Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1997-06-03
1999-12-14
Ballato, Josie
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
324318, 324320, G01V 302
Patent
active
060022566
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
1. Field of the Invention
The present invention relates to a magnetic field pulse generator, in particular for generating an rf pulse for use in a nuclear magnetic resonance (NMR) investigation. The invention is particularly concerned with providing such a generator for use in a down-hole oil well logging application.
2. Description of the Related Art
Conventional NMR pulse signal amplifiers employ linear class A/B amplifiers (typically valve amplifiers).
SUMMARY OF THE INVENTION
In accordance with the present invention there is provided an rf magnetic field pulse generator for generating an rf magnetic field pulse for use in an NMR investigation, the generator comprising; arm comprising two serially connected solid-state switches; each pair of serially connected solid-state switches respectively, the transmitter coil being adapted to transmit the magnetic field pulse in response to an alternating rf signal at the output terminals; arm at a required radio frequency in use whereby the power source is connected with alternating polarity across the transmitter coil and generates an alternating rf signal at the output terminals.
The configuration of switches according to the present invention is known as an "H-bridge". Preferably the solid-state switches comprise class D/E amplifiers. It has been found that this configuration is suitable for use in an NMR investigation (especially a down-hole well logging device) since it is reliable (low voltage stress on the solid-state switches), and a minimum number of capacitors are necessary (capacitors are difficult to manufacture and can be unreliable at operating temperatures of the order of 150.degree. C.).
The design provides greater efficiency (i.e. low power loss in the amplifier components) than the conventional design which employs class A/B amplifiers. It also provides greater simplicity (few components), smaller size and higher robustness and reliability (due in part to the use of solid-state devices).
Preferably, the solid-state switches are MOSFET switching power transistors, such as HEXFETs. These have been found to be suitable for the power and switching requirements of a down-hole NMR application. In addition the MOSFETs are able to withstand operating temperatures of the order of 150.degree. C. albeit at reduced current rating.
The switches may each comprise a single solid-state switch, or may each comprise a number of solid-state switches in parallel.
Preferably, the control means comprises a circuit adapted to generate two control signals which are 180.degree. out of phase with each other and drive the gates of diagonal pairs of switches in the H-bridge. Preferably, the control circuit comprises another H-bridge, controlled by an rf signal generator.
Preferably, the transmitter coil is part of a resonant circuit comprising a capacitance (typically a capacitor) connected in parallel or series with the coil.
Preferably, the resonant circuit comprises a capacitor connected serially with the transmitter coil. The use of a series tuned circuit which has a low impedance at resonance, (rather than a parallel tuned circuit which has a high impedance at resonance) ensures that all nodes in the circuit remain at or below the local supply voltage (typically 250 Vdc), except the connection between the transmitter coil (L.sub.T) and the capacitance (C.sub.T), which may exceed 5 kV depending on the tuned circuit quality factor, Q (=.omega.L.sub.T /R.sub.T). Preferably, this is a permanent connection and the capacitance is located in an oil filled centre section, with the transmitter coil. If it is necessary to tune the resonant circuit in service, due to B.sub.0 drift and/or drift in C.sub.T or L.sub.T, extra tuning capacitors can be switched in between pulse trains. Preferably the capacitor switches are highly specified high voltage and high current relays.
Alternatively, the transmitter may not form part of a resonant circuit. In this case, driving the coil directly without a tuning capacitor will only work if its characteristic time constant is muc
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Ballato Josie
Fetzner Tiffany A.
Oxford Instruments (UK) Ltd.
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