Null-system

Zmuda, H., Toughlian, E. N., Payson, P. Klumpe, H. W. (1998). A photonic implementation of a wide-band nulling system for phased arrays. leee Photonics Technology Letters, Vol. 10, No. 5, pp. 725-727, Issn 1041-1135. 

Measurement of the isotopic ratio. This is normally achieved using either a bridge—amplifier balance system or the infinite bridge null system. The former method provides that the output of the major amplifier supplies a resistor divider network whose output is fed back to the input of the minor amplifier. 

Figure 1.5. The infinite bridge null system for isotope ratio measurement...

The null system can either be made as a reflecting system or as a lens design. Typical IR materials for lenses are germanium, barium fluoride, or zinc selenide. 

In case of concave surfaces the aspherical correction of the wavefront is done with a null system. If a convex surface has to be tested, such as the surface of a secondary mirror, testing is performed against a concave matrix glass. 

Double-Beam Optical Null Systems. Virtually all instruments used for analytical chemical applications utilize a double-beam optical null photometric system. In these instruments an electrooptical. servo-system continually attenuates the energy in the reference beam so that there is no net signal difference between the reference beam and the sample beam. The recording pen indicates the position of the reference beam attenuator and therefore the relative transmittance of the sample. 

Figure 2-8 shows a schematic diagram of an optical null system. Some of the key components in this system may now be described. 

The synchronous rectifier S is mechanically or electrically coupled to the sector mirror. It converts the amplified low-frequency output of the detector to direct current. The rectifier is phased with the optical chopper mirror so that the polarity of the rectified output indicates the condition of unbalance of the optical null system. That is, one polarity indicates more energy in the reference beam than in the sample beam. 

Having examined the working basis and requirements of a doublebeam optical null system, we shall next examine some of the properties of such systems of which the analyst should be aware. The accuracy of calibration of these systems is generally quite constant until the open area of the attenuator becomes very small. For very small openings it is virtually impossible to ensure precise calibration. The principal cause for this is the effect of slit width. If the slit is very narrow, the zero position of the attenuator is sharply defined. As the slit widens, the attenuator will have to move farther to stop the energy and produce zero signal. There is an analogous effect near 100%. 

Measurement of the zero level is not as simple as one might expect. The complication arises from the inherent lack of energy of a doublebeam optical null system at zero transmittance, where both beams are blocked. Under these conditions it is possible for the optical attenuator to drift or coast below its zero position, since the system has no means of returning the errant attenuator to the true zero. In a well-operating instrument there are three potential causes for fallacious zero reading a too rapid approach to zero, an improper electrical balance, and scattered radiation. 

Improper Electrical Balance. The electrical balance control in an optical null system equalizes the reference and sample signal phases, including the effects of spurious pickup signals within the instrument. A negative unbalance will drive the attenuator below zero when both beams are blocked. A positive unbalance will tend to stop the attenuator before it reaches the true zero. The balance control is set for zero drift under zero-energy conditions, i.e., with both beams blocked. Therefore, patience must be exercised to avoid the problem of momentum drift discussed above. 

The subsequent use of the rectified signal depends on the photometric system employed. In single-beam systems it is fed directly to a potentiometric recorder. However, in a double-beam optical null system the rectified signal is remodulated, this time at the line frequency (e.g., 60 Hz). The object of this is to obtain an ac signal which can be amplified to sufficient power to drive a servomotor, which then positions the optical attenuator to establish a null signal. Line-frequency remod-... 

The ordinate speed of a recorder, which limits how rapidly information may be recorded, is ultimately determined by the chopping speed of the spectrophotometer. In general, if the sample beam is blocked, most recorders are capable of traveling full scale in 1 to 2 seconds. This is sometimes referred to as the slewing time. It should be noted, however, that a properly operating null system never runs... 

Documentation of Molecular Spectroscopy 347-348, 354, 357 double beam 8-10, 24 null system 15... 

Thus when all the plane angles exceed 120° a little, one obtains, in some cases, a true film system it happens, for example, with the frame of the polyhedron formed by cutting down the comers of a cube by equilateral sections which join, in a manner that there are only triangular faces and square faces. But, in this polyhedron, all the plane angles are only approximately 125° moreover the tme and symmetrical film system occurs with difficulty, and only when one withdraws the frame by a triangular face when one withdraws it by a square face, it always gives a null system. 

Perfect systems, imperfect systems, null systems conditions under which one obtains them. 205... 

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