How R/C Radios Work

This page explains how hobby-class R/C radios work, what signals ESCs and servos use, what EPA/trim/etc means, and the differences between AM/FM/2.4GHz.

How the radio and receiver work together

Any R/C radio, no matter if they are AM, FM, or 2.4GHz, uses a precise pulsewidth signal to control servos and speed controllers (ESCs).

The transmitter converts user input (steering, throttle, etc) into a set of pulses whose pulse width is in proportion to the user input.
The transmitter modulates those pulses onto a radio carrier frequency using a specific frequency/modulation type (more on this later) and transmits the radio signal over the air to a matching receiver.
The receiver receives and demodulates the radio frequency to get the original pulses and then sends the original pulses to the appropriate output.
The servo or ESC reads these pulses and performs the correct action. For servos, this action is to move the servo output arm to a certain position. For ESCs, this action is to control a motor's speed and direction.

Radio Pulses

What are these pulses I am talking about?

ESCs and servos require a continuous stream of 1ms (milli-second) to 2ms pulses, which occur every 20ms. Actually, it doesn't have to be exactly 20ms as long as it is fairly close to that and consistent.

The voltage level is whatever you are using for a BEC. If using a 5v BEC, the pulses will be close to 5v. If using a 6v BEC, the pulses will be close to 6v. And so on.

The picture at right shows what a typical signal would look like at one of the receiver's outputs.

This interactive demo shows what the signal pulses look like for the throttle and steering of a typical pistol grip transmitter.

Use the mouse to move the slider control to emulate the steering, and move the trigger to emulate the throttle. The output waveforms will change as the inputs are changed.

An aircraft/boat/tank transmitter is basically the same, except there can be several more channels and the some controls do not "snap" back to a neutral/center position when the user lets go.

What do the terms EPA, trim, servo reversing, and exponential rate mean?

Any R/C radio, no matter if they are AM, FM, or 2.4GHz, use a varying pulsewidth scheme to control servos and speed controllers (ESCs).

EPA stands for End Point Adjustment. Normally, signal pulses are between 1ms (min) and 2ms (max), but adjusting the EPA can make these less or more than their normal values.

This might be useful if you have a servo that has too much range. You can reduce the EPA so that instead of 1ms to 2ms pulses, it sees 1.1ms to 1.9ms pulses, and the servo's travel will be reduced. Likewise, if the servo doesn't have enough range, you could set the endpoints to 0.9ms and 2.1ms for greater servo travel.

When setting the EPA, be careful not to go too far as servos can be ruined. If they hit some kind of mechanical limit, the servo motor will stall and pull a lot more current which can damage the BEC and/or servo.
Trim refers to the default center point. Generally, the steering and throttle trims for pistol style radios are set so they output 1.5ms pulses, which is exactly in between 1ms and 2ms. Adjusting this control will adjust the center point to be a little more/less than dead center.

This might be useful for the throttle channel where you may want a tiny bit of braking when you release the throttle. So, you set the trim so that it activates the brakes slightly when you release it. You could also use this to tweak the steering servo if the vehicle still turns slightly right or left when the steering wheel is in the center position. So, you adjust the trim so the tires point exactly straight ahead when you release the steering wheel.

It should be noted that VERY slight changes in the trim should be used. If you need to change the trim quite a bit to get the desired result, you would be better off adjusting the servo linkage and keeping the radio trim at or very near 0 if you can.
Servo reversing does what the name implies; it reverses the direction of the servo. Instead of the radio outputting 1ms-2ms pulses, it outputs 2ms-1ms pulses.

This would be useful if your tires are turning in the opposite direction of the steering wheel. Also, some radios (particularly Futaba) require you to reverse the throttle channel to work with most ESCs.
Exponental rate refers to pulse rate of change. Instead of a steady and constant increase/decrease of pulsewidth over the control range, the pulses change at an expontial rate. It's a little difficult to explain, but here's an example:
- Turning the steering knob 25% from center, the vehicle's wheels turn left or right only 12.5%.
- Turning the steering knob 50%, the wheels turn 25%.
- Turning the steering knob 75%, the wheels turn 50%.
- Turning the steering knob 100%, the wheels turn 100%.
This setting may be useful for high speed vehicles to help control steering. At high speed, even a little change in the steering knob can make a big change in vehicle handling. So, adding exponential steering allows finer control to make smaller changes in steering.

The interactive demo below shows what effect that EPA, trim, amd servo reversing have on the pulse stream. Use the mouse to move the steering wheel and note the pulsewidth. Click the links to set the EPA, trim, and servo reversing, then note the pulsewidth again while moving the steering wheel.

Set EPA to 90%
Set EPA to 100%
Set EPA to 110%

Set trim to -10%
Set trim to 0
Set trim to +10%

Reverse Servo

Pulse width:

How does a "fail-safe" work?

A fail-safe device monitors the pulse stream and specifically looks for the following conditions:

Pulses which are too close together. This may indicate another radio is transmitting on the same frequency.
Irregular pulse frequency. If the pulse frequncey changes from the 20ms figure, it may indicated noise interference or another radio transmitting on the same channel.
Lack of pulses altogether. This may indicate the transmitter is off, or too far away to reach the receiver.

If the fail-safe sees any of these conditions, it assumes there is an error condition and will override the errant signal by generating a stream of pulses to set the servo or ESC to a pre-determined position.

Incidentally, the fail-safe can be a separate external unit, or built into the receiver itself.

What does AM, FM, and 2.4GHz mean?

These refer to the method used to encode the pulse stream into a radio frequency that can be transmitted over the air. To explain these, you first have to understand a few terms; frequency, amplitude, pulse stream, and carrier:

"Frequency" refers to how often the signal changes.

"Amplitude" refers to the voltage swing (the maximum and minimum) of the signal.

"Pulse stream" refers to the series of 1ms-2ms pulses that servos use, explained earlier.

And "carrier" refers to the very high frequency that is used in the actual radio transmitting, and is much higher than the pulse stream that servos use.

AM stands for Amplitude Modulation. This is the simplest means of transmitting, and the most prone to noise interference. The transmitter generates a high carrier frequency (whatever the crystal's rating) and superimposes the pulse stream onto the amplitude of the carrier, which results in the high frequency's amplitude to vary in proportion to the pulse stream. Unfortunately, any nearby noise (power lines, AC motors, etc) also affects the amplitude of the carrier frequency, which causes the pulse stream to be distorted. When the receiver demodulates the signal, it picks up the valid pulse stream along with the noise. This generally causes random servo movements and other undesirable operation.
FM stands for Frequency Modulation. This type is a little more complex, but is more tolerant of noise interference. Like AM, the transmitter generates a high carrier frequency, but this time, the pulse stream shifts the frequency of the carrier signal in proportion to the frequency of the pulse stream. When the receiver gets the signal, one of the first things it does is "chop" the top and bottom peaks off the voltage swing. Since those peaks are where the noise is, it is eliminated if the interference is not severe. The receiver then simply demodulates the shifts in frequency to get the original pulse stream. Even though FM has better noise rejection, it can still suffer from interference if it is strong enough and close enough in frequency.
Both AM and FM are "analog" modulation schemes, while 2.4GHz is digitally modulated. This method uses a much higher carrier frequency, is much more complex, and is "packet-driven" similar to how computer wifi networking works. Both the transmitter and receiver have miniature computers in them to handle the task of handling the signal. Due to its frequency and modulation, it is very tolerant of interference. And if interference does happen to get through, the receiver uses error checking and corrects or may disregard any bad data packets.

This picture helps to illustrate the idea of AM and FM modulation. The data signal should actually be a square wave since R/C radios use square-wave pulses, but the resulting AM and FM signal is easier to see when a sine wave is used for demonstration.

Signal: The first waveform would be the data signal to be modulated (again, it would actually be square wave pulses).

Carrier: The second waveform is the high frequency carrier signal.

AM: The third waveform is the data signal amplitude modulated onto the carrier signal. Notice how the amplitude of the carrier changes in proportion to the data signal.

FM: The last waveform is the data signal frequency modulated onto the carrier signal. Notice how the frequency of the carrier shifts in proportion to the data signal.