WHAT IS 2.4 GHZ AND SPREAD SPECTRUM

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Demystifying terms such as PCM, PPM dual conversion, single conversion, full-range etc., this feature will explain it

Build your own radio gear?: Back in the old days, building your own RC gear was not uncommon and now the arrival of 2.4GHz has made it practical again.

 
Spektrum transmitter
2.4GHz Radio Control Explained
JUST HOW DOES SPREAD SPECTRUM RC REALLY WORK?

If you've got, or you've been thinking of buying, a 2.4GHz spread-spectrum RC set then you'll probably be keen to understand exactly how it works, and hopefully this article will help you do so.

First, a few words about older "narrowband" RC systems...

Traditional narrow-band RC systems on anywhere from 27MHz to 72MHz are fairly easy to understand because they work just like your regular AM or FM radio - sending out a signal that is picked up by the receiver and then sent to the servos.

Unfortunately, just like regular FM broadcast radio, these RC systems require a frequency all to themselves if they're going to avoid interference with each other. What's more, it doesn't take much to disrupt a regular narrow-band signal. A noisy thermostat or electric drill can often cause massive amounts of electrical interference when listening to an AM broadcast and FM isn't always that much better.

But manufacturers of spread spectrum (SS) radio systems are claiming that you need never worry about being shot down by other fliers and that all 2.4GHz systems can get along in harmony, despite apparently using the same frequencies.

So how can that work?

Well to explain this, I'm going to use a series of illustrations that I call "the freeway analogy". Using these diagrams and explanations, I will do my best to convey the complex world of spread spectrum in a form that most people can get their brains around. Of course in doing this I've had to take a few liberties with the details but these are not important.

Which Flavour of Spread Spectrum?
flavors
YES, IT COMES IN DIFFERENT FLAVORS

Before I launch headlong into a detailed explanation, it's worth pointing out that there is more than one flavour of spread-spectrum.

The first and most common type is what we call Direct Sequence Spread Spectrum (DSSS). This involves the transmitter and receiver staying within a fixed part of the 2.4GHz spectrum.

The second type is called Frequency Hopping Spread Spectrum (FHSS) and involves having the transmitter and receiver constantly changing their operating frequency within the alowed limits of the 2.4GHz band.

At the present time, only Futaba and Airtronics use FHSS, the remainder using DSSS.

And right now I can year you asking "which flavor is best?"... to which I have to say... neither and both.

Or, in other words, neither solution is best all the time, there are benefits and drawbacks to both, as you will see. However, it's safe to say that in theory, the Futaba FASST system does give the best of both worlds because it is not only FHSS but also DSSS.

But first, let's see how a traditional "narrowband" FM RC set works on frequencies such as 27, 35, 36, 40, 41 or 72MHz.