Positive-intrinsic-negative diode-based duplexer for microcoil nuclear magnetic resonance

Microcoil nuclear magnetic resonance (NMR), using receiver coils of diamete rs of order 100 mu m, is increasingly employed to observe very small (simil ar to 0.3 nl) samples with high sensitivity. However, many experimental asp ects of microcoil NMR differ greatly from conventional NMR. In particular, the duplexer is a device used to switch between the transmit and receive ph ases of the experiment. The conventional duplexer is a passive device emplo ying crossed diodes, that switch automatically to transmit mode when high r f power is present. In microcoil NMR, however, the transmitter power is nec essarily quite low, with voltages that do not greatly exceed characteristic diode voltage drops. Here we present the complete design and construction methods for a duplexer well suited to the special demands of microcoil NMR. (C) 2000 American Institute of Physics. [S0034-6748(00)00607-9].