Circuit Diagram
The figure above shows the circuit diagram for the RF voltage-controlled current source (VCCS). Rin and Cin set the low-frequency corner and impedance match to an external RF source, such as a DDS. Cin can be omitted if the RF source can tolerate a DC bias and your application requires DC currents. The antenna is represented by Lc, Rp, and Cp. The RF VCCS will work with a variety of antenna designs provided that the antenna is an inductive or resistive load at low frequency. U1 servos the voltage across Rs (and therefore the current through Lc). The DC transadmittance is (1+Rf/Rg)/Rs, which should be set to match the dynamic range of the RF source to the desired antenna current range. The gain compensation inductor Lg flattens the peak in the response caused by the RLC circuit formed by Lc and the output impedance of the current-feedback amplifier. Lg needs to be tuned empirically for each antenna to achieve the best performance. We have found that a good initial guess is Lg = 2*Rg/ωn (see Initial Setup).