Volume 78, Issue 11, November 2007
Index of content:
studies" title="Invited Article: An improved double-toroidal spectrometer for gas phase studies" />
A new spectrometer is described for measuring the momentum distributions of scatteredelectrons arising from electron-atom and electron-molecule ionization experiments. It incorporates and builds on elements from a number of previous designs, namely, a source of polarized electrons and two high-efficiency electrostaticelectron energy analyzers. The analyzers each comprise a seven-element retarding-electrostatic lens system, four toroidal-sector electrodes, and a fast position-and-time-sensitive two-dimensional delay-line detector. Results are presented for the electron-impact-induced ionization of helium and the elastic scattering of electrons from argon and helium which demonstrate that high levels of momentum resolution and data-collection efficiency are achieved. Problematic aspects regarding variations in collection efficiency over the accepted momentum phase space are addressed and a methodology for their correction presented. Principles behind the present design and previous designs for electrostaticanalyzers based around electrodes of toroidal-sector geometry are discussed and a framework is provided for optimizing future devices.
- ELECTRONICS; ELECTROMAGNETIC TECHNOLOGY; MICROWAVES
78(2007); http://dx.doi.org/10.1063/1.2804131View Description Hide Description
A new type of drivers for light emitting diodes(LEDs) is introduced based on the switched-capacitor frequency modulation. In contrast to conventional constant dc current drivers, the current pulse is provided by this new switched-capacitor LED driver. In the present driver, the chargingcapacitor is charged and discharged through a LED and the current flow direction is controlled by a metal oxide semiconductor switch. The input current (and thus the LED brightness) is proportional to the switch clock frequency at relatively low frequencies and becomes saturated at relatively high frequencies. This new driver circuit is simple and robust and maintains high efficiency for a wide range of input powers. In addition, the dimming control is easily realized by modulating clock frequency. Finally, this LED driver consumes no dc current and thus provides inherent protection to LED in standby mode.
78(2007); http://dx.doi.org/10.1063/1.2813342View Description Hide Description
The properties of a differential transconductance amplifier coupled with a four channel measurement system are exploited in order to reach a very high sensitivity in current noise measurements. In particular, it is demonstrated that, in proper conditions, the noise contributions coming from the active and passive devices that make up the transresistance amplifier can be virtually eliminated. Moreover, the proposed measurement method allows the evaluation of the impedance of the device under test from noisemeasurement data. Actual measurement results are also reported that demonstrate the effectiveness of the proposed approach.