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Wideband RF Transmit-Receive Switch for Multi-Nuclei NMR Spectrometers

The utility of wideband high field nuclear magnetic resonance (NMR) spectrometers is significant in many applications. Wideband radio frequency (RF) front-ends are critical for the realization of multi-nuclei spectrometers. To realize such systems, wideband Transmit-Receive (TR) switches and low noise amplifiers (LNA) should be implemented at the RF front-end (FE) of the system. Most of the common designs used nowadays on NMR spectrometers are either inherently narrowband and/or of relatively high noise-figure which limit the overall system performance to a large extent. To improve upon the conventional FE performance, a new design of wideband high-power TR switch which could be devised for multi-nuclei NMR spectrometers is introduced in this brief. The developed wideband TR switch employs PIN diode switching technique and a dual-driver circuit. A prototype was constructed and experimentally characterized. The small-signal measurement results show that the switch has a bandwidth from 30 – 200MHz where the return loss is maintained below 18 dB with insertion loss of 0.09 - 0.2dB in the transmit mode and 0.11 – 0.37dB in the receive mode. It also exhibits a very high isolation of 63 - 80dB over the entire bandwidth. It is demonstrated that the switch can withstand continuous-wave RF input with power levels as high as 125 W while maintaining low insertion loss and high isolation.