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

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sounds like you'll have to redo the pcb design completely. not pretty :(

Ah, such is life. Incidentally, this is why hardware (especially radios) ought to be thoroughly tested - to ensure they don't jam other hardware and are themselves as tolerant as possible of outside interference. Sometimes these things aren't discovered early, so we actually got lucky, I think. :)

Thank you for leting us know about the problem, I hope you can solve the problem fast.

 Keep up the good work. 

I suggest using a TTL cable instead of putting USB directly on the board.  It may well be why SiLabs doesn't put USB on the radio chip even though they could have easily done this.  They do have a reference USB radio design.  You might check that out to see how this problem was overcome.  Also check out the USB power to see how noisy it is.  USB power is often so noisy as to be unusable without a lot of filtering.

Baring some easy solution... I suggest using a cable and using the same board for both air and ground.  You can then dump all the ground boards at a reduced price on people like me who would remove the USB converter and use their own cable.

I was considering using two "air" boards anyway since it is small and should easily integrate into other hardware without leaving chips unused.

Unfortunately USB is notoriously bad for EMC issues - there are often significant challenges simply to meet CE emissions requirements, let alone right next to a radio receiver.

It will almost certainly need filtering on the TTL side of the FTDI chip, and I would not be surprised if it eventually requires a version of the FTDI cable (possibly with ferrites or better) to get some distance between the radio antenna and USB noise source.

I would also be quite happy to use this as a pair of "Air" boards with the logic-level serial link - that's actually better for quite a few applications anyway, like dedicated (non-PC) ground stations.

Thank you for the news; you're right, we expect a good product to replace these crappy xbees. Keep us updated !

Has this configuration been range tested with the 433mhz radios yet? Does this eliminate the interference or is this interference transfered into the ttl level serial through the FTDI cable?

Hi Mike,

I will be range testing with two FTDI radios this weekend. I wasn't able to do that previously as I only had one FTDI 433 radio (I have several 915s that I used for development, but I only had one pair of 433s until this week).

I'll post an update when I have the results. It probably won't be till Sunday my time, as I'm flat out teaching this week.

Cheers, Tridge

Brilliant! Looking forward to the results. Just a thought, if this configuration works well, will you (or the DIYDrones store) likely do a "combo" including 2 of the "pins" radios, an FTDI cable and antennas etc? I would think this would sell well given the flexibility of this configuration and potential for huge range. 

Thanks for your quick reply!


Hi Mike,

That is up to the 3DRobotics people of course. We're still really in the investigative stage of the problem. I'm going to be trying a bunch of different approaches this weekend, and hopefully between all of the people looking at this (Sam, Craig, Seppo, Mike and myself), we'll come up with a good solution.

Cheers, Tridge

A few comments suggseting this wouldn't be an issue with air modules (UART), only the ground module (with inbuilt USB adapter). Is that right? Or is the interference coming from the Bus, rather than the adapter?

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