The theoretical requirement for inverse FFT is for the data to extend from zero frequency to infinity. Side lobes appear around a discontinuity because the spectrum is cut off at a finite frequency. Windowing reduces the side lobes by smoothing out the sharp transitions at the beginning and at the end of the frequency sweep. As the side lobes are reduced, the main lobe widens, thereby reducing the resolution.
Distance‑to‑PIM (DTP) Windowing Examples
In situations where a small discontinuity may be close to a large one, side lobe reduction windowing should be used. When distance resolution is critical, Rectangular windowing should be used.
The types of windowing in order of increasing side lobe reduction are: rectangular, nominal side lobe, low side lobe, and minimum side lobe. Figure: Rectangular Windowing through Figure: Minimum Side Lobe Windowing show examples of these types of windowing.
These four measurements were taken on a line with two PIM sources that were spaced 100 feet apart. Rectangular windowing provides the highest resolution, or ability to distinguish closely spaced PIM sources.
DTP Rectangular Windowing
Rectangular Windowing
Total distance is 200 feet. The PIM sources are separated by 100 feet.
This view of Rectangular Windowing shows the maximum side lobe display and the greatest waveform resolution.
DTP Nominal Side Lobe Windowing
Nominal Side Lobe Windowing
Total distance is 200 feet. The PIM sources are separated by 100 feet.
This view of Nominal Side Lobe Windowing shows less side lobe resolution than Rectangular Windowing and more side lobe resolution than Low Side Lobe Windowing. This level of windowing displays intermediate resolution.
DTP Low Side Lobe Windowing
Low Side Lobe Windowing
Total distance is 200 feet. The PIM sources are separated by 100 feet.
This view of Low Side Lobe Windowing shows less side lobe resolution than Nominal Side Lobe Windowing and more side lobe resolution than Minimum Side Lobe Windowing. This level of windowing displays intermediate resolution.
DTP Minimum Side Lobe Windowing
Minimum Side Lobe Windowing
Total distance is 200 feet. The PIM sources are separated by 100 feet.
This view of Minimum Side Lobe Windowing shows less side lobe resolution than Low Side Lobe Windowing and displays the lowest side lobe and waveform resolution.
