revisiting the 434 MHz band

Ron K. Jeffries rjeffries at gmail.com
Sat Jul 24 12:17:43 EDT 2010


The discussion of alternatives to add some
form of wireless to Nanonote is interesting,
but is missing fundamental information.

What user problem do we wish to solve?
---
Ron K. Jeffries




On Fri, Jul 23, 2010 at 23:13, Werner Almesberger <werner at openmoko.org>wrote:

> Wolfgang Spraul wrote:
> > No no, not hookup to an FPGA.
>
> I think a CPLD would be sufficient to beat the bitstream into
> shape. This wouldn't have to be a large chip. What it would do is
> just find out where the packet starts and ends, find byte
> boundaries, and do a bit of buffering so that the CPU has
> something it can work with.
>
> You may be able to do those bit-level operations at low speeds
> (802.11b) also entirely in software, but you'd really spend a lot
> of resources for a task hardware is much much better at.
>
> Maybe the DSP could do it, though. Also, maybe there are some
> chips that integrate some of these functions.
>
> > I definitely want to hook this up to a regular ASIC XBurst CPU.
>
> I think this would be quite wasteful, even if it works. Kinda
> like the ZX81, where the CPU spent most of its time refreshing
> the screen, instead of running your program.
>
> > I was encouraged to look into SiGe chips by Henry Hallam
> > (http://www.pericynthion.org), who says he works on a solution with a
> > Blackfin DSP and SiGe SE4120L to implement the GPS stack,
>
> Okay, but the SE4102L outputs a digital Q/I stream. Even with
> the MAX2830, you still need the ADC. Plus, unlike GPS, you also
> have a transmit stream to take care of.
>
> Regarding the ADC/DAC, in case you were thinking of using the
> things you find inside the CPU: they only do a few kilosamples/s,
> which is too slow as well. I don't know what sample rate 802.11b
> or even 802.11g need, but I would be surprised if it was less
> than a few hundred kSa/s, even if you do tricks like scaling
> 802.11b down to 1 Mbps. (Would be nice for Lies and Dece..., err,
> marketing, though. Much like all those "USB 2.0" devices that
> only do full-speed :-)
>
> Last but not least, unlike WLAN, the GPS data stream moves at an
> unhurried 50 bps.
>
> > The one thing that worries me about IEEE 802.15.4 is the range. Only a
> > few meters? That's what I still like about 433 Mhz - it seems to work
> over
> > hundreds of meters, possibly even more...
>
> Yes, range may be a problem. You can use more power, see e.g.,
> [1], but then you're adding extra chips and you go deep into
> RF design land. They also say they've achieved 378 m without
> such aids [2].
>
> [1] http://www.freescale.com/files/rf_if/doc/app_note/AN2975.pdf
> [2] http://www.mwrf.com/Articles/Index.cfm?Ad=1&ArticleID=9379
>
> 433/434 MHz can go a long distance, but at least in the lands
> under FCC's harsh rule only with crippling duty cycle
> restrictions.
>
> - Werner
>
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