Missions to Titan are severely limited in available mass and power because spacecraft have to travel over a billion miles to get there, consuming large masses of propellants. Thus low-mass, low-power instruments are a high priority need for Titan missions. A miniature, liquid-phase, high-resolution, pulsed proton-NMR (1H-NMR) spectrometer was developed with low mass (1.5 kg), requiring low power, that can be operated cryogenically on the surface of Titan. This work focuses on new pulsed electronic circuits, optimized for a nuclear magnetic resonance (NMR) spectrometer for analysis of hydrocarbon liquids on Titan.

The miniature, low-temperature-compatible NMR Electronics Board. The circuit is capable of generating and processing signals up to 50 MHz.
A breadboard pulsed-proton NMR circuit was designed and fabricated, and initially tested at room temperature. The NMR functionality was checked by obtaining a Fourier transform NMR signal from a water sample. As a first step toward developing a low-temperature-compatible circuit, functionality of all the electronic components (each mounted on a test board) was evaluated, starting from room temperature down to 77 K, by blowing cold N2 gas on the components.

From this experience, a new, pulsed NMR circuit was developed with all low- temperature-compatible components. The NMR circuit is powered by a USB interface and consumes less than 0.5 W total electrical power during normal operation. Central to the operation of the circuit, a direct digital synthesizer generates the main frequency source, which is coupled through a transformer, followed by a four-pole, low-pass filter to allow generation of any frequency up to 50 MHz. A clock source was used to set the frequency of the output drive and input frequency of the receiver.

Since bipolar transistors failed below –150 °C due to a carrier freeze-out phenomenon in silicon, they were replaced with CMOS (complementary metal-oxide semiconductor) operational amplifiers and a voltage regulator. To maintain a constant input voltage to the NMR circuit at all temperatures, a temperature-compensated voltage regulator was constructed by inserting a thermistor (resistance temperature detector) and an adjustable voltage divider into the circuit. The new NMR circuit with all low-temperature-compatible components performed properly down to 77 K.

The low-temperature-compatibility of the NMR system is critical for chemical characterization of hydrocarbons in their natural liquid state in the Titan environment. Such an instrument will facilitate simplified spacecraft design because it will not require a warm electronics box. The NMR will provide non-destructive, first-hand identification of organic molecules, their functional groups, and relative abundances, thus allowing complex chemical analyses to be accomplished by a low-mass, low-power instrument.

This work was carried out by Soon Sam Kim, Shannon P. Jackson, and Mohammad Mojarradi of Caltech; and Christopher T. Ulmer for NASA’s Jet Propulsion Laboratory. NPO-49367

This Brief includes a Technical Support Package (TSP).
Low-Temperature-Compatible Electronics for a Miniature Nuclear Magnetic Resonance Spectrometer

(reference NPO49367) is currently available for download from the TSP library.

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This article first appeared in the October, 2014 issue of NASA Tech Briefs Magazine.

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