Anritsu S251C manual Appendix B Windowing, Examples, Introduction

Appendix B

Windowing

Introduction

The Distance sub-menu (page 2-8)provides for setting the cable loss and relative propaga- tion velocity of the coaxial cable. The WINDOW key opens a menu of FFT windowing types for the DTF calculation.

The theoretical requirement for inverse FFT is for the data to extend from zero frequency to infinity. Side lobes appear around a discontinuity due to the fact that the spectrum is cut off at a finite frequency. Windowing reduces the side lobes by smoothing out the sharp transi- tions 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.

In situations where there may be a small discontinuity close to a large one, side lobe reduc- tion windowing should be used. When distance resolution is critical windowing can be re- duced.

Examples

The types of windowing in order of increasing side lobe reduction are: rectangular, nominal side lobe, low side lobe, and minimum side lobe. Figures B-1thru B-4show examples of the types of windowing.

DistanceToFault

-5
-10
-15
-20
-25
-30
ReturnLoss(dB)
-35
-40
-45
-50
5	10	15	20	25	30	35	40	45	50	55	60
					Feet

Figure B-1. Rectangular Windowing Example

B-1