Two-photon excitation allows for excitation at half of the Larmor frequency. In Earth’s magnetic field, this corresponds to a Larmor frequency of about 2 kHz and an excitation frequency of about 1 kHz. By transmitting at 1 kHz and receiving at 2 kHz, we could transmit and receive at the same time by using filters to protect the receiver. While some harmonic distortion prevented us from fully separating the transmitted from the received signal during excitation, the always connected and unsaturated receive circuitry allowed us to eliminate receiver dead time, which can be quite long at low frequencies.
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