communicationradio
My son got a
quadcopter for his birthday, a "NiGHT LiONS TECH® X9 Flying Car 4 Channel 2.4Ghz RC Quadcopte Drone".
On the box, it says that it communicates at 2.4 GHz on the "100-200 band"; on the website it says:
2..4 Ghz spread spectrum transmitter uses multiple frequencies across 100-200 band
What is the 100-200 band? Does this refer to 2.4 GHz + an extra 100-200 Hz?
Generally we distinguish two types of stations:
See Amateur Radio Stations for more information:
(left: fixed; right: mobile)
The government decides the frequencies. In the United States, it's decided by the FCC in coordination with the ITU, which is the international governing body (since radio waves don't magically stop at the borders of countries).
There's nothing particular about FM that makes it more suited to those frequencies compared to others. In fact, ham radio operators and others use FM on many different frequency bands. Probably when FM broadcasting needed some spectrum, the 88 - 108 MHz was the most convenient at the time, for technological or political reasons.
Best Answer
I smell a translation error. "Spread spectrum" is a term for technologies that take a narrowband signal (e.g. the very low-rate signal needed to communicate a few button/joystick levels) over a large bandwidth. You can do that in a number of ways – including FHSS (Frequency-hopping spread spectrum), that is, taking a single transmit symbol (e.g. the value "4") and quickly jumping to multiple frequencies ("hop frequencies", or "channels"), and sending e.g. the same symbol there.
Now, "channel" is a term very narrowly related to "band". I could imagine that for a non-native speaker, a "100-channel frequency hopping spread-spectrum system" becomes a "multiple frequencies across 100-200 band"; notice that in any case, the grammar is slightly off in that sentence.
As a technological comment: it certainly doesn't mean +100-200 Hz, because if you have an oscillator at 2.4 GHz, even generating a tone that is a couple hundred Hertz close to its specified frequency is very nontrivial. Consider the relative oscillator accuracy needed to hit exactly within 100 Hz of a wanted 2.4 GHz tone:
$$\frac{10^2}{2.4\cdot10^9}\approx 40\,\text{ppb .}$$
That is parts per billion. I did a quick search on Mouser for oscillators that have a better accuracy than 50 ppb – and the cheapest one starts at roughly 38€ (without taxes), so about the same price as your complete drone.
For this very reason, and because things can be moving relative to each other (Doppler!), frequency synchronization is a crucial part of every wireless communication device; the oscillators within a cheap 2G telephone certainly aren't great enough to "hit" the center of the 200 kHz GSM channels, but with a couple of tricks, the oscillators can be "trained" to run at much more exactly the same rate as the base station's oscillators, at least for a short duration of time. For 30-years-old GSM that was already a necessity – now imagine things like 3G (which, at least in Europe, uses a really offset-sensitive modulation) where symbol rates are much higher, and a smaller frequency offset has even greater effect, or 4G, which has 1200 subcarriers in a 20 MHz uplink, meaning that you need to know very exactly what frequency you are at – otherwise, you'd be receiving the wrong things.