Learn the Basics of Spread Spectrum R/C

Entering the world of radio-control (R/C) hobbies can be a little daunting. To make it a bit easier to get started, we’ll cover the basics of 2.4GHz spread spectrum radio control, and the different pieces of a typical R/C system.

Transmitter

The role of the transmitter is to convert movement of the control sticks into digital signals, which are sent via radio waves to the receiver. Transmitters offer multiple channels to control multiple components. For example, if a transmitter has 6 channels, you can control up to 6 servos or motors. These channels are not to be confused with the sub-frequency
“channels” of a radio spectrum.

While older R/C systems communicated over frequencies such as 72MHz and 75MHz, modern systems use the 2.4GHz microwave radio spectrum, plus a collection of clever wireless protocols for more reliability. At the heart of these new R/C systems are Frequency-Hopping Spread Spectrum (FHSS) and Direct-Sequence Spread Spectrum (DSSS) technologies. Both work to nullify the interference and frequency conflicts found in the older R/C systems. While the actual wireless technologies used vary between transmitter brands, all of them include some combination of DSSS and FHSS to avoid interference and frequency conflicts between R/C systems.

Receiver

The receiver collects control-stick data from the transmitter and distributes it to the servos and motors on the R/C vehicle. Transmitters and receivers are frequently sold together, though you can buy them separately. If you do buy them separately, make sure that each unit is compatible, as different brands use different proprietary technologies, and will often not work with one another.

The receiver shown here is a bare board, but most receivers come in plastic cases. The short wires protruding from the receiver are the antennas. If your vehicle is made of metal or carbon fiber, be sure to mount the receiver in a place where the antennas will not be obstructed, as 2.4GHz radio waves will not pass through these materials.

Servos

Servos are geared motors designed for precision control over movement. Inside a servo you’ll find a circuit board, a small DC motor, and a series of gears. The receiver outputs a pulse-width modulation (PWM) signal to the servo, which the circuit board translates into precise control signals for the DC motor. The circuit board also takes input from a feedback potentiometer attached to the output shaft of the servo to detect its rotation. It then compares the desired shaft position, based on the PWM signal, to the actual position to know which way to turn and when to stop.

Electronic Speed Controllers and Motors

To control a motor, you’ll need an electronic speed controller (ESC). Its job is to take the low-power signal from a receiver and turn it into high-current control signals for driving a motor. ESCs come in two varieties depending on your motor type: brushless and brushed. Brushed speed controllers send a PWM signal to a brushed motor, while brushless speed controllers switch power between the three leads required for brushless motors.

When selecting an ESC, you’ll need to know the maximum current rating for your motor. The maximum current rating on your ESC should be 5 or 10 amps higher than the maximum rated current of your motor, as motors often draw more current than they’re rated for. Many ESCs are also programmable, allowing you to set different attributes for controlling your motor.

FHSS

Frequency-Hopping Spread Spectrum (FHSS) technology constantly changes the channel that a radio signal is transmitted on, in order to lower the likelihood of signal corruption due to interference on a single channel. The pattern of frequency hopping is pseudo-random, but the transmitter and receiver go through a binding process before any signal is transmitted, ensuring they jump to the same frequency at the same time.

DSSS

Direct-Sequence Spread Spectrum (DSSS) technology spreads the radio signal across a wider range of channels (sub-frequencies) than the old narrowband single-channel systems. This means that even if several of the channels are subject to interference, the signal will still get through on the other channels.

Old R/C

Previous to the 2.4GHz spread spectrum revolution, most R/C systems worked on 27, 50, 53, 72, and 75MHz. With these systems, only one transmitter could be used on a given frequency at a time. Using multiple transmitters on the same frequency would interfere and cause loss of control of the R/C vehicles. Operators also had to be cautious of areas that were prone to noise on the frequency they were using. As a result, almost all R/C systems sold now use the much safer 2.4GHz spread spectrum system.

R/C Over Wi-Fi and Bluetooth

Many new multirotors and toy cars can be controlled via Wi-Fi or Bluetooth wireless technologies. One major benefit of this is that you can control the vehicle using apps on smartphones and tablets, eliminating the need to purchase a separate transmitter. The drawback, though, is that range is limited, and you don’t get the same tactile feedback from a screen as you do from a typical R/C transmitter’s joysticks and switches.