Doctoral Dissertations

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Electrical Engineering

Major Professor

James M. Rochelle

Committee Members

William Bugg, Paul Crilly, Vaugh Blalock, Donald Bouldin


A constant-fraction discriminator (CFD) is a time pick-off circuit providing time derivation that is insensitive to input-signal amplitude and, in some cases, input-signal rise time. CFD time pick-off circuits are useful in Positron Emission Tomography (PET) systems where Bismuth Germanate (BGO)/photomultiplier scintillation detectors detect coincident, 511-keV annihilation gamma rays.

Time walk and noise-induced timing jitter in time pick-off circuits are discussed along with optimal and sub-optimal timing filters designed to minimize timing jitter. Additionally, the effects of scintillation-detector statistics on timing performance are discussed, and Monte Carlo analysis is developed to provide estimated timing and energy spectra for selected detector and time pick-off circuit configurations. The traditional delay-line CFD is then described with a discussion of deterministic (non statistical) performance and statistical Monte Carlo timing performance. A new class of non-delay-line CFD circuits utilizing lowpass- and/or allpass-filter delay-line approximations is then presented. The timing performance of these non-delay-line CFD circuits is shown to be comparable to traditional delay-line CFD circuits.

Following the development and analysis of non-delay-line CFD circuits, a fully-monolithic, non-delay-line CFD circuit is presented which was fabricated in a standard digital, 2-μ, double-meta], double-poly, n-well CMOS process. The CMOS circuits developed include a low time walk comparator having a time walk of approximately 175 ps for input signals with amplitudes between 10-mV to 2000-mV and a rise time (10 - 90%) of 10 ns. Additionally, a fifth-order, continuous-time filter having a bandwidth of over 100 MHz was developed to provide CFD signal shaping without a delay line. The measured timing resolution (3.26 ns FWITh1, 6.50 ns FWTM) of the fully-monolithic, CMOS CFD is comparable to measured resolution (3.30 ns FWHM, 6.40 ns FWTM) of a commercial, discrete, bipolar CFD containing an external delay line. Each CFD was tested with a PET EGO/photomultiplier scintillation detector and a preamplifier having a 10-ns (10 - 90%) rise-time. The development of a fully-monolithic, CMOS CFD circuit, believed to be the first such reported development, is significant for PET and other systems that employ many front-end CFD time pick-off circuits.

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