Need your help on GSM RF Connection/Antenna

HI
I am using U-blox LEON G100 GSM module in one of my project.
Antenna used - TG.09.0113 from Taoglas
GSM RF connection is (as recommended by manufacturer)

Component details are

It is working fine for one mobile operator SIM. If other SIM cards are used, processor goes hanging.
Observations:
1. GSM module draws more current (unusual amount) during power on (delay between GSM LDO enable and processor hanging is ~30s).
2. I added extra decaps (multiple times more than vendor recommendation). Power droop introduced during hanging disappears now.GSM voltage rail is within operating ranging. Still issue persists
3. If I remove antenna, board never hangs up.
What am I missing? Is it due to Antenna? Is there any mismatch between antenna and GSM module RF filter circuit?
I would appreciate very much your thoughts.
Best regards,
Ramesh

note: please refer the attachment for the circuit

I had a similar issue quite some time ago.

My problem was the dc-dc converter used to drive the circuit could not respond fast enough to the immediate power demands of the GSM module when it is switching bands and registering on the mobile network.
It was transmitting RF at 2W in very short bursts and the dc-dc could not keep up. My solution: another dc-dc with parameters that suit the application.

Calm down, one step at a time..dont panic.

1. If I've understood it correctly that your system works properly with one mobile operator system ...question is: are you checking any parameters on tester to see your system actually working?
2. Lets assume you have checked your system and it works with one mobile operator system, it tells you that your HW is functional.
3. You need to make sure that your device/system can be used/supported any SIM card? Look at your mobile, if your mobile is SIM LOCKED, do you think you can use any other operator SIM card without unlock your system? Find out...
4. Filter output to ANT...if your filter output is 50 ohms and ANT input is 50 ohms then we should not see any mismatch (of course its a ideal case)...if not you could use resistive 3-dB attn. circuit to minimize the mismatch.

Bottom line is:
with the working SIM card figure out your system performce..check out SIM card signals (measure signals on SIM card via scope) and then do the same exercise with not working SIM card..create a delta list for your visibility.

Dear ‘not-a-moderator ’, MK28,
Thanks for the response
Fast transient response parameters for LDO – already checked and TPS75801 meets these requirements
GSM module is controlled by uP. uP communicate with GSM module through UART and through AT commands, I checked all the parameters like signal strength, GPRS connection, GSM UART parameter settings, SIM card details, GSM module FW/HW versions, etc. Everything is fine. Signal quality is around ~ -58dBm to -60dBm
SIM is not a locked SIM.
One more observation:
Board is booted up without GSM antenna. Voice Call can be made to the GSM number. But signal quality is poor comparatively ( -72dBm to -78dBm). If I connect antenna, signal quality is around -54dBm to -51dBm. (Maximum value given by U-blox).
In brief, Presence of antenna during power-on of GSM is affecting somehow the whole board and uP freezes.
After this observation, I tried few values and zeroed on 5.1nH@L103, 3pF@C114.These value provides signal quality of ~ -60dBm. So far (18hours), it works fine for all the SIM cards.
There is no details regarding GSM ANT pin impedance in the datasheet. I was using the filter circuit from their reference design but different antenna.

My concern is Is this called ‘mismatch’ between antenna and filter circuit? Though these values works fine for now, i want to understand the reasons for the issue.
I want to understand how to arrive these values through design calculation. can you please help me on these?I am novice on RF design.

best regards
Ramesh

Hello Nramesh,

Like I have said earlier, if you fear that it's a mismatch then modified your reference board, build an attenuator (3-6dB, it's a 3 registers circuit) between filter output to ANT input (assuming you do not know the ANT impedance). This will minize your mismatch.

BR
MK28