25 mars 2009


The software of RCHv2 is growing slowly by surely :) After many thoughts, I decided that the configuration of the mixers will be done through "xml" like commands. "xml" is a structured language to specify datas, very useful when datas start to be complex and a lot imbricated. And this is the case with the software of the radio, because I wanted it to be powerfull and generic enough to allow users to do powerfull mix without headaches. If I fail, then I will provide 'aspirin' to anyone who wants to build and use an RCH system :)

So, given that there is a lot to do to reach that objective, I am trying to proceed small steps by small steps, which is not always obvious. My next step is to allow the configuration of RCHv2 from a PC with an XML configuration file.
You should say "ok, but what's the benefit of that ? " answer is: to be able to provide almost any kind of parameters to the radio, thus to allow to program anything. You could argue that "xml" has a cost, and that it takes SRAM ressources as well as CPU. Answer: Yes that's not wrong, but I 'll give a try on an AVR8bits, and I am pretty sure that CPU resources will be very small, and that there will be still sufficient SRAM memory to proceed.
Once config from XML will work, it will be a great step forward.
I have also planned to add a second avr644P for LCD and keyboard handling.

Please comment, if you think what I am doing is stupid, obvious, or bad way, thanks in advance :)

03 mars 2009

Some Spectrum measurements

Last monday I had the opportunity with friends to do some measurements of spectrum on various 2.4Ghz modules.
The initial intention was to measure the difference between 2.4Ghz antennas, but the environment (in a shop) reveals that it was impossible.
So, no results about the antenna efficiency, but we could observe the spectrum of various transmitter. Thanks to flash-rc who makes this possible to test different transmitter from various vendors.
First, here is the spectrum print for the Xbee module that is used in some home made RC. You can see that the spectrum is not very sharp around the 2.46Ghz channel, and there are many harmonic around.

Then we tried Assan module. Very interesting. Its Spectrum is very sharp and neat. It uses DHSS technology with two channels simultaneously as Spectrum. It looks clean.

Then we tried several transmitter together to compare their Spectrum. The picture below shows Assan, Spektrum and Xbee. The Xbee transmit only on one single channel at a time.

Finally, we tried two Futaba transmitters simultaneously. On the picture you can see that they use the entire spectrum 2.4Ghz. They use the FHSS technology. Don't be afraid by this picture, the spectrum here is remanent and it does not mean that the radio continuously emit on all these channels at the same time.

In conclusion of all this, if I have to buy 2.4Ghz module, I would seriously investigate Assan as their Spectrum looks very clean, which make feel confident in it. But of course it is just one aspect. In addition, it looks that the DHSS technology is more suited to RC because it seems to be more robust against jamming. DHSS techology is based on CDMA, made for the Army for Missile guidance and comms. More info on DSS techno here. FHSS primarly purpose is communication crypting for army to prevent comms hijacking. [edit] But it looks that FHSS used by Futaba is not exactly same as FHSS used by the army. Subject is complex. There is an interesting thread about this on rcgroups