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GPS Issues - Do I need a GPS Ground Plane & EMI Ring?

Posted by Doug Walmsley on March 11, 2015 at 8:32am

So I read an interesting article about GPS antennas called "Adding a GPS Chipset To Your Next Design Is Easy".

A few points to bring up that I have concerns with dealing with my M8N antenna.

1.  Active vs Passive Antennas.  Two paragraphs within the article describes the difference between Active and Passive antennas.  According to CSG Shop's specification for the NEO-M8N it comes with a low-noise regulator and RF filter built-in.  So I'm assuming that it is a active antenna. 

 

2. Antenna's requiring adequate plane.  If I read that document correctly, these GPS modules may require a GPS plane as they are installed on a PCB that does NOT have 40mm of side to them.

    Quote: "Generally, patch antennas in the 15- to 25-mm size range with a least a 40-mm (on a side) ground plane will give the best performance in portable equipment, but this may be too large for your application.  This could force you to look at smaller antenna topologies such as linear chip antennas."

 

3. The next concern is to mitigate the noise interference from FC, ESCs, and PDB.  Since my Y6B is set up with a clam shell cover and my M8N is attached under and close to the all the electronics, I may need to develop a shield "ring" connected to the shield can and then connect that ring to RF ground through an inductor at a single point.  

 

     Quote: It's common in VHF and UHF RF shielding to connect all points of the shield can to the PCB's ground plane.  This can be a mistake at GPS frequencies, since the open-air wavelength of a GPS signal is so much shorter than UHF.  Depending on the size of the shield can, if there is current flow across the can, the shield can will be able to resonate near GPS frequencies resulting in interference or de-tuning of the GPS RF.

 

By developing a shield "ring" connected the shield can and the inductor, the inductor will filter any EMI-induced current flow.  The ring connected to the shield can will prevent any current flows or resonation issues. 

 

I'm not an electrical engineer and need guidance from those out there who are.  Did I interrupted this correctly? and if so I could use some help with developing the "ring".

 

Thoughts?

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Replies

      • AKcopter > AKcopter March 17, 2015 at 8:53am

        And by the way those flying Inverted Lawnmower things are AWESOME!!

        peace :)

        • AKcopter > AKcopter March 17, 2015 at 9:02am

          So this basically means we are stuck with the CSGshop's design or whatever other vendor's design of the M8N and if we try to add a ground plane we may increase the directivity but the impedance matching and hence the SWR will go to hell !!

          • AKcopter > AKcopter March 17, 2015 at 9:04am

            I have a question though, does having a smaller ground plane necessarily mean better impedance matching as suggested by your tests above ?

  • tshado March 16, 2015 at 1:23pm

    the content of my post just above

    http://diydrones.com/xn/detail/705844:Comment:1934202

    And Randy's reply have been suppressed

    Why?

    It was stated that this post was the one to also discuss about INAVERR errors with 3DR and M8N GPS

    Please tell us is this changed, for everyone to be aware

    Thanks

    • DG > tshado March 16, 2015 at 5:52pm

      I did not see either post. 

      There is a lot of information coming out, but as far as I'm concerned, nothing yet conclusive or even much help getting to the bottom of this "problem". As I found and others have reported, a high INAVErr is not exclusive to the M8N but also affects the LEA-6H. It may not be as widespread, but is happening. Therefore I think before talking about shielding and such for the M8N, the high INAVErr issue (whether innocuous or not) needs to be nailed down as to what is the exact cause.

      I'm confident it will be, just maybe not on the timescale many of us would like. It would be great if there was nothing to worry about, but based on comments from a few devs, it doesn't leave me with warm fuzzy feelings.  

      I now believe Craig Elder's explanation for high INAVErr as being caused by use of the M8N is not 100% correct. That's just my opinion but I think there is enough evidence to support that opinion.  If Doug wishes to not use this group to discuss the unanswered questions, and only focus on shielding and EMI rings, that's fine with me.

      • Doug Walmsley > DG March 17, 2015 at 5:56am

        @DG,

        It's fine that folks are using this to discuss the error rate issues.  I'm starting to see that the error rate is possibly from a timing issue between the GPS unit and Pixhawk processor.  So a message drops and is registered as a nav error but the airframe is stable and isn't flying erratic, then I believe the firmware the DEVs have developed is catching and ignoring it in favor of other sensor information.  This is good as no one sensor should have total control but a fusion of data helps the FC make the right decisions for flight.

        My concern with bad nav errors needs to be addressed to whether its the firmware on the M8N, interference, or something else.  Seeing that much discussion out there is on GPS plane interference, I thought this discussion was a good idea.  After all, who's to say that the design of the GPS unit isn't causing the nav errors due to inadequate GPS plane or EMI ring?   I feel that either of these may help reduce the problem many are reporting as errors.

         

        I'm still planning to setup a few tests in the days ahead as soon as I get a few more parts, so I can test my theory and report on it.  But at this time weather is holding me up.

  • MarkM March 16, 2015 at 6:22am

    Don't know if they will be helpful but I've just gone through my old logs and have some from AC 3.1.5 and AC 3.2 on an APM with the M8.

    As well as AC 3.1.5 on an APM with a neo-6m.

    Having used the log analyser tool, it shows more errors occurring as the AC release number increases, I'm assuming that's because there is more stringent error checking as the code improves.

     

    m8 apm 3.1.5.log

    m8 apm 3.2.log

    neo 6m apm 3.1.5.log

  • DG March 15, 2015 at 9:05pm

    In case anyone missed this:

    http://diydrones.com/forum/topics/gps-issues-do-i-need-a-gps-ground...

    He is saying this is definitely a case of clock drift. 

    • AKcopter > DG March 15, 2015 at 11:29pm

      Hi DG

      I think I am having two problems both of which do not seem to be affecting autonomous flight in any manner, the first is the common INAVerr which as suggested by Thorsten seems to be occurring because of clock drift, and the second is most probably interference from my RFD900 radio, which may be producing out of band emissions, these coupled with the wide front-end bandwidth of the M8N may be causing bad GPS health messages...but why is the reported hdop so low even after getting these messages is a bit confusing..this is why I am a bit more keen on shielding rather than improving the sensitivity of the gps by providing it with a ground plane..

      also my RFD is 20 cms away from the secondary gps  and 35 cms away from the primary   

      • T3
        Thorsten > AKcopter March 16, 2015 at 12:02am

        Hi AKcopter,

        I am not sure if clock drift is the cause. Most probably it is missing samples. The question is where these missing samples come from (GPS, driver, clock drift, ...?).

        Yes, the HDOP is very low. But remember AC is reporting the PDOP, but names it HDOP. HDOP is approx. PDOP*0.6.

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