Let us take a look at what PWM & PPM is, along with FrSky Receiver PPM Output in this article. PWM or Pulse Width Modulation has been the de-facto choice for receiver out for decades and for good reason - it was direct, easy and simple to use. But as the need for more channels came necessary, it wasn’t logical to keep adding PWM channels, each taking an extra pin - enter PPM or Pulse Position Modulation which brings in multiple channels into a single pin.
What is an RC signal?
So, how do the servos on your RC plane or the rotors on your drones get the signal to move the rudder or control rod? Obviously from the receivers. So there must be a standard communication “talk” between the receiver and the servos, right? This is called as the RC signal. It is what enables servos or any peripherals such as Flight Controllers to understand what input the receiver is giving them.
What is a PWM signal?
The first generation of receivers used PWM or Pulse Width Modulation signal. So what exactly is a PWM signal?
A Pulse Width Modulation signal is a way of creating analogue signals from a digital source. Analogue signals are sinusoidal in nature while digital is in binary, ie. 0s and 1s. So to generate an analogue signal(continuously changing signal) from a digital source in a PWM signal, we change the frequency and duty cycle of the signal.
The duty cycle is the amount of time that the signal is HIGH (ON) versus the total time it takes to complete the cycle in terms of percentage. Frequency, on the other hand, is how fast one cycle is completed. For example, 100Hz (Hz or Hertz is the unit of frequency) means that the signal completes 100 cycles per second.
By combining duty cycle and frequency rates we can change how long a signal is in the HIGH state and how long the signal is in the LOW state. By varying these two values, you get an analogue signal (not pure analogue though)
So now that we have got that out of the way, let us talk about why PWM was widely used.
As previously mentioned, PWM just consists of two variables that is needed to be changed, which makes PWM signals extremely easy to be created and decoded by simple devices like your servo.
But as time passed by, pilots started to require additional channels for drones and auxiliary devices. Due to the hardware requirement of an additional PWM pin for each channel, it was quite obviously not feasible to have more than 6 to 8ch via separate PWM signal wires. Hence, there was a requirement to condense multiple channels into a single hardware signal wire.
This is where PPM came into play.
What is a PPM Signal & Why use it?
PPM or Pulse Position Modulation is another scheme used to send signals, but this has the added advantage of sending multiple channels/signals via a single PPM signal wire. How? Read on!
In PPM, each channel is lined up back to back on a single signal train (technically this is not how it works, but for simplicity, we will follow this schema). So, channel 1 is followed by channel 2 which is followed by channel 3 and so on. Once the final channel data is also sent (say channel 8), the pulse train goes back to the first channel and gets refreshed with new channel values.
So with this, we have essentially cut all the extra channel wires down to a single wire thus saving space, hardware pins and complexity of wiring.
So what are its downsides?
The issue is that standard servos do not support PPM signals but rather PWM only. This would not be a deal-breaker for the drone fliers out there but for planes and fixed wings, this could be an issue as they might not be using a flight controller which accepts PPM but rather directly connect receiver outputs to servos and ESCs.
Of course, you can get around by using a PPM to PWM converter module which is available quite cheaply.
Where does FrSky stand?
FrSky obviously has a vast number of receivers, each capable of outputting different types of data as PWM, PPM or the newer S.Bus. Few examples of FrSky Receiver PPM outputs are:
X8R
X8R-Pro
X4R
XSR
Among others.
In this article, we have seen what are radio receiver signals, what is PWM & PPM and the differences between the two and how FrSky Receiver PPM Output can help you save a lot of wires on your build and how clean the build is with PPM over PWM.
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