PPM coding and decoding are generally realized by integrated circuits, while PCM coding and decoding are realized by analog-to-digital (A/D) and digital-to-analog (D/A) conversion technologies. ? In the eyes of many aviation model enthusiasts, PCM proportional remote control equipment is an expensive high-end product, which is beyond our power. There are two main reasons for this phenomenon. On the one hand, in the past few years, the price of single chip microcomputer was very high, and its function was not strong enough. On the other hand, the imported PCM proportional remote control equipment has many functions and low cost.
At present, the two most commonly used pulse coding methods in proportional remote control equipment are PPM and PCM, and the two most commonly used high frequency modulation methods are FM frequency modulation and AM amplitude modulation. The most common combinations are PPM/AM pulse position modulation coding /AM, PPM/FM pulse position modulation coding /FM and PPM/FM pulse modulation coding /FM only. The usual PPM receiving and decoding circuits are all composed of general digital integrated circuits, such as CD40 13, CD40 15, etc. For this circuit, as long as the rising edge of the input pulse reaches a certain height, it can be flipped. In this way, once the input pulse contains interference pulse, it will cause output confusion. Because the number and position of jamming pulses are random, the effect produced at the receiver output is "rudder jitter". In addition, the distortion of coded waveform caused by poor contact of potentiometer will also affect the receiving effect, resulting in "rudder jitter". For narrow interference pulse, general PPM circuit can be eliminated by filtering; However, for wide interference pulses, the filter circuit is powerless. This is also the reason why the common PPM proportional remote control equipment will cause misoperation in a strong interference environment or beyond the control range. Especially in the case of co-frequency interference, the model will often be completely out of control.