GPS signal through PCB? - Page 1
Forum section: electronics / projects

author: pyrohaz
author_rank: Regular Contributor
timestamp: December 23, 2014, 05:10:48 pm
content: Hey,To those who know, I've been building a range of smart watches and this one I'm currently building is the Eon Ultra (http://hsel.co.uk/2014/12/21/eon-ultra-it-livessss/). It includes WiFi, GPS and a GSM modem. As of yet, everything works apart from the GPS! What I'm wondering is: can a GPS signal not penetrate a few layers of plastic and PCB fibreglass? I've got a couple of pictures to show the kind of things the signal has to penetrate. I've tested the GPS module not connected to my watch and it works.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 23, 2014, 05:20:44 pm
content: Are you testing it in a basement? Or are you testing it outdoors?Remember that display with touch-sensor is probably a conductor and forms a shield.Remember also that the GPS signal is much weaker than WiFi or typical cellular signals.What exactly are you using for an antenna for the GPS receiver? Your photos reveal nothing.

author: pyrohaz
author_rank: Regular Contributor
timestamp: December 23, 2014, 05:36:40 pm
content: Quote from: Richard Crowley on December 23, 2014, 05:20:44 pmI'm doing the testing on my window sill. The screen isn't actually touch but I assume it could still act as a shield? How does the GPS section of mobile phones work? Do they have better antennas than the ceramic ones you get with most GPS modules?

author: Wilksey
author_rank: Super Contributor
timestamp: December 23, 2014, 05:45:32 pm
content: In short, yes it can go through that, it can go through a plastic enclosure, anything metal can shield it.The antenna is ideally placed facing upwards, I would try it outside rather than inside.What kind of antenna are you using?

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 23, 2014, 05:54:40 pm
content: The screen in your photo appears to have a touch-surface whether you are using it or not. Those are conductive (or they wouldn't work).If that is your GPS module on the BOTTOM of the stack, then the antenna seems to be facing toward the ground!Few, if any, GPS receivers would work without a clear view of the sky. You are working with very high frequencies (and very weak signals) here!

author: 8086
author_rank: Super Contributor
timestamp: December 23, 2014, 05:57:40 pm
content: Your GPS patch antenna is directly facing the PCB (and presumable ground plane), the joystick metal parts, etc.I think this may be your problem.

author: pyrohaz
author_rank: Regular Contributor
timestamp: December 23, 2014, 06:07:35 pm
content: Quote from: Wilksey on December 23, 2014, 05:45:32 pmI'm using an active ceramic antenna included with the module - which is a cheapo VK16E (http://goo.gl/tfvQbR).Quote from: 8086 on December 23, 2014, 05:57:40 pmI made sure there was no ground plane underneath the antenna. The joystick however does have a small metal part (not grounded). Could it be this that is impeding the signal?

author: Wilksey
author_rank: Super Contributor
timestamp: December 23, 2014, 06:18:25 pm
content: Oh right,If you are sure that there is no ground plane around the antenna and that it is facing the right direction, then maybe you have a faulty GPS module, do you have another to try?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 23, 2014, 07:06:54 pm
content: There's also that big loop of copper going around the edge outside the joystick thing???Also, doesn't look like you've stitched any of that ground fill. I can't tell for sure since it's not a closeup of the entire board...Tim

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 23, 2014, 07:58:41 pm
content: It is not reasonable to expect that a GPS module with a small patch antenna will work with anything metallic above it.I would use a smaller GPS module on the TOP board, adjacent to the joystick.Like one of these:https://www.sparkfun.com/products/11571GPS Receiver - GP-635T (50 Channel) US$39.95https://www.sparkfun.com/products/10890GPS Receiver - GP-2106 SiRF IV (48 Channel) US$49.95

author: electr_peter
author_rank: Unknown
timestamp: December 23, 2014, 09:50:15 pm
content: Quote from: pyrohaz on December 23, 2014, 05:36:40 pmInside house near window? Some windows have slight metal glazing which attenuates GPS signal. Also, not every window is the same due to satellites' position in the sky.I have tested quite a few GPS receivers with various antennas. Small/weak antennas do not work at all inside, bigger ones can lock to satellites after 1-5 minutes near the window. Initial lock is essential - it is easier to maintain GPS signal than to establish it with typical consumer device.Don't expect too much from small GPS antenna - GPS signal is very weak and works only in good conditions.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: December 23, 2014, 10:17:27 pm
content: You could have an interference source on the board. You could start by trying the GPS on the board, but connected to a PC, and everything else powered off.Once I was working on a board which had an embedded microstrip for a GPS signal. This trace passed close by an Silicon Labs C8051xxx MCU, although separated by an powerplane and a groundplane. As soon as the internal clock PLL of this MCU was switched on, the GPS signal was completely gone. PLL off and all was fine.

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 24, 2014, 01:17:48 am
content: Quote from: Short Circuit on December 23, 2014, 10:17:27 pmWhich is another excellent reason to have the GPS antenna facing AWAY from your PC board, and NOT trying to operate THROUGH your board!

author: rob77
author_rank: Super Contributor
timestamp: December 24, 2014, 11:44:57 am
content: that design will NEVER workyou have a square "window" in the ground plane with the antennae below the board... and a big joystick with metal parts in the centre of the window blocking the weak GPS signal... you know it's like: "i placed a big black sticker in the centre of my window, and now i'm not able to see the street outside"GPS signal is extremely weak and you should avoid any obstacles in front of your antenna especially with cheap modules (usually those have bad SNR and they tend to operate outdoors only ).

author: pyrohaz
author_rank: Regular Contributor
timestamp: December 24, 2014, 05:27:03 pm
content: Quote from: T3sl4co1l on December 23, 2014, 07:06:54 pmYep there is, is it this ground plane that could be adding such attenuation to the GPS signal?Quote from: Richard Crowley on December 23, 2014, 07:58:41 pmOne of those modules actually looks great! I've got switched power and USART RX/TX which I think should help. Would you recommend a module with a standard RF connector and use an external antenna?Quote from: electr_peter on December 23, 2014, 09:50:15 pmThat's true actually, after the initial lock, re-locking is fine. I assume that's because the receiver has to receive the satellite information about the atmosphere and almagnac etc. and once it has, it can then relock relatively easily.Quote from: Short Circuit on December 23, 2014, 10:17:27 pmYou have a good point there! My processor is an STM32F1 with a 9x PLL supplying the clock. I'll try it with the PLL disabled!Quote from: Richard Crowley on December 24, 2014, 01:17:48 amI've 3d printed a case to fit around the PCB and didn't really make any useful place to put a GPS antenna. Would you also recommend an external antenna?Quote from: rob77 on December 24, 2014, 11:44:57 amThank you for your words! I'll be honest, radio design isn't my thing at all so sorry if all of my assumptions seem so naive! I thought RF signals could penetrate various mediums dependent on their frequency though I couldn't find a listing of materials for GPS. Would I be correct in now assuming that no frequency RF can penetrate metal?Thanks everyone and merry Christmas!

author: pyrohaz
author_rank: Regular Contributor
timestamp: December 24, 2014, 05:50:47 pm
content: There is a space above my Joystick which I could cut out for an antenna to poke through. Would a module like this:http://goo.gl/JpMB3lbe more suitable for this project?

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 24, 2014, 06:06:50 pm
content: Metal (or metallic-like materials) are used to SHIELD against radio waves. NO radio waves can pass through metal.Most certainly, an external antenna would perform MUCH BETTER than the internal antenna on any GPS module.Any GPS antenna, internal or external, needs an unobstructed view of the sky.Even a good external GPS antenna will fail in a big city with tall buildings because too much of the sky is blocked.As many have said frequently here, GPS signals are VERY WEAK, and very high frequency.You must go to extraordinary lengths to provide good "visibility" between the sky and the antenna (internal or external).Burying the GPS antenna and module UNDER other parts of the gadget is practically a guaranteed FAIL.Yes, at least try to get the GPS module/antenna ON TOP of the gadget so it has an open "view" of the sky.Don't expect decent performance indoors. Especially in commercial buildings with structural metal.

author: pyrohaz
author_rank: Regular Contributor
timestamp: December 24, 2014, 06:09:35 pm
content: Quote from: Richard Crowley on December 24, 2014, 06:06:50 pmThats fine, I wasn't expecting any indoor performance anyway! How do phone, laptop and tablet manufacturers get around implementing GPS without dock off ceramic antennas inside? My phone (Sony Xperia M) has fine GPS which works relatively far indoors!

author: Richard Crowley
author_rank: Super Contributor
timestamp: December 24, 2014, 06:14:07 pm
content: Quote from: pyrohaz on December 24, 2014, 06:09:35 pmThen you are extrordinarily lucky. I have never seen ANY cellphone that demonstrated even halfway-reliable GPS performance indoors, unless you were standing right next to a large window out in the suburbs or open countryside. I don't think you appreciate how really fragile the GPS signals are.

author: SeanB
author_rank: Super Contributor
timestamp: December 24, 2014, 06:18:49 pm
content: Phones and laptops will have either a plastic case, or a spot under the glass next to the speaker, to place a small patch antenna for GPS, and then they can have a good chance of getting reception. They often cheat in using tower data to work out a rough location and then using that, along with the cellular system clock, to work out a rough almanac and likely decorrellation stream for the module so it will lock faster. As well the GPS signal is almost never used alone, it is used with tower location info ( from the carrier, who will know the tower location very well) to give a position even if GPS info is spotty.If I cold start a GPS module ( powered off for a while and no battery inside) it will take about 20 minutes to start giving valid data, while the phone GPS has the benefit of being always on ( if you turn it on in settings if available to you) and always updating so long as the phone is powered in either standby or active.

author: pyrohaz
author_rank: Regular Contributor
timestamp: December 24, 2014, 07:38:51 pm
content: Quote from: Richard Crowley on December 24, 2014, 06:14:07 pmUsing the cell tower locater sat about 3 meters away from my window on a second floor house (with occupied loft space), I've got a GPS signal with an accuracy of 27m (which I assume is worst case error).I'll be completely honest and agree! I don't have a clue how fragile GPS signals are, hence why I'm asking here for professional help!Quote from: SeanB on December 24, 2014, 06:18:49 pmMy watch does actually have a GSM modem and I can find an approximate location through GPRS using the CIPGSMLOC command. Could I somehow feed this data to my GPS modem to help get a lock?

author: ConKbot
author_rank: Super Contributor
timestamp: December 24, 2014, 07:40:56 pm
content: Quote from: pyrohaz on December 24, 2014, 06:09:35 pmPhone antennas are just about the darkest of the dark arts that is RF. The fact that is has to coexist with 3-6 or even more radios or receivers on a device, the whole thing is liable to be next to either your head or your hand on the back, or both at the same time, no space for high dielectric constant materials, and first and foremost, it has to be CHEAP. They have everything working against them and nothing for them except maybe a limited temperature range.That being said, good GPS receivers can lock down to -160dBm maybe lower on some, GPS signals hit the surface at ~-130 dBm, -10dB of antenna gain on some poor tiny mutilated antenna in a phone, and a 30dB pre-amp makes for 50 dB of link margin, which is pretty healthy. 20 dBm without a preamp isnt as great, but still good, but it would be really easy to lose even that, especially with a receiver that doesn't have a sensitivity of -160 dBm. Now, going indoors could easily attenuate it that much or more, and putting a metallic shield in front of it might not just add attenuation, it could de-tune the antenna (capacitive loading) so far that you cant even receive any signal, even with the aperture in the ground plane.

author: rob77
author_rank: Super Contributor
timestamp: December 24, 2014, 07:57:03 pm
content: Quote from: pyrohaz on December 24, 2014, 07:38:51 pmYour GPS module must be supporting such a assisted lock...here is a link to a interesting article to readhttp://gpsinformation.net/main/gpslock.htmit's really not a trivial task for the poor little GPS receiver

author: Howardlong
author_rank: Super Contributor
timestamp: December 25, 2014, 04:37:40 pm
content: I will also add that placing a bit of FR4 (without any copper) directly on top of that ceramic patch antenna will detune the antenna due to its permitivity so the module is very likely to become less sensitive as a result.Phones and tablets with GPS receivers will be designed in such a way as to either minimise any interraction with the antennas, or designed in such a way that the enclosures become part of the antenna design.As others have stated, link budget for GPS is generally right at the noise floor, it doesn't take much to wreck reception.

author: Howardlong
author_rank: Super Contributor
timestamp: December 25, 2014, 04:51:26 pm
content: Quote from: ConKbot on December 24, 2014, 07:40:56 pmMost of what you state is right, but I would question the perceived benefit of a preamp in this case. Remember that a preamp is amplifying noise as well as signal (as well as adding a little noise itself), it doesn't miraculously drag a signal out of the ether unless either you already have a deaf receiver or a lossy feeder between the antenna and receiver. As the antenna is directly attached to the receiver, we can discount feeder loss almost entirely in the link budget calculation, and you'd hope the front end on that GPS receiver already has a reasonable noise figure.This is why sticking a premp on your terrestrial TV setup at the TV end to improve reception is rarely effective. It needs to be at the antenna to offset feeder loss.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 25, 2014, 06:51:39 pm
content: Yes, the FR4 and copper are in the antenna's near field, detuning it and lowering the radiation resistance (which is already low enough on account of the small antenna).When it comes to antennas, size matters. Even if you can squeeze room for a 1/4 wave whip or some analogous sort of patch or strip antenna, you'll be doing better than that goofy ceramic block.Tim

author: ConKbot
author_rank: Super Contributor
timestamp: December 25, 2014, 09:00:12 pm
content: Quote from: Howardlong on December 25, 2014, 04:51:26 pmVery true, a preamp will help signal levels but it wont be improving CNR. GPS CNR will be a lot more critical to if you have a lock or not than signal level, as the receiver will lock on at -160dBm

author: Howardlong
author_rank: Super Contributor
timestamp: December 25, 2014, 09:07:38 pm
content: Quote from: T3sl4co1l on December 25, 2014, 06:51:39 pmYes, and I should add that it's not going to do much for the radiation pattern either.

author: eas
author_rank: Frequent Contributor
timestamp: December 26, 2014, 08:20:40 pm
content: As a point of reference, I decided to dig out an ~5-6 year old Holux M-241 GPS logger I had in a drawer and see how long it took to get a GPS lock indoors. I am sitting at the north west side of my house, about 3' from an exterior wall. The inner surface of the wall is plaster and lath, the outside is wood siding. I am about 15 feet from the south wall of my house, with an interior plaster and lath wall in between.It took about 5 minutes to acquire an initial GPS reading. On my iPhone 6, using a GPS app, which takes advantage of the phones A-GPS to acquire the almanac, it took less than 30s.

author: pyrohaz
author_rank: Regular Contributor
timestamp: December 27, 2014, 04:42:21 pm
content: Quote from: ConKbot on December 24, 2014, 07:40:56 pmI'll be quite honest, I find the whole idea of radio black magic myself! I think I'm going to look for a GPS module with an SMA connector that I can interface an external antenna to.Quote from: rob77 on December 24, 2014, 07:57:03 pmThank you for the link! It was a good readI find GPS in itself a magnificent invention and the fact it is "free" for the good of humanity just makes it even better! Thinking in depth about how it works however is another matter...Quote from: T3sl4co1l on December 25, 2014, 06:51:39 pmI've been doing a bit of research into antenna theory as I've never needed to learn it and after looking, a quarter wave whip is actually pretty plausible. Using 1.575GHz as the GPS frequency, I'm looking at an antenna length of 47.6mm. The problem I wonder though: How can I interface this with my GPS module? I could just solder a small SMA connector to some insulated copper wire and be done. Would this be the correct method of interfacing the antenna? If I'm using a module with a GPS module on a PCB, am I safe to assume that up to the SMA connector, all transmission line effects would have been solved by the module engineers?EDIT:Also, isn't the ceramic antenna already a form of patch antenna or am I on the completely wrong end of the stick here? That little ceramic block seems pretty magic to me as it is!Quote from: eas on December 26, 2014, 08:20:40 pmI've left my watch on my window sill pointing to the sky for the past 3 days and I've not managed to get any form of lock so there is a definite problem with my design! I'd be happy at a 5 minute lock though!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2014, 05:37:52 pm
content: Quote from: pyrohaz on December 27, 2014, 04:42:21 pmThe ceramic block thing most likely qualifies as a patch antenna, yes. The point is usually that you have a relatively wide radiating surface (a patch), to make up for the fact that you don't have any length behind it. You're coupling to electric field, but not magnetic field (indeed, most of the current flow and associated magnetic field may be trapped under the patch, via some short connecting wires or traces). Added capacitance (such as from a ceramic block) will act to shorten dimensions even further, but, the reactive current flowing through that capacitance is entirely internal, and does not couple into space. So you get an even lower resonant impedance (which makes things even worse against the already low inductance, necessitating some sort of matching network), more internal losses, and even lower radiation resistance. (The matching is usually on the part itself, so the transmission line sees 50 ohms or whatever as appropriate -- at least, at one narrow range of frequencies.)The logical extreme of a very small antenna (patch or otherwise) is a resonator in a box: exclusively internal losses, with no radiation resistance whatsoever.Electrically, the two cases are indistinguishable -- they both can be matched to 50 ohms at resonance, and have a narrow bandwidth; but one, the power just goes into heat, while the other actually communicates with the outside world.As for 1/4 wave whip antennas, the problem is, to make a textbook case, you need the whip part to be vertical above a horizontal ground plane. Often, the ground is provided by an array of in-plane 1/4 wave grounding rods, or a PCB or something; solid ground is not required, but it does need to carry current radially.The dipole replaces the ground plane restriction with a 1/4 wave in the opposite direction. Replaces one obstacle with even more length...The free space you necessarily want around these antennas also ranges from 1/4 wave (can be helpful -- acts as a reflector) to 3/4 or more (avoid 1/2 or 1 wave, that's a null). You need volume as well as length to couple into space effectively.You'll never be able to accommodate a proper, high efficiency antenna in a compact device, so you'll have to settle for something that's still close in (so it experiences proximity / loading capacitance, and has a smaller physical length, which means less to couple into space), and make it as much to the outside as possible. Hopefully, that's still good enough for useful GPS reception.As for design or selection, that's really hard to say. Which is why the internals of cellphones are all so peculiar: they can afford the research. If you have a 1.5GHz signal generator and voltmeter, you can try testing it for resonance and matching, but it's just a shot in the dark* otherwise. You can get self-adhesive and patch antennas for various close quarters, but if they specify what proximity they are supposed to operate within... you really don't have much choice but to match your mechanicals. And if they say "keep clear up to at least X", it's probably because they're not trying any proximity at all, so have fun with that...*Ha, because you don't know where the electromagnetic energy (light?) is. Get it?...The one advantage your ceramic patch antenna has is, it's designed for board mounting, so one side should be in proximity to a board. The rest should be clear. But that's still better than trying to blindly fudge a more close fitting antenna.Tim

author: pyrohaz
author_rank: Regular Contributor
timestamp: December 28, 2014, 05:42:20 pm
content: Well thank you all for your help. I now know never to do such a stupid antenna design again! I've now changed my GPS module to a cheapo one off eBay which I originally wrote my GPS driver/NMEA parser for as this module has an onboard IPEX connector which I can interface to external antennas. I'm going to use a small 10mmx10mm patch antenna (unless I can find a rectangular one!) in the top right corner of the PCB - currently deciding on:http://uk.rs-online.com/web/p/gps-modules/7165365/I've tested this with the stock 25mmx25mm patch antenna with good results getting up to 8 satellites by just placing it on my window sill! Hopefully from now, I can get much better results.

author: itdontgo
author_rank: Regular Contributor
timestamp: December 29, 2014, 11:04:37 am
content: You need to use a chip antenna for a watch project. They need a fairly large ground plane though. Here is a speedometer I designed a few years ago:That is not ideally laid out but it has good performance. NMEA SNR indicator can be up to 50.Performance is all about the ground plane. If you look at the picture I've intentionally moved as much away from the aerial as possible and stitched both layers together. However on a previous design there were parts all around the ground area and it still worked.That PCB has an error in that you should remove the soldermask around the RF parts as in the later PCB as it affects the capacitance and gives a noticeable improvement in performance:With regards FR4 transparency - sometimes you get a higher SNR (signal to noise) if you tip the PCB upside down so the antenna is looking at the floor (obviously no satellites there). This board is 0.8mm thick in order to match the impedance of the aerial trace to the module but the FR4 has been cut away to improve performance. It is also at the edge of the PCB. When we used a 1.6mm PCB performance suffered. It was something to do with the FR4 as when we peeled back the laminate to make it thinner it improved performance.You also should appreciate when designing for GPS the satellites are not just above you. You may get a better signal from one in front of you. You don't need to look up to see the sky! That is why these chip antennas give better HDOP readings as they have more even gain around their axis than a patch antenna. A patch antenna has high gain in the direction you point it which is normally just up and have less gain for the satellites on the horizon which are equally/more useful for velocity.

author: pyrohaz
author_rank: Regular Contributor
timestamp: December 29, 2014, 03:15:04 pm
content: Quote from: itdontgo on December 29, 2014, 11:04:37 amWhat an awesome project! Thank you very much for your input! Where did you source the rechargeable Lithium batteries? That chip antenna actually looks ideal. I'd likely have the actual GPS module on the underside so how does crossing sides of the board come into designing around a chip antenna? Also, do you have any information that could help me on the designs with these chips?Thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 29, 2014, 08:57:22 pm
content: If the side-crossing is done with impedance matched vias, it doesn't know any better. In practice, that's a crock, but 1.2GHz isn't enough to notice very much from one or two vias. Just use impedance matched trace where possible, load the surrounding ground with vias, and make sure there are ground vias flanking any layer transition (vias either side of the active via will do a sort of three-wire shielded section of transmission line, hopefully having a similar impedance). Such construction (give or take some study of the correct via size and separation) should be enough to show merely small bumps on a picosecond range TDR.That design is simple, and unless there's more to the chip component than I can see from the picture, it looks rather useless to buy a component at all -- it's just a hunk of metal! Having a known, reliable layout is more important, and in this case, requires knowing the dielectric constant of the substrate, its thickness, and the dimensions of the ground cut-out, trace, length and so on. It should be almost as good if replaced with solid trace (no chip component). But... it won't be exact, there will be a small change in length required then.Likely, the observed difference with thickness was due to loading capacitance shifting the resonant frequency down. Which could also be solved by shrinking dimensions slightly (the length of the chippy part, or the length of the trace extension). But that's just a very loose guess -- it all depends on where that resonant frequency is, and that's hard to calculate without building ten variations and measuring them, or calculating it with a microwave analysis suite!That's the downside to this. If everything, from ground plane up, is on the purchased part, you can be moderately confident that its properties are as specified. If part or all is on the PCB, it's up to you to get the dimensions right.Tim

author: ConKbot
author_rank: Super Contributor
timestamp: December 30, 2014, 11:28:59 am
content: Quote from: T3sl4co1l on December 29, 2014, 08:57:22 pmCeramic antenna, not just a hunk of metalSeehttps://www.sparkfun.com/products/9131as an example