Intensity control is accomplished by varying the frequency of the three subcarrier oscillators that operate in the 100 to ?00 kHz frequency range. A 100 MHz oscillator is turned on and off by the pulse train from the subcarrier oscillators. The duration of this pulse is determined by the pulse-duration switch. These bursts of 100 MHz RF energy are received by (2) the receiver-stimulator which is carried ?? the subject, measures 3.7 cm x 3.0 cm x 1.4 cm, and weighs 20 grams. The solid-state circuitry is encapsulated in epoxy resin which provides it with very good mechanical strength and makes it waterproof. Space for the ? volt mercury battery is included in the size mentioned above. After RF detection, the resulting subcarrier frequency is demodulated into an amplitude. This amplitude controls the current intensity of the stimulation pulse by means of a constant current transistor in the output circuit of the receiver. This method makes the pulse intensity independent of biological impedance changes over a wide range. Under average stimu- lation conditions, the battery life is approximately one week. Operating range is up to 100 feet. [EW: This is prototype equipment, unclassified, and operating sometime in the 1970s. Imagine classified equipment at the end of the 20th century!] Three channels of stimulation are available. The pulse intensity of each channel can be controlled individually from the transmitter. The pulse duration and repetition rate are the same for all three channels. Encephalographic (EEG) Telemetry A miniature FM-FM amplfier-transmitter combination and telemetry receiver are used for this purpose. (1) The transmitting circuitry, carried by the subject, consists of an EEG amplifier with a gain of 100, input imped- ance of 2 megohms, frequency response from 2 to 200 Hz, and a voltage- controlled oscillator (VCO) for each channel. The VCO operates in one of the frequency bands assigned for subcarrier oscillators by the IRIG stan- dards. In these studies, a three-channel system was used which operated on IRIG channels (Delgado & Hamlin, 1962; Delgado & Mir, in press; Fonberg and Delgado, 1961). The outputs of all three subcarrier oscillators were summed and connected to the single RF transmitter module. The miniaturized RF transmitter operates at 216 MHz and it's range is 50 to 200 feet, depending on the environment. The size of the three chan- nel unit, including the battery, is 4.5 cm x 4.5 cm x 1.5 cm and it weighs 50 grams. -132-