Constant signal mode

Fig. 2.7 Schematic of the imaging modes in SPM. In constant signal mode, feedback control is used to keep the interaction of probe and sample at constant value. This results in the probe moving at constant distance above the surface, and the output is the controlled vertical position. In constant height mode the probe is scanned without feedback, and the signal is the varying interaction between probe and sample. As shown, if the sample is not sufficiently smooth, this interaction may be destructive.

Two working modes are used for the STM first, the constant height-mode, in which the recorded signal is the tunneling current versus the position of the tip over the sample, and the initial height of the STM tip with respect to the sample surface is kept constant (Fig. 22(a)). In the constant currentmode, a controller keeps the measured tunneling current constant. In order to do that, the distance between tip and sample must be adjusted to the surface structure and to the local electron density of the probed sample via a feedback loop (Fig. 22(b)). 

Fig. 3.2 Triangular velocity reference signal top) and drive error signal bottom) of a Mossbauer drive operating in constant acceleration mode. The error signal is taken from the monitor output F of the drive control unit (see Fig. 3.1). Usually it is internally amplified by a factor of 100. Here, the deviations, including hum, are at the 2%o level of the reference. The peaks at the turning points of the triangle are due to ringing of the mechanical component, induced by the sudden change in acceleration (there should be no resonance line at the extremes of the velocity range)...

Images can be made in variable or constant force mode. In the latter case the difference signal from the photo detectors is used to adjust the distance between tip and surface, such that the force between the two, and thus the deflection of the cantilever, remains constant. An important advantage of working in constant force mode is that the overall orientation of the surface with respect to the z direction is not so critical, because the z piezo compensates for any inclination of the sample. 

Two different operation modes are commonly used. In the topographic mode, images are created by scanning the tip in the xy plane and recording the position required to keep It constant. A three dimensional map z(x, y, It = const.) is recorded. In the constant height mode, the probe scans the surface while the signal at the z-scanner is kept constant, and a three-dimensional image It(x,p,z = const.) is created. 

Images can be made in either variable or constant force mode. In the latter case, the difference signal from the photo detectors is used to compensate the dis-... 

Even more interesting is a new spectrometer that has been built with a novel moving system that synchronizes the movement of the radioactive source with that of the detector (206). This system allows the accumulation of Mossbauer spectra either in constant acceleration or constant velocity modes, therefore giving better signal-to-noise ratios and also reducing the time necessary for accumulation of a spectrum. Narrower line widths were also obtained with this equipment design. 

The power supply to the electrolyser was a Model 710 from The Electrosynthesis Company, Inc. of Lancaster NY. It was operated in constant current mode rather than in constant voltage mode. The maximum current and maximum voltage available was 50 amperes and 20 volts, respectively. In addition to current measurement provided by the power supply, a calibrated shunt was connected to the output to allow for independent measurement of current. Voltage taps independently connected to the cell electrodes were connected to the data acquisition system (DAS). The instrument signals from thermocouples, pressure gages, and flowmeters were connected to the DAS, which was comprised of a Dell computer with special acquisition boards and Labview software. Observations and some data were manually recorded in a laboratory notebook. 

The step size is given by the size of the scan area and the pixel resolution of the image . Tire acquired signal - either the voltage needed to move the tip up and down in z direction (constant current mode) or the tunneling current itself (con-... 

Fig. 17 Example of application of 4D HNCACO technique, (a) Pulse sequence. Evolution for CO is in the real-time mode, and for N and CA in semi-constant-time mode (a, = (/, -l- A)/2, 6, = t,(l-A/Wi)/2, C = A(l- 6Amaxi)/2) or constant-time mode (a,- = (A -I- /,)/2, 6,- = 0, c,- = (A—/,)/ 2), where A stands for An ca and Aca-co. respectively, t, is the evolution time in ith dimension and imaxi is the maximal length of evolution time delay. Delays were set as follows An h = 5.4 ms An-ca = 22 ms Aca-co = 6.8 ms. (b) Coherence transfer in the peptide chain. Amide nitrogen and proton frequencies (filled colored rectangles) are fixed during Fourier transformation. Each plane contains CO-CA peak for i and i—1 residue, (c) 2D spectral planes for CsPin protein obtained by SMFT procedure performed on the 4D HNCACO randomly sampled signal (Poisson disk sampling) with fixed Hn and N frequencies obtained from 3D HNCO peak list (d) 2D spectral planes for MBP obtained in the same manner. Reprinted with permission from [81]...

Instrumentation. A cantilever with a sharp tip interacting with the surface under investigation is used. The actual bending of the cantilever is measured with a laser beam deflected from a mirror-like surface spot on the back of the cantilever towards a position-sensitive photodetector. The measured signal is used to control the piezo actuators. A constant force mode in which the cantilever-surface distance is kept at a preset interaction force and a constant height mode of scanning operation are possible. The principle of operation is schematically outlined in Fig. 7.9. 

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