8 Nov
2023
8 Nov
'23
4:25 p.m.
Hi Mychaela,
as usual: Sorry for the late response.
On Sun, Oct 22, 2023 at 03:40:49PM -0800, Mychaela Falconia wrote:
But what would be the right way to support 2 A current
spikes during
GSM Tx bursts? I reason that a large capacitor will need to be placed
on the output of the 3.5V step-down regulator, one that will store
enough energy to feed the PA during that hungry burst - any thoughts
as to which type and size of capacitor would be most appropriate here?
In the past several people have powered calypso phones that way, e.g. via
https://wiki.cuvoodoo.info/doku.php?id=osmotoserial
I'm not sure if any of those people ever looked at the transmitted spectrum
or envelope and verified it passes conformance specs, though.
I would go about this experimentally: Take a few large (1000uF or
larger) electrolytic capacitors from a box, configure the MS to transmit
at 900 MHz at full RF power and look at the DC input voltage at the PA
during the burst transmission, comparing it from battery operation with
USB operation. Or, even better, if you have a Racal 6103 or equivalent:
Look if the power envelope of the transmitted bursts is working out.
You could also start with theory: calculate how many coulombs you'd
need, etc. - but I'd find that tiresome and given the ESR/ESL and other
real-world effects one would have to do the practical test anyway.
One thing you will need for sure is some kind of USB-specific power
circuit that limits the inrush current. The USB spec has a clear limit
of how much capacitance a device may present to the host. It wasn't
much, IIRC somewhere in the 10..100uF range. So you need to limit the
inrush current.
Furthermore, if you really want to be correct and ensure spec compliant
operation, you actually must not draw more than 100mA until a descriptor
with more current requirement has been selected by the USB host.
And the most important question, for people with more
EE experience
than me, and/or people who have already considered this problem: is
this capacitor solution expected to work, or is the problem intractable
in the sense that a USB-powered GSM MS, with the USB host limiting to
500 mA, will never be able to produce proper GSM Tx bursts at max power
without tripping overcurrent shutdown on the USB host port?
I would expect it to work, if the capacitor is reasonably large and fast.
--
- Harald Welte <laforge(a)osmocom.org> https://laforge.gnumonks.org/
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