An increase of a down-hole nuclear magnetic resonance tool's reliability and accuracy by the cancellation of a multi-module DC/AC converter's output's higher harmonics

Abstract

Described in this paper is a method for improving higher harmonic cancellation in nuclear magnetic resonance transmitters, which are used in oil and gas well logging tools operating at 175 °C. Multi-module multi-level topology which combines the outputs of several identical power modules operating at 50% duty cycle at the fundamental frequency provides the versatility needed for both low harmonic sine voltage synthesis and amplitude control. Cancellation of the output voltage higher harmonics is achieved by creating fixed relative phase shifts between the individual modules of the multi-module converter. The amplitude control employs the Chireix–Doherty outphasing modulation principle with added feed forward correction circuitry. The possibilities of a 20% increase of the tool signal-to-noise ratio, as compared with that of a two-module transmitter has also demonstrated significant increase in the tool life expectancy.