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?
Replies
Thx Jor for your excellent, as usual, explanations.
For the sake of simplicity and clarity, can you graphically point out, on the ceramic patch antenna pictures you posted, where is the "RF ground" : is it the top silver layer , is it the bottom pin ?
When people say to solder the RF ground to a ground plane extension, what part of the ceramic patch antenna must then be soldered to the ground plane extension ?
Hi Hughes,
The ceramic patch in the photo - the top shows the pin soldererd to the active element. The bottom shows the pin protruding - that is the RF connection to the antenna = goes to the GPS Chipset RF input, either directly, or via the center of your coax, or via the LNA. The underside of thes type patches, where the silvered 'ground' is, does not connect directly to your PCB copper plane - it connects capacitively, and the RF received 'sees' a ground due to the low impedance of that capacitor at those frequencies. So, your ground reference connection is your PCB copper plane, nowhere else.
I do not understand the concept of a 'ground plane extension' - any connection to this ground plane other than to get power to the GPS from your battery source, serves no purpose at all. This ground plane is the GPS chipset ground, and the Patch antenna ground plane and reference only.
Sorry, back on Monday....
The Nampilot...
ok, if I understand correctly what you're saying, if I want to extend my M8N ground plane from the 35x35 size to a 70x70 size (what seems recommended in the specs of the manufacturer of the chipset), I just solder pieces of copper to extend the PCB size to a 70x70 rectangle (soldered on "scratched PCB" borders to reveal the ground copper layer).
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
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.
@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.
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.
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.
I am also posting this here per DG suggestion
Several threads here on DIYDrones and RCGroups have been opened to discuss about the explanation and what represent these INAVERR errors in performance monitoring in logs
This problem was reported end of January
Still no answers from 3DR/Dev guys, to tell us something about this INAVERR in logs
A ticket was opened in Github about that
I know that they are probably busy, but could be great if somebody could tell if this INAVERR error if indicating a problem or is not
I found a post from Randy who in last November was thinking INAVERR at 255 was not a "very good thing"
http://diydrones.com/xn/detail/705844:Comment:1482305
Marcus,
Thanks for the info. Your copter missed one GPS message and went 0.07seconds without a fresh GPS update (you can see the 400ms jump in the GPS time between lines 2593 and 2606). You were in Loiter mode but it was so short lived it didn't upset the navigation at all. If it was -rc6 you would have seen a very slight twitch. If that INAVErr was 255 meaning it had maxed out the recording of errors I'd be more concerned. For now, this is consistent with what I'm also seeing which is that we're getting occasional missed messages which we still need to get to the bottom of but in low numbers it's not dangerous.
@tshado,
The Inav errors mean that inertial nav has not received a GPS update within 0.3 seconds. So that probably means that the GPS is running too slowly (i.e 3hz or less) or messages are being missed. I've never used one of these GPSs so I'm really not sure what exact technical issue is that's causing the missed messages.
It's hard to say how badly this will effect inertial nav's position and velocity estimates but it will have a negative effect I think.
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