delay in shipping 433MHz radios due to USB interference problem
Hi All,
I just wanted to give everyone who is waiting for a 3DR 433MHz radio a heads up about a shipping delay. A few days ago I discovered an interference problem with the 3DR 433MHz radios that can cause the range to be much less than expected. We're still investigating the problem, but meanwhile I've suggested to Sam that they delay shipping the 433MHz variant of the radio until we've solved it.
Our current theory is that the 480MHz clock in the USB host controller on some PCs is the source of the noise, and that 480MHz is close enough to the 433 MHz radio band to cause the problem. This theory is supported by testing with the USB host controller forced into USB 1.1 12MBit/s mode ("full speed") instead of the more common "high speed" 480MBit/s mode.
The above graph shows a particularly extreme version of the problem on one of my laptops. This graph was produced with a modified firmware that uses the Si1000 as a simple scanner, looking at the background signal level at frequencies from 420MHz up to 950MHz. The graph shows the background noise levels in "433-scan.txt" (the red dots) for a USB version of the 3DR 433 radio. The green dots are for the FTDI version of the radio connected to the same laptop. Those noise levels are far too high, and would severely limit range when connected to this laptop.
We have found some workarounds - in particular running the USB host controller in 'companion' mode, where it forces the controller to run all ports in "full speed" 12MBit/s mode. That helps, but we are not very happy with the solution, and the noise is still higher than we would like.
So we are still investigating the best solution and meanwhile I've suggested to 3DRobotics that they hold off shipping the 433MHz radios until we have properly solved the problem.
The 900MHz radios are unaffected by this and we've got great range on those with lots of different ground station laptops.
I'll keep you all updated via this forum as we make progress on the noise problem. Meanwhile all I can do is apologise for the delay. We thought it was better to ensure the problem is properly fixed rather than having some people disappointed in the range.
Cheers, Tridge
Replies to This Discussion
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DeveloperPermalink Reply by Andrew Tridgell on -
Hi Chris,
We don't know for sure yet. I've now got two air modules, and I plan on doing some range testing soon on those to try and narrow things down. If you want to be involved in that testing then I'll probably do it either at the local park (where you flew the Hopter) or at CMAC on Sunday.
Seppo in Brisbane is also doing some analysis of his modules in his RF lab, in the hope of narrowing things down. He's already confirmed the output power levels, and is looking at noise and sensitivity soon.
Cheers, Tridge
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Permalink Reply by Gil on -
Hi Andrew, i´ve already ordered 1 pair of these 433 modules and now you tell me this, my question is... can you guys change my order for the 900 version instead ???
Gil
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Permalink Reply by Chris Gough on -
Hi Gil,
I suggest you contact the shop you placed the order with - Tridge's developing the firmware, but he's not the hardware vendor.
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3D RoboticsPermalink Reply by Alan Sanchez on -
I'm sure there should be no problem, please send an email to sales@3drobotics .
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Permalink Reply by Andke on -
Quality first.
Ironically , this order also holds my APM2's - so I am waiting very long now .. and it feels even longer.
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Permalink Reply by Gil on
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me too, but my APM2 order is older than the 433 one, i´ll think better about making pre-orders in the future !!!
Gil
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Permalink Reply by Tom Worthington on -
Some thoughts:
"USB FM Transmitter MAX2606" suggests:"A common-mode choke is connected in series with the USB connections in order to avoid interference between the circuit and the PC supply."
"The interference from the USB module is mostly gone now that I have a RF shield around that part."
Note a shield between the USB components and the rest of the circuit board may not be possible if one integrated chip is being used for both functions.
Another thought might be to build a very large, lightweight antenna into the aircraft, exploiting the fact that the aircraft is mostly dielectric material. The antenna could be some bits of fine wire taped inside or a metal film stuck on.
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DeveloperPermalink Reply by Andrew Tridgell on -
Hi Tom,
Hugh also suggested some shielding, and I'll give that a go this weekend. I'm also quite hopeful that Seppo will find something in his RF lab.
Nice to see you at lunch today!
Cheers, Tridge
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Permalink Reply by Tom Worthington on
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The tricky bit with the shielding is that it has to go between the USB and RF components.
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Permalink Reply by Gabriel on -
not sure how shielding will work since from my understanding you have high frequency switching noise from the USB and the radio but no isolation of the 5V or GND with the radio. so the fix would be to treat the radio as needing a separate analogue gnd and 5V and connect it to the usb using ferrite bead for the 5V and a 0ohm resister to separate the gnd planes.
I may be completely wrong I'm only a beginner at PCB design. I also can't find the pcb design for the unit so I can only guess from the pictures what's there.
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Permalink Reply by Tomas Soedergren on -
For what it is worth, not having pinned the critical point of the problem, I provide this link as food for thoughts:
http://www.icron.com/products/usb/usb_rover_200.php
Just to introduce opto fiber as a resource in your tool box.
I look forward to the future solution, since this new radio modem seems very good.
I was just about to order one of them, so I am grateful for this heads up.
/ Tomas
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