Development of a Compact Tunnel Diode Oscillator Circuit for High Magnetic Field Applications

We have developed a new design for a small tunnel diode oscillator (TDO) circuit which minimizes eddy current pickup in changing magnetic fields, minimizes parasitic reactance for achieving high frequency operation, and is small enough to fit in the confined spaces available in high magnetic field dilution refrigerator environments. The circuit size is slightly less than 3 x 8 mm². The TDO circuit has been developed in collaboration the National High Magnetic Field Laboratories and available for use in Tallahassee and Los Alamos. The circuits all have two diodes with a peak current of 250 or 500 uA and the ability to select either of the two diodes or both in parallel. Therefore depending on the size of the two diodes, peak currents of 250, 500, 750 or 1000 uA are available. Future versions will have 100 uA diodes. We have tested the circuits with coils from 0.2 mm in diameter and two turns to 1.0 mm in diameter and 6 turns. We have also worked out how to calculate the resonant frequency of a particular coil including the contributions of the twisted pair or coax between the coil and the TDO board, and the parasitic capacitance of the coil and TDO board. We will show test data of oscillators between 200 and 1500 MHz. The circuit has been tested at Clark University up to 45 T and Los Alamos up to 60 T in pulsed magnetic fileds. In addition we have run experiments in the 32 T all superconducting magnet in Tallahassee. In all cases, due to the circuit’s small size it was able to continue to work at field center despite the high fields and high dB/dt. We will show examples of applications such as measuring superconducting transitions and quantum oscillations.