Is the underclocking affecting the digital domain only? Or would there be some analog frequency response curves that would start falling off too? If it's a digital-only underclock I don't see why there would be any degradation (aside from the obvious speed decrease). Is this easily testable somehow, with a single clock variable?

Yes, the subcarriers would get really narrow, but the sampling time would increase proportionately, so the FFT resolution would stay the same. I wonder if we'd be exceeding the hold time of the S&H circuit(s)... I don't really know anything about these chips, just making wild ass guesses based on generic modem architecture. :)


On 08/27/2014 03:53 PM, Adrian Chadd wrote:

So, I'm not sure if we can underclock it _that_ far. The 5/10MHz
channels are implemented by underclocking various parts of the chip,
which results in everything being some fraction of 20MHz (or 2x
clocking it, resulting in 40MHz.) Bringing it all the way down to
200KHz is a pretty tall ask.

I know people have gone down to 2.5MHz on these chips, but I don't
know of anything lower than that. The OFDM subcarriers start being way
too narrow and I doubt you'd end up with anything sensible.

It's likely that you'd want to create a custom DSP based solution to
do 200KHz side stuff at that symbol rate.


On 27 August 2014 15:34, Bart Kus <> wrote:

I'm wondering if you can tell me if it's possible to modify the FBSD TDMA
code to make this card:

legal to use in its intended spectrum.  By default the card violates two
Part 97 rules:

1) Emissions are limited to 200kHz bandwidth
2) Symbol rate is limited to 56kSym/s

Is it possible to slow down the subcarrier symbol rate in that Atheros chip?
Is it also possible to then space the subcarriers tighter together to
respect the 200kHz emission bandwidth limit? They'd have to come closer
together anyway to uphold OFDM subcarrier orthogonality.

Thanks for any clues,


PS: I'm asking in the interest of the project.  We're trying to
find a more mobile solution, which penetrates through forests and some
buildings.  440MHz seems to fit the bill, but hardware is really hard to
come by.

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