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Andrew W
Joined: 01 Jun 2005
Posts: 220
Location: Melbourne, Victoria
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Some of you guys might be aware that the spektrum radios use 2.4GHz. The 2.4GHz specturm is very overcrowded atm. Some devices that are using this spectrum include wireless AV senders, wireless camers, 2.4GHz cordless phones, microwaves, wireless routers and Bluetooth (just to name a few).
I am involved with Melbourne Wireless (Wireless community broadband network, and before you ask there is no Internet access ) and we have found that the 2.4GHz spectrum is already getting very busy and noisy. One guy from our group went into the city (Melbourne) and found that each wireless channel in 2.4GHz was maxed out and the noise was very high on all the available 5.8GHz channles .
Here is some information from Spektrum's website:
"DSM SYSTEM SPECIFICATIONS
* Frequency Band 2.400–2.4835GHz
* Channels 79
* Channel Spacing 1MHz
* Range 3000 ft
* Latency 5.6 ms
* Resolution/Channel 4096 steps"
This is interesting though, also from Spektrum's website:
"FCC learned valuable lessons from other bands and applied those lessons to the 2.4GHz band. And unlike the 27 and 75MHz band, the FCC limits the power output in this 2.4GHz to a maximum of 1watt, thus preventing other 2.4GHz systems from overpowering each other. In other words, there are no “high power” commercial users interspersed in the 2.4 GHz band."
2.4GHz in the Australia and AFAIK in the US is limited to 4 watts for computerised wireless devices (wireless routers and access points). Although there is a 1W limit for the other types of 2.4GHz devices (AV senders etc).
I wonder if these devices will change channel if they encounter interference during operation, if they don't and you fly a plane or heli into an area which is highly populated with 2.4Ghz equipment you could find it crashes.
Some devices will coexist others don't. Take wireless A/V senders and wireless cameras, both these devices have analog modulation and split the available 2.4GHz spectrum into only 4 channels.
I have 2 wireless cameras, 1 running on the AV channel 2 and the other on channel 4. I've turned both of these on before and found it has completed destroyed my wireless network. They cover from wireless channel 1 right up to at least channel 12 (which is all the available channels for wireless devices).
These noisy devices don't just cover a small area (especially since you can connect large antenna's to the AV senders). I have picked up someone elses wireless AV sender from several houses away with a 8dB gain omni directional antenna (larger versions of the ones that come with the AV senders and recievers).
Sorry about the long post
Andrew W
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Wed Apr 19, 2006 1:18 pm |
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Valen
Experienced Roboteer
Joined: 07 Jul 2004
Posts: 4436
Location: Sydney
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I was looking at the 2.4Ghz chips from nordic semi/laipac. Problem is i couldnt get the dern things working they were cheap and small everything was nifty except for the not working part. (TRF2.4G i think the part number from laipac is)
I wound up going with maxstream 9xcite modules (the cheap ones) because they are meant to work out of the box (and they did). They are running in spread spectrum mode, i'd like to move them to single frequency because it will reduce latency when locking onto signals. Down side to them is they cost $50 each
Thing with any propriaty radio system is that your dependant on that supplier not changing anything. The plan for the next version is to go with either zigbee or bluetooth, with any luck it should get the cost down to about $10-20 for a complete "module" that will run on a network, lets you put a few of them in the bot (say one for when your bot is upside down, the other advantage to having 2 widley spaced antennas is your less likey to get into a dead spot from reflections).
Downside to that is its harder to do the software that runs the bluetooth/zigbee stack.
Also I haven't had time to look for conveniant modules to play with to try and get something working.
Driving a robot dosent actually need that much data, you can manuver your bot with 4 status updates per second, and 2 per second can get your bot moving to somewhere with better signal. The advantage you get with a dedicated digital bot controller is you know when you have recieved valid data, and you can get your bot to behave how you want in marginal signal conditions. (IE enhanced failsafes on weapons if you want, though with a decent checksum you should recieve static that looks like a valid signal something like twice in the lifetime of the universe ;->)
I just used the laptop because it was the easiest way for me to get the thing working, it also lets you do 2 way data. _________________ Mechanical engineers build weapons, civil engineers build targets
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Wed Apr 19, 2006 11:03 pm |
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