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Thread: PZ0420 and GPS interference

  1. #1

    PZ0420 and GPS interference

    I love my PZ0420s, and that's why I have three . However, I made a disturbing discovery today while setting up a new quad. The camera emits strong RF interference at exactly 1575.0MHz, which is very close to the GPS L1 frequency of 1575.42MHz, and well within the typical +/-10MHz bandwidth used by most GPS receivers, GPS SAW filters, etc. This interference seems to jam GPS receivers.

    As it turns out, 1575.0MHz is exactly 55 times the 28.6363MHz frequency of the crystal on the PZ0420 boards, which is also the pixel frequency of NTSC video. I think one of the DSP chips on the PZ0420 must have a 1575.0MHz clock frequency, which is very unfortunate for anyone using this camera anywhere near a GPS receiver, which is probably at least half of people using it for FPV.

    For now, I've ordered the 690 TVL Pixim Seawolf from GetFPV to see whether it has the same problem. Has anyone else experienced this, and if so, did you find a nice alternative camera that didn't do this?

    Demonstration

    The first clue was that I couldn't get a GPS lock, even with my GPS receiver 6 inches away from the FPV cam. I'm using the 3D Robotics V1 GPS receiver, and it has built-in external LNA and SAW filters which makes it pretty robust to interference, so I was surprised it wasn't working well.

    I connected u-Center and looked at the "CW Jamming Indicator". Here's how things look with the FPV camera turned off. The indicator is at 1.6%, and I have 6 satellites locked (indoors - in a concrete building - not bad if you ask me!)

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    When I turn the FPV camera on, the indicator goes to 49.8%, and I lose all 6 satellites. Note also the "phantom" satellites, which I think are the receiver locking onto 1575.0MHz and other nearby frequencies from the FPV camera.

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    I have an RTL2832U SDR radio from Amazon (http://www.amazon.com/gp/product/B008S7AVTC/). Using the HDSDR software as a spectrum analyzer, I was able to visualize the interference directly. Here's a screenshot. Ignore the bottom half, only the spectrum in the top half of the screen is useful:

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    The green/red dots in the spectrum show the interference. I turned the PZ0420 off between 7:36:12 and 7:36:16 to demonstrate it's coming from that camera.

    You can see the interference falls directly in GPS's frequency range, which is a really, really unlucky coincidence. It passes straight through all the SAW filters and gets right to the u-Blox chip.

  2. #2
    Warrior Monk Kyodai's Avatar
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    Nice find! This explains quite a lot about some issues I've been having, actually...
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  3. #3
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    Same here, I had to move my PZ0420 to the back of my quad before I would acquire any satellite lock.
    Good work!

  4. #4
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    Ah yes I noticed this a while ago too, 30cm of separation distance seemed to solve the issue

  5. #5
    His Royal Sassiness ssassen's Avatar
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    Yeah, familiar problem, and one I've been informing people about for quite some time.

    That's not the only issue with that camera though. There's several revisions, or rather, several manufacturers and/or cameras being sold under the same PZ0420 monniker, some have a quirk in their firmware where sync pulses are missing, causing for ICs using these sync pulses to time video frame generation (like the MAX7456 OSD IC) not being able to sync, and thus OSD info disappears (any flashing text in the EzOSD for example) or ends up scrolling vertically over the screen.

    This again is a testament to ass-hat, lazy and stupid manufacturers not making *any* effort to make sure the stuff they sell plays nicely with the equipment the end user is likely to own and use. I would suggest to avoid this camera if you can.
    Purveyor of the FPV secret sauce at ImmersionRC

  6. #6
    Shedding light over FPV HugeOne's Avatar
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    Strange, my 420 is within an inch of my gps and no problems.
    I got 7 of them and no problem with either minim OSD or eagletree stuff.

    Hugo

  7. #7
    His Royal Sassiness ssassen's Avatar
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    Quote Originally Posted by HugeOne View Post
    Strange, my 420 is within an inch of my gps and no problems.
    I got 7 of them and no problem with either minim OSD or eagletree stuff.

    Hugo
    As I already explained Hugo, there's many of them sold under the same name, but these are not the same cameras, frankly it'll be difficult to track down the 'original' camera.
    Purveyor of the FPV secret sauce at ImmersionRC

  8. #8
    Kiwi in Germany whakahere's Avatar
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    I would agree there are different types of this camera. I have been using this camera for years now and I have seen about 4-5 different placement of components. I should really measure how noisy my new ones are but since I don't use gps very often and they pass my range test .... I don't give a flying ****..
    Whaka's Tinkerings blog --> http://fpvlab.com/forums/showthread.php?36623-Whaka-s-Tinkering&p=631753#post631753
    Battle H Virus (home build basher quad tutorial) --> ]http://fpvlab.com/forums/showthread.php?12544-Tutorial-Battle-H-Virus
    Range record on 5.8ghz 200mw 24km --> http://www.rcgroups.com/forums/showthread.php?t=1441664

    The one thing I know how to do well with FPV - crash. All in a day of fun!
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  9. #9
    That's all really interesting to hear. I bet the different versions use slightly different clock frequencies for the DSP too, so it could be that some interfere with GPS while others don't.

    Are there any similar $40-90 cameras out there that never give people interference problems? I'm going to try out the Lumenier 690TVL Pixim from getFPV when it arrives on Monday.

    Or maybe we can lobby SC2000 to change the clock frequency multiplier. If I'm right that it's 55 * 28.3636MHz, increasing it to 57 or reducing it to 53 would both be enough to move the interference out of the GPS band. It's also likely that the PAL version uses a different frequency, for those who are in Europe or don't mind using PAL in the US (it's lower framerate than NTSC at 25Hz rather than 30Hz, but higher resolution at 576 lines vs 480).

  10. #10
    His Royal Sassiness ssassen's Avatar
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    Problems don't end there, that same camera (or one of its iterations) hoots all over the UHF band, which makes you wonder why you would even consider buying it when you use UHF for RC control.
    Purveyor of the FPV secret sauce at ImmersionRC

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