IMPROVING MEASUREMENT QUALITY60Improving measurement qualityRemoving interfering signalsInterfering signals can be recognised because they have a fixed frequency,usually a multiple of the local mains frequency (50 or 60 Hz) throughoutthe image. Thus, they are manifested by straight lines that run throughoutthe image.Possible interference sources are:• Mechanical vibrations from machines or heavy transformers in the envi-ronment (e.g. pumps).• Infrared light sources (light bulbs, sample illumination in an invertedmicroscope).• Electrical interference (in the electronics, or in electrical forces in the tip-sample interaction).Mechanical vibrationsMeasure the frequency of the vibrations to find out if the interference is dueto mechanical vibrations. These vibrations have a frequency that is (a mul-tiple of ) the rotation frequency of the source. This frequency is usually nota multiple of the local mains frequency, and may change slightly over time.Try the following to find out if the interfering signal is due to mechanicalvibrations:- If possible, turn off all rotating machines (i.e. pumps) in the room.- Change the vibration isolation by putting the Scan head directly on thetable, instead of on the Sample stage.To reduce the influence of these vibrations, either improve the isolation ofthese machines, or improve the isolation of the instrument by using a vibra-tion isolation table (e.g. the optional TS-150 active vibration isolationtable).
