SIM900, CPIN NOT READY and PCB Antenna

[franconyx]

Hi All,

I'm designing a device based on a SIM900 with a PCB antenna.
The device comunication works fine except when the GSM signal get very poor (typ CSQ = 2/4), in such cases, I observed that the modem (not always) sends a message CPIN NOT READY, after that the VSim goes to 0V.

I read many Thread on Edaboard forum which trait this issue, many of them give a reason related to conflict due to AT commands sent vs the Modem .... moreover I think that the cause could be related also to the HW design, in particular my fear is related to the nearness between antenna and Sim Holder ... What I ask you is to confirm or not (I hope) my doubt.

In order to give you a further element, I attach to this post a screen shot where I highlighted the Sim Holder (on the PCB TOP Side) and the PCB Antenna (on the PCB BOTTOM Side).

Thanks in advance for your reply.

Francesco

 

[FvM]

SIM-card interface of the GSM module and the SIM card iself can be expected to have considerable immunity against RF interferences. But I agree, that if the circuit layout is really bad, you will be able to cause SIM card operatiopn failure when the module is transmitting.

- - - Updated - - -

SIM-card interface of the GSM module and the SIM card iself can be expected to have considerable immunity against RF interferences. But I agree, that if the circuit layout is really bad, you will be able to cause SIM card operatiopn failure when the module is transmitting.

If understand the screenshot right this is a two-layer PCB, so the PCB antenna is coupling directly to the SIM card holder. The large ground loop further increases the coupling. Yes, this design is likely to bring up SIM card failure.

 

[franconyx]

Hi FvM and thanks for your reply,
I understand that the Sim holder is very coupled with the Antenna, so for this reason I put an oscilloscope probe on the Sim card signal pad, in order to see the waveforms quality in the stress condition ... I noticed that the signals rising and folling edge are quite noise-less, the second order effect are small, so no overvoltage are present... nevertheless, the error message CPIN: NOT READY continue to appear after a while when the GSM signal strenght go down.
But my big question, which answer I haven't found up till now, is: What does it means CPI: NOT READY? which is/are the cause/causes? The Simcom AT command document doesn't give an explicit definition of it.

Many thanks,

Francesco

 

[E Kafeman]

A simple way to check for near-field interference is to temporary replace internal antenna with a short coax and external antenna.
For existing antenna, route pcb pattern as long away as possible from chip inside SIM, especially first part of antenna.
As antenna efficiency probably is very low, less then 10% at 900 MHz, does it mean that 90% of TX power is reflected back in PCB structure, which may cause unstable digital levels.
If antenna location or routing not can be changed, can decoupling of all SIM card wires down to a stable ground help. Capacitor values typical 100pF - 1nF which probably just will cause a minor load at digital wires.
If a such decoupling will result in any improvement can probably be checked on existing PCB layout.
Decoupling of SIM card will most likely cause antenna mistuning as SIM card will load antenna heavier. A retuning will be needed to keep antenna as effective as possible, which also reduces internal EMI.
CPIN READY - SIM-card unlocked. If not, can be many reasons, including weak antenna signal or internal EMI. Guess both are equal possible.

 

[Batang]

Hi franconyx,

Having the SIM socket/card directly on top of the antenna is just asking for trouble.

I also notice you have several signal lines crossing the antenna as well.

Just by having anything on top of or crossing the antenna will load the antenna which will result in a high VSWR which only compounds the problem further.

Another point to note is the case for the assembled unit will also load the antenna.

Your problem occurs at a low CSQ because as the received signal level reduces, the TX power increases i.e. as you move away from a cell tower for example the TX power increases in 2 db increments until full TX power is realized.

As a general rule of thumb the transmitter output should be kept as far away as possible from any digital/audio signals.

Cheers.