I'm thrilled to report that my radio kit arrived and it works well. Andreas is likewise happy with his kit.
Writeup here:
http://eastbay-rc.blogspot.com/2012/04/review-3dr-900mhz-radio.html
Fixing the air unit network id:
http://eastbay-rc.blogspot.com/2012/04/setting-network-address-on-3dr-air.html
Thanks 3DR, Tridge, and everybody else that worked in producing this unit!
Comments
Just received two pair of these beauties yesterday, one pair 433 mhz (two air radios, to use one as a ground ftdi style) and one 915 mhz. The problem I have is determining which of the air radios is 433 and which is 915. I've looked the boards over pretty well but can't determine which is which. Any help will be appreciated, and if the answer is just plug them in and see, I would like to be a little more certain before I power mine up. Both pair came in the same box so no help there.
Thanks in advance for any help
Dennis
@ Andrew
Heres a link to the TLOG: https://www.dropbox.com/s/a56mp35fs3r7y2m/2012-04-23%2007-36-03.tlog
@John,
With that much margin you should be getting excellent link quality. Can you post a tlog?
My only guess as to what may be wrong is you may be running an older ArduPlane/ArduCopter version that didn't support the flow control that the 3DR radios use. The default settings on the 3DR radios oversubscribe the link, so unless you have a current APM/ACM version then you may get some packet corruption if you try to send more telemetry data than the link can support. With the newer APM firmwares it will auto-adjust the rates to suit the link.
Cheers, Tridge
@Andrew How are you able to extract the RSSI data from the TLOG outside of APM Planner?
@Robert Could you post a picture of your box?
I just did my first flight with the 3DR radios today and at 390m with a clear LOS I was getting a 13dB margin on my ground radio and 17 dB margin on the remote radio with a 60% link quality on the APM HUD. With the XBee I had before I was able to get a 99% link out to 1000m before I ran out of clear LOS room. My first thought is my aerial antenna placement it a little to close to other electronics, the XBee had a little more room.
Hi Robert,
The basic answer is that we don't know for sure. I haven't noticed any difference in APM behaviour between using an Xbee and using a 3DR radio, but that doesn't mean there isn't a noise problem, it just means its small enough that I haven't noticed it.
Seppo did some RF analysis of the 3DR radio prototypes and they were fine, so we were fairly confident they were OK.
One difference with the XBeePro900 is that the 3DR 900 can put out twice the power (100mW versus 50mW). So if you have something sensitive to 900MHz then you could potentially get more interference, but the simple answer to that is to lower the power output of the 3DR radio. The 3DR radio has better receive sensitivity then the XbeePro900 so you should be able to get better range even with lower power. I've done a lot of my testing at very lower power levels (down to below 2mW).
A think a bigger issue is the radio being affected by noise from the rest of the plane, rather than the APM being affected by the radio. See the graph I posted above showing bad noise on our 3m Mugin.
Cheers, Tridge
So do these things radiate less noise than the Xbee does into the airborne electronics?
Hi Andrew,
Thanks for the comments! The 900MHz radio was indeed close to other emitters in the plane, in particular it was less than 1m from a 5.8GHz Ubiquity bullet used for digital video. I originally thought that may be the cause of the noise, but I'm not so sure now. The 5.8GHz radio was turned on well before takeoff, but the graph shows the high noise levels started on takeoff.
I also thought it might be the ignition, and we do indeed have an electronic ignition system, but that was also started well before we took off. It may be that it only produces noise in the 900MHz range when running at high revs. That theory is supported by the drop in noise levels as the plane was coming in to land (when the engine was at low revs).
For the flight in the above graph we weren't using the amplifiers, so the LNA wasn't the cause.
We don't know for sure if the noise is radiated or conducted. My guess is radiated as the engines ignition is separately powered from the radios, so the only conduction path would be a very tenuous one via the throttle servo and the APM telemetry port.
It certainly is a puzzle. We'll need to do some more experiments to narrow it down.
Cheers, Tridge
@Andreas, I would suggest you put the ground side into a little plastic "project box". That's what I did with my Xbee. It protects the electronics from static, as well as protects the USB connector. I now have a USB cable coming out of the box permanently. It is a great strain-relief for the USB connector.
Im fussy! when will the air radio be all purple? I want to keep it all the same color! :)
Hi Tridge,
Great job on the radios.
There are a number of reasons why the receive performance on a small mobile platfrom can never be as good as on the ground, but it is worth exploring them to try and minimise the difference. You have noted an increase in the noise floor, so this is worth considering first.
Number one, there is the increased proximity of other RF emitters, which can be physically separated on the ground. Although out of band, if strong enough or close enough out of band emitters can penetrate the input filters of the receiver in question and degrade its sensitivity. Image frequencies are particularly troublesome. Separation is difficult to achieve in an aircraft, but every little helps.
What sort of ignition does your petrol engine have? Electronic ignitions are notoriously noisy electrically. Is this being radiated or conducted into the radio?
I recall that you said adding an LNA inproved the receive sensitivity of your base radio. This suggests that the front end of the radio is not quite optimal (not intended as a slur on the design - there may be nothing you can do about it with such a broadband front-end). Again there are a number of reasons this could be the case. Impedance match between antenna and input stage is one possibility. If the LNA does a better job of matching to the antenna at the frequency of interest than the radio does, then this will improve the receive perfromance. Noise figure itself is another possibility. The LNA is likely to have a better noise figure than the radio, and together with a bit of gain this will improve the receiver sensitivity, dB for dB (almost). An LNA on the aircraft might improve the receiver performance, but it might not be practical.
On the subject of LNAs, they should be located as close as possible to the antenna. That is why they often accept power up the output coax - to simplify installation if they end up being mounted up a pole. Every dB lost before the first amplifier worsens the receiver sensitivity, dB for dB.
There are no doubt other factors that I have missed.
Cheers,
Andrew.
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