In the previous post I fired up my Varioprop 6 R/C transmitter and examined the transmit signal. The model aircraft control works by using PWM Pulse Width Modulation for the Rudder, Elevator and Engine Speed. There are 4 pulses; one for each control and for carrier sync. The width of each pulse is a linear function of the control stick position. PWM is a hybrid modulation technique. It is immune to amplitude noise as the noisy receive signal can be amplified and clipped because the information is not in the amplitude but the pulse width. PWM is commonly used in motor speed control and in Class D audio amplifiers.
Figure 1 shows the Scicos simulation schematic. This simulation just shows how one particular pulse could be modulated. The Varioprop 6 signal has a period of 11.6msec (86Hz) between bursts of the 4 pulses. A sine signal is used to represent the control position, maximum positive to one side, maximum negative to the other side. This is compared to a sawtooth signal with a period of 11.6msec. The voltage +/- of the sawtooth is adjusted to set the maximum PWM say at 90%/10% depending. Figure 2 shows a train of baseband pulses whose width corresponds to the control voltage. Figure 3 shows this pulse train modulated with the 27.145MHz carrier.
Please send your comments, questions and suggestions to:
#1. – “R/C Varioprop 6 Signal Decode_a”
#2. – “Learn Telecommunications by Simulation”
#3. – “HF Radio Telecommunications Learn by Simulation”