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?

Views: 16545

Reply to This

Replies to This Discussion

Perfect!

The external baro should help as well.

I have no experience with the CAN bus so far. Is it correct that only one cable is required for all components, i.e. GPS + compass + baro, but more than one cable can be used for "redundancy"?

Yes.

Thanks!

Looking forward to testing it!

Good day to you Nampilot,

I just happened to come across your post while doing some research, and being the author of this article, I thought I might give some background here.

In open disclosure, I haven't researched the issue at hand in this post, so I can't say whether any of the design guidelines presented in my article would have hindered or helped.  That will take more time than I have available to discern at present.  But, let me state that I think our design objectives are different here.  You appear to be speaking from an antenna design perspective (and perhaps a passive antenna plus GNSS module design perspective?), while I was primarily concerned with addressing GNSS chipset design into larger multi-processor and multi-frequency system solutions.  My limited objective (as given to me by Electronic Design) was to demystify some of the design goals and solutions for the average digital designer who is faced with a GNSS chipset design, not to turn them into GNSS antenna designers (a specialty, which I'm sure you'll agree, cannot be achieved through a single magazine article).

Unfortunately, magazine articles offer limited space in which to reach your objective, so I couldn't get very specific in any one area or else my article would have run over (and would have been edited down by a copy editor with less of a command of the subject).  I also had to resort to generalities, that of course would not hold true for all customers, but were generally true for the vast majority of customers attempting a chipset design.  Also, as printed, the article was not verbatim what I wrote (there were some edits that occurred after my submission that I could not control), but in general, the concepts were mostly preserved, so I couldn't reasonably object given the medium.

Let me give some context here.  I work for a semiconductor manufacture that supplies GNSS chipsets (multi-constellation GPS receivers if you prefer) into automotive, consumer, and industrial accounts.  We don't make GNSS modules, only chipset solutions, so I am often faced with customers who are trying to "drop in replace" their current module solution with a dedicated chipset design.  This is further complicated by the fact that our solution is a Telematics system on chip (SoC) solution that includes a number of interfaces (e.g. CAN, SPI, I2C, SD-Card, USB, and audio codec interfaces) not typically found on your single frequency GNSS solutiions.  We're also the only manufacturer that allows custom software integrations on our solution.  All of these high-speed interfaces add many more noise sources than is typically found in the average GNSS module, so some accommodation is needed to keep the edge-noise of these interfaces and subsystems from raising the local noise floor, or worse, producing a jamming signal in either the RF or IF signal paths.  To give you an idea of the type of product this solution is used in, take a look at the eTrex 10, 20, and 30s.  That solution uses our GNSS processor as the GNSS receiver and the main micro in the system.  It does everything, including the map reading and map drawing!  This is a highly complex mixed signal solution.

Given that context, I'd like to address your comments (many of which I agree with, but given the limited space of the article in question, I had to make do with a generality that would make life easier for the average designer):

Thank your for your descriptions of the patch antenna.  That in itself was outside of the scope of my article (beyond the quick generality about linear versus patch antenna benefits), and it was well presented.  Can I suggest you write an article of that for RF Design, GNSS Today, or GPS World?  They're always looking for quality articles and I think that one would be a benefit to the readers...please consider it.

Now on to the individual points (I have put the original poster's comment in italics):

“Passive antenna” designs are more complex and can be susceptible to noise coupling into the antenna ground plane if not correctly isolated from other noise-producing components on the PCB.

This is untrue - The antenna is simply that - the actual antenna ( the patch element, the ceramic substrate, the ground plane) is the same for a passive or active antenna. To make the antenna an active one, an amplifier is added - that is all. The antenna is no more or less susceptible to noise pickup - in fact , the active antenna is more susceptible, since the amplifier will now actively amplify any noise within its passband.

For designing an antenna module by itself, I agree with you.  But when you have a designer who is now needing to design the antenna, the RF path that is inside the GNSS module (i.e. the LNA, SAW Filters, impedance matching components), in addition to the digital sub-systems of his own design onto a single PCB, the issue becomes more complex.  Take as an example where a designer has flooded a digital ground and power plane underneath a passive patch antenna.  In this example, digital noise will capacitively couple into the patch antenna's ground plane and (potentially) be amplified by the first LNA (in effect raising the noise floor at the LNA).  Since the SNR is fixed by the first LNA in the chain, this would have a disastrous effect on the c/n0 of the system.  LNA gain cannot make up for a noisy ground plane before the first LNA stage.  This is what I meant by the design for passive antennas being more complex than say an active antenna design were additional LNA gain "may" be able to compensate for a noisy RF path further down the chain.

There’s one thing to note when deciding between chip and patch antennas, though. Patch antennas will provide the best signal performance for their size, as they receive signals on all sides of the patch. Linear GPS antennas (chip or dipole) will generally only receive signals along one of their axes. This results in linear antenna designs being at least half as sensitive (i.e., around –3 dB) compared to patch antennas, and most will probably be around 25% as sensitive as a patch (or about –6 dB).

Again, not true. The radiation pattern for the patch and the chip antenna can be designed to be very similar. The patch does not 'receive on all sides' and the chip not - they 'receive' according to the designed radiation pattern, and a chip antenna with a pattern in only one specific direction would not work for GPS and would never sell...And this is NOT the reason for the -3dB of the chip antenna directivity - the reason is that the Patch is designed for circular polarisation, while it is very difficult to do so for the chip antenna. So the GPS signal suffers the 3dB loss because of cross polarisation.  What is important with ALL antenna, regardless of type, is to understand what sort of radiation pattern you need, and what the antenna can provide. For GPS we would like a pattern that is a half sphere upwards, with maybe a -3 to -6dB loss on the horizon ( to eliminate pickup of man-made RF noise from the horizon). Most GPS patch antenna deliver this, if used with a suitable ground plane...

All GPS receiver manufacturers ( of the chip level device) , such as the various Ublox devices mentioned - Neo, Max, etc, have taken care of ALL emc issues related to signal integrity and noise effects to the device itself. External devices, the LNA, etc, are the responsibility of the board level integrator. 

Ah generalities and editing.  No real disagreement here; of course the polarization is the reason for the signal loss, but also the ends of the linear's antenna pattern present nulls that patches do not suffer.  The main point here being that "all things being equal" with a sufficiently large ground-plane, a patch will out-perform a linear antenna.  Your version is more correct, but the performance generalization still stands.  Also note that the uBlox is not dealing with high-speed mixed signals...more on that later.

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 wavelengths 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.

Absolutely untrue. The first sentence is correct, but the concept of a shield can is used on every GPS Chip and many other RF devices - Video TX, RCS TX, etc. The can must connect to the ground plane correctly, ie, the spacing between connections to ground, if not solid, must be a very small portion of the wavelength we are trying to shield against. These small GPS module's cans are soldered every 3 or 4 mm, so the frequencies that may enter' these gaps are in the region of 20GHz.....And if the can could ever resonate ( not possible - it is connected to ground..) the resulting current flow in the can material would be on the outside, and penetrate very little into the material ( skin effect) and would NEVER enter the interior of the can. Imagine a copper sphere, hollow inside. Make a small hole, and stick some coax cable into this hole. Solder the screen of the coax to the sphere. Connect the other coax end to your transmitter and transmit. Now try to measure the rf exiting the sphere...If we could measure any useful signal out of the sphere  we could place antenna inside metal fuselages...

I think you may be thinking of antenna shield cans over LNA components and not dynamic current scenarios.  In this article, I am talking exclusively about the shield can resonating under the current flow from other digital devices in the system.  This is a phenomenon that may be more prevalent in system on chip solutions, but I can tell you it is very real (having measured the resonance with spectrum analyzers in the lab).  I had one customer who lost a third of his initial production lot to shield can resonance (the reason it wasn't 100% was due to the variability of how the shield can was placed when soldered down and how complete the soldering was...more complete soldering only fixed about 50% of these failed cases).  The issue here is you have several ground planes in use, a GNSS antenna ground-plane, a GNSS RF ground plane, and maybe multiple digital ground planes.  Shorting them all together with a shield can allows for noise and current to flow across the planes and the can (which is what this customer did).  The easiest fix was to isolate the shield can through an inductor to keep stray currents away from the can.  This fixed his issue and he was able to move forward with production.

Specifically to shield can resonance you say:

And if the can could ever resonate ( not possible - it is connected to ground..)

No grounds are perfect (even 30-mil vias impart a 10-ohm impedance mis-match in the signal path) and even if the grounds were perfect, this ignores the self-resonant frequencies of the components and structures.  In other words, ground is not absolute.  If it helps to understand the shield can resonance problem, maybe you can consider it a grounded loop antenna?  It's the AC current flow through the loop that sets up the resonance.  

The shielding can is solidly connected to the chipset PCB ground plane, and this ground plane is brought out to many 'pads', on the side of the chipset PCB ( just check any Ublox GPS chipset datasheet for example..)- these pads are then soldered the  main PCB ground plane, which forms part of the Patch antenna ground plane in an integrated GPS. If this grounding is maintained, then all is well. And a connection to any part of the ground palne is as good a ground as you will get.

And this approach will get you into trouble in a mixed signal system.  The uBlox chipset isolates their digital sub-system inside their module and inside their device package (in effect creating the ground isolations I discuss in the article).  System-on-Chip solutions with high speed interfaces (SD-Card, external memory buses, et cetera) do not have that luxury since the return current for all those high-speed edges MUST return to the device package.  uBlox can survive with their methodogy because their interfaces are limited in number and are either low rate (RS-232), or are double-ended driven (e.g. USB where the current flowing out is mirrored by the current coming in).

And then the discussion got lost with fast edges, and clocks, etc - nothing to do with fitting a GPS chip-set to a PCB and antenna...

I disagree, it is a crucial point when designing mixed signal, high-speed systems.  All those switching signals have to dump into ground somewhere and taken in aggregate, they can severely effect the noise floor of the system.  Remember, GNSS signals are already 20dB or more below the open air noise floor; it doesn't take much ground-plane noise from the digital side of the system to raise the noise floor or create harmonics at GNSS frequencies.  I once had a customer's asynchronous 4-Mhz clock, via a 392nd harmonic, create 35dB c/n0 jammers in the RF.  It doesn't take much.  I could tell you more over a beer sometime...

Adding an LNA to increase gain does not increase directivity, and can worsen signal to noise ratio if not done well (shielded, etc).

It also does nothing if it's not helping overcome coax loss or high RF front end noise floors.  The signal's SNR is fixed at the first LNA and assuming that LNA has sufficient gain to overcome the losses down-stream, adding additional LNAs downstream will not get you more signal, and in fact may only be adding noise figure losses to the system.

In closing, please do consider writing an article for one of the above mentioned magazines.  If you have an interest, I would love to see a comparison of linear-chip, folded-F chips, and patch antennas.  One comparison I've been meaning to do is to compare a limited ground plane chip (say 25x25mm) with a patch of limited ground plane (say a 12x12x4mm patch with a 20x20mm ground plane) in terms of directional response, de-tuning effects (think ground planes on wrist watches in close proximity to the human body) and absolute gain.  All the antenna manufactures love to give cell-phone sized ground planes (80x60mm) and most of my customers want ground planes 1/4 that size.

Best Regards,

Jeff Wilson

Hi Thomas,

Can you use the 3DR configuration file for all the ublox M8x series?

I thought this was exclusive for the LEA-6H GPS module.

Regards,

Geert

Does AC 3.3 support an external baro?

Doug and all,

I just posted some test results regarding the influence of the ground plane:

http://diydrones.com/profiles/blogs/u-blox-m8n-ground-planes-antenn...

Thorsten

Thought I'd post my initial look at the Ublox NEO-M8T Time & Raw GNSS Beidou Triple Band EMI Protection GPS I received today from CSGShop.com.  Looks impressive and now I can start my comparison tests in the coming days.

Wow... does that have a compass also? Just wondering what the EMI protection is exactly and if it has a compass perhaps it would help with that as well? Is that a micro usb for the connection? 

The GPS unit is the UBLOX NEO-M8T which does not include a compass on it.  For me that's fine.  I plan to install it on my OctoQuad as a primary GPS under clam shell and the Lea-6H and compass will remain external on a mast as secondary GPS and primary compass.

The connection is a Mini-B type plug and not the Micro version.

Today I installed my NEO-M8T onto the outside of my Octoquad.  Just couldn't find room inside the clamshell to install it but it is working great on the outside. 

I also noted that the M8T acquired satellites very quickly after being unplugged for the last 6 days.  While indoors it acquired 15 satellites and an HDOP of 1.33 within a couple minutes.  

I also ran power tests on the FPV, ESCs, Pixhawk, and LiDAR and noted that the Lea-6H was negatively affected but the M8T did not degrade but actually kept acquiring more satellites while sitting only inch above the Pixhawk and 2 inches above the 2x Quattro 25x4 ESCs.  Tests are early but the EMI shielding and large ground plane on this antenna is working very well. 

Should explain that the Lea-6H was the left readout at 9 satellites, while the M8T is on the right at 15 satellites.  The Lea-6h when there was no power applied held 11-12 satellites and an HDOP around 1.3-1.5 but degraded once power was applied.

Reply to Discussion

RSS

© 2019   Created by Chris Anderson.   Powered by

Badges  |  Report an Issue  |  Terms of Service