GSM/GPRS Modem

[Wilksey]

Good afternoon,

I have been looking at the Telit series of GPRS modules, and I was wondering if anybody has any suggestions for any other manufacturer or another Telit module (I am currently looking at the GE864 (GPS or QUAD, dont need GPS)) that is low power.

I am not expecting it to be 2mA for example, the 864 has an idling current of 16mA, but the GPRS transmission current at 900MHz is a killer for low power applications!

Any suggestions will be greatly appreciated, oh, and it has to be widely available in the UK if possible!

It has to have an built in TCP/IP stack also! (I know, I don't want much!! )

Thanks!

Wilksey

 

[Nigel Goodwin]

Such chips are bound to be current hungry when they transmit, otherwise they wouldn't reach the cell towers.

You meed a decent size battery, and a 'best capacitor' across the supply, these are commonly used on GSM modems to cope with the high current spikes involved (which are fully explained in the datasheets).

For a low power app that 16mA is a killer, have you considered using a GSM chip rather than a module?.

In my current project shutting the chip down reduces it's consumption to almost nothing, and then I switch the power off to it anyway with an FET.

 

[Wilksey]

Hi Nigel,

Thanks for your input,

Yes I understand that the power is going to be high when transmitting, I do think that 16mA is quite high for idling, the SIM900 is 22mA!!

All I really need is a TCP/IP Stack, and I have had a quick look at some GSM chips which don't seem to have one on.

I would shut the chip down, but I want to retain GPRS connection as it can take too long to reconnect sometimes.

It will be run off of a 12V leisure battery charged with a solar cell, the solar cells are quite big for high current, so I want to try and reduce the power consumption as much as possible, to reduce the cell footprint. I will probably use something like a 4700uF "super cap" for the inrush spikes.

If you have any suggestions to which GSM chip or a handful of chips I may look at then please do let me know!

Many Thanks

Wilksey

 

[Diver300]

The current spikes on a GSM phone are about 1.9 A for around 580 μs, and they start and stop in a few ns. You need some ceramic capacitors as well as some large capacitors. Your 4700 μF capacitor will be just about large enough to support the transmission pulse, but it is physically quite large.

Make sure that the ESR of the capacitor is less than about 0.1 Ω or it might as well not be there.

A switch mode power supply is needed for efficiency if you are running from 12 V.

If you turn on power saving in the GE864, with the command AT+CFUN=5, the power drops to 1.5 to 3.9 mA (well that is what Telit claim). I think that you have to raise and lower the RTS line at the right times in that mode to get the best power consumption.

You can get the Telit stuff from https://www.adaptivem2m.com/

 

[Nigel Goodwin]

Hi Nigel,

Thanks for your input,

Yes I understand that the power is going to be high when transmitting, I do think that 16mA is quite high for idling, the SIM900 is 22mA!!

All I really need is a TCP/IP Stack, and I have had a quick look at some GSM chips which don't seem to have one on.

I would shut the chip down, but I want to retain GPRS connection as it can take too long to reconnect sometimes.

It will be run off of a 12V leisure battery charged with a solar cell, the solar cells are quite big for high current, so I want to try and reduce the power consumption as much as possible, to reduce the cell footprint. I will probably use something like a 4700uF "super cap" for the inrush spikes.

If you have any suggestions to which GSM chip or a handful of chips I may look at then please do let me know!

The chip we're using is the AirPrime_WISMO218

But we're only using SMS services.

 

[Wilksey]

Nigel,
Thanks for the info, i've also found a module from Cinterion, the MC55i-W, which looks promising and is B2B not BGA which would make it easier!

Driver,

Yes, I have a good deal with the guys at Adaptive, been using them for years!
I think the application notes give information about capacitors to use etc.

I did notice some power states were lower consumption, ideally I don't want the connection to drop out as sometimes it takes a long time to lock onto the signal again.

Thanks for the information, all very useful, even for clarification it's vital!

Wilksey