Let us take a look at what PWM & PPMis, along with FrSkyReceiver PPM Output in this article. PWM or Pulse Width Modulationhas 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 camenecessary, it wasn’t logical to keep adding PWM channels, each taking an extrapin - enter PPM or Pulse Position Modulation which brings in multiple channelsinto a single pin.
What is an RC signal?
So, how do the servos on your RC plane orthe 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. Itis what enables servos or any peripherals such as Flight Controllers to understandwhat input the receiver is giving them.
What is a PWM signal?
The first generation of receivers used PWMor Pulse Width Modulation signal. So what exactly is a PWM signal?
A Pulse Width Modulation signal is a way ofcreating analogue signals from a digital source. Analogue signals aresinusoidal in nature while digital is in binary, ie. 0s and 1s. So to generatean analogue signal(continuously changing signal) from a digital source in a PWMsignal, we change the frequency and duty cycle of the signal.
The duty cycle is the amount of time thatthe signal is HIGH (ON) versus the total time it takes to complete the cycle interms of percentage. Frequency, on the other hand, is how fast one cycle iscompleted. For example, 100Hz (Hz or Hertz is the unit of frequency) means thatthe signal completes 100 cycles per second.
By combining duty cycle and frequency rateswe can change how long a signal is in the HIGH state and how long the signal isin the LOW state. By varying these two values, you get an analogue signal (notpure analogue though)
So now that we have got that out of theway, let us talk about why PWM was widely used.
As previously mentioned, PWM just consistsof two variables that is needed to be changed, which makes PWM signalsextremely easy to be created and decoded by simple devices like your servo.
But as time passed by, pilots started torequire additional channels for drones and auxiliary devices. Due to thehardware requirement of an additional PWM pin for each channel, it was quite obviouslynot 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 hardwaresignal wire.
This is where PPM came into play.
What is a PPM Signal & Why use it?
PPM or Pulse Position Modulation is anotherscheme used to send signals, but this has the added advantage of sendingmultiple channels/signals via a single PPM signal wire. How? Read on!
In PPM, each channel is lined up back toback on a single signal train (technically this is not how it works, but forsimplicity, we will follow this schema). So, channel 1 is followed by channel 2which is followed by channel 3 and so on. Once the final channel data is alsosent (say channel 8), the pulse train goes back to the first channel and getsrefreshed with new channel values.
So with this, we have essentially cut allthe extra channel wires down to a single wire thus saving space, hardware pinsand complexity of wiring.
So what are its downsides?
The issue is that standard servos do notsupport PPM signals but rather PWM only. This would not be a deal-breaker forthe drone fliers out there but for planes and fixed wings, this could be anissue as they might not be using a flight controller which accepts PPM but ratherdirectly connect receiver outputs to servos and ESCs.
Of course, you can get around by using aPPM to PWM converter module which is available quite cheaply.
Where does FrSky stand?
FrSky obviously has a vast number ofreceivers, each capable of outputting different types of data as PWM, PPM orthe 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 areradio receiver signals, what is PWM & PPM and the differences between the twoand how FrSky Receiver PPM Output can help you save a lot of wires on yourbuild and how clean the build is with PPM over PWM.
