PPM is NOT analog. It is a digital pulse signal where the position of the pulse is varied slightly.

http://www.answers.com/topic/pulse-position-modulation

http://cnyack.homestead.com/files/modulation/modppm.htm

Narrowband RF (Radio Frequency) channels with low power and long wavelengths (i.e., low frequency) are affected primarily by flat fading, and PPM is better suited than M-FSK to be used in these scenarios. One common application with these channel characteristics is the radio control of model aircraft, boats and cars. PPM is employed in these systems, with the position of each pulse representing the angular position of an analogue control on the transmitter, or possible states of a binary switch. The number of pulses per frame gives the number of controllable channels available. The advantage of using PPM for this type of application is that the electronics required to decode the signal are extremely simple, which leads to small, light-weight receiver/decoder units. (Model aircraft require parts that are as lightweight as possible). Servos made for model radio control include some of the electronics required to convert the pulse to the motor position - the receiver is merely required to demultiplex the separate channels and feed the pulses to each servo.

More sophisticated R/C systems are now often based on pulse-code modulation, which is more complex but offers greater flexibility and reliability.