The transmitter has been designed in several configurations; 1, 2, and 4 channel. Each channel takes an 8-bit (greycode) binary value from a shaft encoder and transmits this value by generating PWM pulses indirectly to the reveiver via the RC radio. The connection between the transmitter and the radio is actually an analog voltage with 4 "steps". the analog value mimics the voltage from a potentiometer (the traditional controller for RC radios). This allows for easy integration of the transmitter board into the radio (pull out the pot and put in the transmitter and controller).
The receiver also contains driver(s) for the servo motors, in the form of an H-bridge. The receiver will decode the incoming PWM pulses into a new angle for the servo and then switch tacks to controlling the servo motor using a proportional control algorithm. The main configuration for the board is single channel, and it is designed to directly plug into the RC Radio Receiver (where the servos plug in). Power for the PIC chip is derived from the radio, while power for the servo motor can be taken from another source (for high current motors). The PIC chip is clocked at 16Mhz, to provide enough speed for accurate PWM measurements.
The physical size of the transmitter and receiver boards (single channel) is 1.5" x 2" x 0.25". The receiver board is usually contained in a metal enclosure to provide shielding and a heat sink for the high current version of the H-bridge. The transmitter board has a small RF shield to cover the crystal (4Mhz) and the PIC chip.