[gta02-core] Modem schematic / RF help needed

Rask Ingemann Lambertsen ccc94453 at vip.cybercity.dk
Mon Sep 14 03:02:39 CEST 2009

On Fri, Sep 11, 2009 at 04:10:41AM -0300, Werner Almesberger wrote:
> Rask Ingemann Lambertsen wrote:
> >    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.

   Assuming a battery internal resistance of 295 mOhm on top of the 55 mOhm
in the power switch, we'd have a 216 Hz square wave with an amplitude of 2 A
* 0.35 Ohm = 0.7 V and a DC bias of 3.85 V. That will look a lot better when
smoothed by the 100 uF capacitor. The Telit documentation also suggests that
the main worry is power supply ripple affecting the rest of the system rather
than affecting the GE865 itself. Power switch or not, the rest of the system
will, in the worst case, see 2 A * 0.295 Ohm = 0.59 V ripple on the battery
voltage without the large capacitor.

   There's an error in the Hardware User Guide when it talks about using a
step-down converter from 12 V. The schematics show a 1000 uF capacitor while
the text says 100 uF. It looks like a copy-paste bug, so are both the other
places it says 100 uF correct? Just wondering.

Rask Ingemann Lambertsen
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