[gta02-core] Modem schematic / RF help needed

Werner Almesberger werner at openmoko.org
Fri Sep 11 09:10:41 CEST 2009

Rask Ingemann Lambertsen wrote:
>    Why?

Very good question :-) In theory, there should be no problem at all.
In practice, our hardware team found that the Calypso didn't work if
also the RF amplifier was connected through the switch. As far as I
know, the reason for this was never clearly determined.

So this may have been a quirk of the Calypso, poor characteristics
of the switch, or something else, which may or may not happen with
the Telit module as well.

>    The Telit GE865 Hardware User Guide has examples of how to power the
> modem from an LDO as well as from a step down converter. Why would it be
> a problem to have a power switch between the battery and the RF amplifier
> when you are supposed to get away with an LDO?

I was wondering about that too. Telit's documentation could be read
as saying that you need at least that 100 uF capacitor, plus whatever
it takes to handle the peaks, but that would be weird at best.

I'll feel much better about having the switch if it cuts all supplies
and not leave the modem in a probably completely undesigned-for

We could try the following approach:

- bring back the switch (probably best to switch to the NTGD1100L
  while we're at it) and a pull-up

- route both VBATT and VBATT_PA through the switch

- add a 0R resistor parallel to the switch. Put it somewhere where
  it can be reworked. Since we feel confident, we make it NC for SMT.

- we then need to free one of the PMU's GPIOs. MODEM_nON might be
  the best choice, since the switch would override it anyway.

- put a 0R into MODEM_nON from the CPU so that, in case of trouble,
  we can fall back to the current configuration

Does that sound good ?

By the way, it seems that the ON/OFF insanity is systemic. All three
load switches you found use that stupid name we already complained
about with the Si1040X :-(

- Werner

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