Harald Welte laforge at openmoko.org
Mon Aug 25 09:32:48 CEST 2008

On Wed, Aug 20, 2008 at 09:39:14AM +0100, Andy Green wrote:

> IMO it doesn't make any harm to see MOSFET switches on the power rails
> to whole GSM side so there is no question.  

no, it actually causes problems

> This was done on a previous prototype I saw in .tw, but the low Rdson switch
> they used had built-in currently limit (1.6A IIRC?) that got triggered at low
> temperature and highest TX setting destroying TX action.  So they removed it
> from PA side... maybe we can bring this back in the future.

to the best of my knowledge it is industry standard practise to have the GSM PA
cunnected directly to the battery. the RdsON of the mosfet switches (even of
those optimized for very low RdsON) is not constant but current-dependent.
This causes spectral impurity and can easily have (and actually has in
practise) effects on violating the regulatory requirements on the extremely
strict regulations on GSM/GPRS/3G signals.

So I really vote strongly against tryign to do this for no apparent reason.
The GSM PA is an analog circuit without any digital logic.  Thus, it can never
lock up internally.  It has an input pin that switches it off/on depending on
the logic state, i.e. a built-in power switch.  This is what should be used
(and is used in GTA02) to ensure acutal power-off.

- Harald Welte <laforge at openmoko.org>          	        http://openmoko.org/
Software for the world's first truly open Free Software mobile phone

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