We found this circuit that converts a direct voltage into a proportional frequency signal. The circuit is from an Electronics Now from April 1993. The JFET can be of any type.
This circuit uses a voltage comparator and can produce signals up to 20 MHz. The resistors R are...
It can be obtained from Mouser Electronics the new LM290xAQ operational amplifiers from Diodes...
This circuit was obtained from a Texas Instruments 1985 document. The circuit is powered by 1.5 V...