[amsat-bb] Re: [Discuss-gnuradio] OLPC - next generation with SDR?
gnu at toad.com
Mon Dec 24 14:11:34 PST 2007
As preface, I'm not a radio engineer. I'm a software guy with
pretentions to understanding digital hardware. I have a few signal
processing books on a dusty shelf. You lose me as soon as you start
talking "Q signals".
The Odyssey board operates at 10MHz IF; so wouldn't it need an external
> I am in agreement with Frank that we can currently do it for a few tens
> of dollars ~$50 in small quantities and that include parts and boards.
> We can even put together a prototype which will allow HF shortwave
> reception from low bands through about 21 Mhz covering these bands:
> [15m thru 120m]
What kind of antenna would this require? Something external to the
laptop? Or something that could be built into the plastic case?
> The dsPIC33 has more than enough horsepower to provide good
> (synchronous) detected AM and even some modest AGC.
We won't need a processor; the laptop will come with a processor much
faster than 40 MIPS. (The current XO CPU is a Geode LX 433 MHz x86,
with MMX, 3DNow, and some SSE instructions.)
> We need a DDS and a QSD (we do not need the QSE for the receive only
> version) if we are going to tune the HF shortwave broadcast bands and
> get reasonable performance at low cost.
I think that single chips are available that do broadcast-band AM and
FM decoding for cheap; has nobody done this for the television and
shortwave bands? Or is the problem that nobody's done this digitally?
If we can provide something that gives real benefit for the target
kids, we shouldn't be dogmatic about analog versus digital.
Alternatively, if OLPC provided a million-unit order for a digital
tuner chip that would target all these bands, others could then buy the
cheap chip for a variety of projects.
> This would provide a clear example of how it could be done. It does not
> meet the price point, but it shows the capabilities and then we can
I'm glad you-all are pointing out low volume prototypes. I hope we'll
get someone interested who has designed high volume digital radio
electronics. High volume ~= million-unit. (Do any people like this
exist? Perhaps Matt's bluetooth design has shipped in that quantity;
WiFi does too.) There's already an entire high speed digital radio
transceiver in the existing XO: it's the Marvell "Libertas" WiFi
88W8388 controller chip and 88W8015 radio chip. It's reprogrammable,
though the ARM code that runs in it isn't open source yet (the high
level code can be open sourced, but it runs on a proprietary RTOS).
I think the best strategy for a $50 laptop's radio would be to have
either an internal antenna or a single connector; a small number of
cheap analog components; perhaps *one* analog/digital chip (multi
channel DAC/ADC "radio chip"); and stuff *everything* else into a
corner of the digital system-on-chip that implements the rest of the
laptop. It's hard to prototype such a thing, though perhaps using an
FPGA that come with a fast embedded MIPS or ARM CPU would be the
The current XO uses two custom chips (the DCON display controller, and
the CAFE camera/flash/SD controller), some very custom "mesh" firmware
for the ARM core inside the WiFi chip, and some very custom firmware
for the EC embedded controller battery charger chip. A $50 laptop
version would probably mash all these chips together with the CPU,
GPU, and its "southbridge" support chip, leaving only one
system-on-chip, plus flash, DRAM, a few external analog chips, and a
pile of analog electronics for power supply and such.
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