GSM SIM800L modules from Allegro - antenna line quality and project application

Question

Cheap SIM800L GSM module boards have been available on Allegro for a while now. Usually mounted on a red laminate, they are characterised by their small size and, as far as I can see, are quite popular among Arduino users. Or at least this is indicated by the not inconsiderable number of...

czareqpl · Answer

And searching for topics on the electrode tried?
There are several topics where people have used these modules (including me).

They perform quite well. You just have to take care of a good power supply.

czasnagli · Answer

The sim800l works fine, but for about $4 on aliexpress you can buy a sim800c. See how many useful pins are brought out in the sim800c. It also has bluetooth. .

Anonymous · Answer

. Yes, I have, I have dealt with them. I won't shimmy as I see the problem. In the absence of an external antenna and a badly directed antenna (the wire one) the module reports something like OVERVOLTAGE PROTECT and goes (maybe for 10s) into OFF mode (maybe a non-radiated/reflected wave). After a while it tests the voltages again unlike the old SIM900 software which will no longer stand up on its own (read: will not connect to the network without a reset). The SIM800L will try to log into the network by itself (when the conditions are right and maybe that is why it leaves everyone). This diagnostic message from the SIM comes with a 100% good 4.0V power supply so I submit this for consideration. It is worth listening in your software to what the module is writing.

simw · Answer

From what I've read in the literature, a very common cause of problems with gsm modules is bad PCB and power supply design, as the thread author also wrote about. The module takes relatively a lot of current in the operating mode and therefore, in a nutshell, it is easy to generate pins on the power supply. So much for the literature. I haven't run these modules myself yet, so I can't confirm this. Colleague niveasoft seems to be suggesting that the modules themselves are tref, since he writes about 100% good 4.0V power supply, at least that's how I perceive it myself, but I could be wrong. Please elaborate on this thought. The PCB for this module is a handmade one or is it some kind of spider? Was the power supply checked on an oscilloscope? I am very curious...

czareqpl · Answer

I'll add a piece of my solution:

Diagram excerpt:

.

A coil in the ground and power supply cuts off buzzing during data/audio connections. In this way I have ensured silent operation for the audio path.

PCB fragment:

.

Greetings and best wishes for success.

Anonymous · Answer

. Probably not the best idea. Added after 48 : . The SIM800 can synchronise time from the GSM network.

czareqpl · Answer

Not the best but effective.

Anonymous · Answer

Or maybe the problem is in the PCB design (ground) or power filtering. Nowhere have I come across a solution for eliminating interference, with a coil on the ground. The ground should have one potential, the coil, in certain situations, separates these grounds. You have to admit that this should not be done (I disregard the conscious galvanic isolation).

Anonymous · Answer

. A colleague will work with radios longer..that's for sure..and on Elektroda a shortened course Radio has a long tradition and engineers. When there is no power reception (non-radiated energy) there is no miracle, energy has to show up somewhere. And it wreaks havoc on the CB if you have no antenna to radiate it. I am thin/ I know what I need to know and I can handle it/ in radios, but I know that they can blow on the board when there is no power reception. Going back to the SIM I think it tests the voltages on each pin. Maybe my statement looks chaotic, but those initiated into the subject know what I am writing about. ..and about adjusting the transmitting power to the signal strength... yet I will mention.... It's such a flavour. The module/cell determines how far away it is from the transmitter by counting retransmissions (a retransmission when there is no confirmation of successful transmission). This is why the module transmits at full power when logging in, and phones in areas where coverage is poor drain the battery. Really interested parties should read up.

czareqpl · Answer

. Some advice on how to route the ground plane would be useful in that case: In blue I have marked the position of the SIM800L module. In red I have marked the connection point of the coil to ground. In yellow colour I have marked the paths with the audio signal, which is sensitive to hum from the GSM module. In the upper left is the audio section. . Any advice from a colleague?

krzbor · Answer

I have this module, it works very well. However, I bought it with a PCB antenna plugged into the socket. What surprised me was the audio working well - I thought with that much power in the antenna the unshielded cables to the microphone and speaker would be a problem. Just one piece of advice - I did tests on a contact board and it wouldn't work - it turned out that the contacts on this board have too much resistance (the Chinese made them out of steel, I think). This circuit really draws a lot of current. If anyone is having trouble with the stability of this circuit, start by checking the power supply (its efficiency).

simw · Answer

The maximum current for such modules can be up to 2A in about 0.5ms. The power supply must be designed so that the supply voltage does not drop by more than 300mV - data for the SIM800f module (if I remember the model correctly). It turns out that the problem may be the thermopads themselves at the pins, and other thin paths on which, at 2A, too much voltage drop may occur - by the way, a similar effect is observed with poor quality usb cables supplying power to mobiles or even RPIs.
With errors in the above, even connecting to high-efficiency sources such as batteries or laboratory power supplies will have no positive effect.

Atlantis86 · Answer

Among other reasons, this is why I do not use thermal pads on my boards.
And of course I intend to put at least two suitably large tantalum capacitors at the module power pin.

BTW I have read that it is condemned to put small filter capacitors on the RX and TX lines of the module. Is it advisable to use similar filtering also on the second UART interface (debug)? What about the other communication interfaces (SPI, USB, etc.)?

Anonymous · Answer

. I would rather write that not advisable. Presumably these capacitors were to mask the problem of incorrect design/. I'm afraid that with high speed transmission of large blocks of data, the error would still show up. I don't have any capacitors. Added after 3 [minutes]: . . Will you be using debug? Capacitors on SPI? Do you want to slow down the communication? On SPI, if level translation is necessary, give 74LVCxx. If you give resistor dividers, you may lose transmission speed. Do not mix anything in USB. Stick to the manufacturer's application. Added after 6 [minutes]: . . Try connecting with 0R resistors as shown: . and short the coil on the module ground. Write if it got better or worse. Maybe the problem is in the power supply? Ground, shielding, earthing, are taboo topics.

Szaflik · Answer

Has at least the minimum filter on the audio lines recommended by the manufacturer been implemented on the PCB ? (Figure 30 of the hardware design: Speaker reference circuit) ? In the picture of the PCB it looks as if the audio paths are coming directly out of the module.

neo_84 · Answer

As long as you don't need an audio track, everything looks fairly easy. Unfortunately I managed to make one board on which the microphone worked really well . Recently I received new boards (different shape pcb) ready beautiful non-thermal transfer but unfortunately failure the problem is probably with the antenna as some colleague wrote does not radiate power. If I cut my hand between the antenna and sima covers as if the plate interference disappears and the current decreases

GSM SIM800L modules from Allegro - antenna line quality and project application

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