Noise in phototransistor due to packaging has been investigated via theoretical models and measurements. A new design of package for phototransistor to reduce noise is proposed and tested for commercial photoelectric sensors. Generally, there are two kinds of materials, namely, metal and plastic used for the package, and a metal case made of aluminum or copper is the more popular. However, a metal case could pick up noise from electromagnetic fields, such as radio frequency noise caused by micro controllers. The noise could go into a phototransistor because of the metal case connected to the collector of the phototransistor, and amplified further, and could turn on a photoelectric sensor. Surface current induced by electromagnetic waves on a metal case due to skin effect is analyzed using theory of electrodynamics. Comparison of phototransistor packaged using the new design in photoelectric sensor and one in commercial is carried out. The latter is locked on under radio frequency noise environment, but, the former is not.
We report on a bridge structure PZT [Pb(Zr<SUB>x</SUB>Ti<SUB>1- x</SUB>)O<SUB>3</SUB>] thin film microtransducer with proof mass that has been fabricated successfully at the Microtechnology Laboratory (MTL) of the University of Minnesota. The bridge microtransducer is made on silicon wafer using bulk micromachining of microelectromechanical systems (MEMS) and special techniques for deposition of a PZT thin film. The bridge is 300 micrometers wide, 1000 micrometers long, and a few micrometers thick. A proof mass made from the silicon wafer is loaded under the bridge at the central region, its area is 300 X 300 square micrometers and its thickness is 475 micrometers (same as the wafer). Used as an accelerometer, the microtransducer is calibrated using a Vibration Test Systems (VTS), which is a commercial accelerometer calibration instrument. The sensitivity of the microtransducer is constant over the range of frequencies from zero to 10 kHz, 240(mu) V/g at 0.5g with a dc bias voltage of 0.2 volts and a deviation of 5%. The Brownian thermal noise equivalent acceleration is 9.072(mu) g/(root)Hz. Design of a bridge structure with mass loading is modeled using ANSYS. Simulation analysis shows that the fundamental natural frequency of the microtransducer is 11.352 kHz, which is close to the measured resonant frequency of 12.28 kHz.