Contact is Chris/Mirco for Aephraim Steinberg's Lab.  The TOP coil modulation and drive is done with a signal generator and amplifier.  This circuit generates the AM (Amplitude Modulation) control for the signal generator.
The voltage range for the AM input on the signal generator (make? model?) is +6V for full signal to -3.7V for zero signal, a 10V range.  An external analog voltage from an arbitrary function generator which supplies step voltages inside that range is fed into this circuit.  The circuit simply tracks the step voltages with a controlled slew rate (AKA dV/dt).  The slew rate is adjustable with a user accesible potentiometer from 1Sec/10V to 30Sec/10V. This controlled slew rate can be overridden with a TTL signal. When set to "fast", it will track at <100uSec/10V.

How To Use

Connect the supply ±12V at <25mA, connect the input to the arbitrary signal generator, the output to the AM input of the TOP signal generator and the TTL "fast" control input to a TTL source.  With "fast" low (false), set the output slew rate with the pot.