Engineering topics
Publications and source records attributed to Williamson, Cynthia K..
A 6-MHz 16-bit waveform digitizer was evaluated for use in atmospheric differential absorption lidar (DIAL) measurements of ozone. The digitizer noise characteristics were evaluated, and actual ozone DIAL atmospheric returns were digitized. This digitizer could replace computer-automated measurement and control (CAMAC)-based commercial digitizers and improve voltage accuracy.
Signal-induced noise is generated when a photomultiplier tube (PMT) is subjected to an intense light pulse. The PMT signal does not return to the dark current level after the signal is removed, but decays slowly (i.e., signal-induced noise). This is of practical significance for DIAL (Differential Absorption lidar) measurements where signal-induced noise decays are superimposed on the on-line (absorption) and off-line signals. Errors in the ozone density calculation result for stratosphere measurements. Other researchers have implemented mechanical choppers that block the intense pulse which may be from near field return scattering or scattering from a cloud. This configuration cannot be implemented for the DIAL system employed for aircraft measurements since the on-line and off-line pulses are 300 microseconds apart. A scheme has been developed in this study to electronically attenuate the signal induced noise. A ring electrode, external to the PMT photocathode, is utilized to perturb the electron trajectories between the photocathode and the first dynode. This effect has been used for position sensitive PMTs and suggested for gating PMTS.
A new 16-bit 6-MHz compact, lightweight waveform digitizer module has been tested using actual 300-nm lidar atmospheric returns. The noise level of this digitizer was tested and found to be substantially below the ozone number density to be measured. The digitizer is inexpensive and compact enough to be deployed in UAV aircraft and spacecraft environments. With 16-bit digitizers a resolution of 0.046 mV/step can be achieved, substantially improving resolution over 12-bit systems. This digitizer will find widespread use in future DIAL receiver systems.