instrument does not clear it on the other instrument.

Note: in custom FFT mode, the FFT instrument receives from the safety service 1k points covering the bandwidth on display. For each instrument, the safety service works out the portion of the FFT to send out and the bin reduction to perform. Bin reduction is done in the power domain

  • by summing over adjacent bins when using Hanning window
  • by picking the highest bin when using Flat-top window

For example, if the safety FFT is 8k, and the user is showing the whole bandwidth, each group of 8 bins will be reduced to 1 bin. If the user is showing 1/8 of the whole bandwidth, no bin reduction is necessary. If the user is zooming further than x8, FFT resolution decreases as the service won’t be able to send 1k points.

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