Impedance mismatch and GPS antenna signal issue

[Mahsheed]

Hi,

I have been working on GSM/GPS board and I am having extreme issues in GPS fixing. It only returns data in Roof top (Open air) with bigger antenna. I got the PCB made from PCBWAY, and I asked them to control the impedance of the track to 50 Ohms. There is a bend in my track and I am worried if that bend is generating Impedance mismatch. I am attaching picture of the track for better understanding. Also, PCBWAY response is also attached for reference. Please help me how can I fix this thing. I am extremely worried as I have spent a lot in PCB fabrication

[chitransh92]

Hello,
No, this should not affect the impedance mismatch.

Impedance mismatch is of high concern when the length of track is comparatively long then the electrical length of the signal.

GPS are strict devices and ideally they need at least 4 satellites to get locked and send signals..
Some manufacturers design them in such a way that even for 3 or 2 at times they get synced at a reduced accuracy..
Depends on what manufacturer are you going with..

Your issue is somewhere else and not this highlighted trace length in the your attached image.

Thank you.

[Mahsheed]

Hello,

Impedance mismatch is of high concern when the length of track is comparatively long then the electrical length of the signal.

.

But the track length is very very large when comparing to signal electrical length since GPS antenna signal would be at 2.4 GHz. Also does length matter in case of microstrip line? I heard only width matters and in case of bend, width changes so does the impedance

Hello,
.
Some manufacturers design them in such a way that even for 3 or 2 at times they get synced at a reduced accuracy..
Depends on what manufacturer are you going with..

I am using SIM 868 module of SIMCOM.


Thank you so much for your kind response

[chitransh92]

Hello.
Impedance of a trace is a function its width and distance from reference plane.
From the PCB way image, impedance is maintained under tolerance for 50 ohms.

That bend is not generating any impedance mismatches that could cause signal distortions.

When I said length, I meant that if your signal is traveling a longer distance on your board that imepdance is of high concern.
For smaller traces as you have highlighted this does not affect much.

There should be some other issues..
Get an O-scope and probe the signal.. you will find the root cause..

Thank you.

[Mahsheed]

Hi,

PCBWAY did not measure directly on that track, rather they used test coupon to measure it on straight track with that width. Please see the attached pictures shared by PCBWAY. Now, are you still sure this bend would not have induced impedance mismatch in my track? ( if this is the case, then I would be really happy)

Hello.
There should be some other issues..
Get an O-scope and probe the signal.. you will find the root cause..
.

Can you please explain how to test this thing? I am not an RF person, so I do not know how to test it. Please assist.

[Mahsheed]

Pictures attached here

[chitransh92]

Hello,
That is how most of the fabrication house test the impedance of the track using test coupons..

The convention is simple..
For ex : if the spec for your 50ohms track on a PCB is : width(W) = 5 mils and distance from reference plane (H) = 8 mils.
All the tracks designed with this criteria will be automatically 50Ohms (with some tolerance) hence coupons are measured for the impedance.


To find the root cause.
O-scope is one way but you will need a greater bandwidth O-scope.

I would suggest reach out to the GPS module provider for support. The have some internal error codes that can be read to get to the issue..

Thank you.

[robertferanec]

I would focus on two things:
- sensitivity of that module and antenna. Not every module and every antenna are good
- Do not forget, that the module has also GSM and that could also cause problems (e.g. noise)

How the module performs, when GSM is OFF? Is there difference between different antennas?

[Mahsheed]

Hi everyone,

So after struggling a lot. The problem was with the antenna. I consulted antenna manufacturer company and shipped my assembled boards to them. Now antenna is tuned according to my board and GPS is working perfectly fine I hope it might help someone in near future.

[robertferanec]

Thank you Mahsheed for sharing your solution.