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Copyright (c) 2022 Eli Mattingly, Monika ?liwiak, John Drago, Erica Mason, Matthias Graeser, Lawrence Wald
This work is licensed under a Creative Commons Attribution 4.0 International License.
A harmonically pure and stable drive field is an essential component of any MPI system. Here, we present a filter topology and design methodology tailored for FFL MPI driven at a single frequency. It is a balanced and differential design that minimizes Ohmic losses by transforming the load impedance to a high value for the filtering stages and then back down with a transformer to present an input impedance of ~6 Ohms to the drive amplifier. We implement resistors in tuned elements of the drive filter to damp the high-amplitude side resonances these coupled,tuned elements would otherwise generate in the filter design. This helps damp the undesired resonances to avoid noise in the received signal. In addition to the pass-band at the drive frequency, we place notches at the 2nd and 3rd harmonics (total 100dB and 140dB attenuation, respectively) to specifically target these important harmonics. Finally, the circuit elements are picked to maximize stability of the drive current to temperature-induced tuning shifts. The design software (written in Julia), which calculates component values given load parameters and notch locations, has been made available at OS-MPI.github.io.
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