UmTRX aka OpenBTS HW update

Alexander Chemeris alexander.chemeris at
Wed Oct 5 14:42:35 UTC 2011

On Wed, Oct 5, 2011 at 17:50, Robin Coxe <coxe at> wrote:
> Hi Alexander.  I took a look at the schematic last night.  Protel is hard
> for me to read, probably because I'm not used to it.  Overall, the board
> looks very nice.  I have a couple of questions/comments:
> 1) Is there a particular reason why there's an external SRAM on the board?
> I notice that there is one on the Ettus N210 board as well, but do guys have
> a particular purpose in mind for it?   The Spartan-6 FPGA has ample on-board
> Block RAM resources. If it's not strictly necessary, it would remove an ~$10
> part from the BOM.

It's used as a FIFO for TX packets with timestamps. I.e. if your
packet has timestamp in future it will be stored in this RAM for a
while. It's 9MBit RAM, so Block RAM can't compete with it here.
Whether this is strictly needed or is still open - we didn't do deep
investigation. We may remove it later or just not populate if decide
it's not needed.

> 2) Maybe it was just because it was really late, but I couldn't figure out
> what was happening to the 1 PPS GPS timing pulse.   How are you planning to
> use GPS information to discipline the 26 MHz oscillator?

It goes to FPGA and then we have to implement PID controller there
which will pull TCXO by adjusting DAC output (DAC121S101CIMK). We
haven't done any work on this yet, because PID regulator coefficients
must be tuned to specific DAC/TCXO and for testing purposes we can
tune manually. I saw PID regulator at OpenCores, so I don't think its
implementation is a big deal - tuning will take time OTOH.

> 3) One general recommendation:  add lots of test points, particularly around
> the DC power sources, clock signals, and digital signals of interest going
> to and from the FPGA.  Probing BGAs is a pain without them.  Also,
> strategically placed ground test posts that you can use to clip on a scope
> probe ground connection will make debugging much easier.

Yeah, I'm going to talk with Andrey about debugability tomorrow.

> 4) 6.5 V is kind of a weird voltage.  I'd be inclined to go with a 12V
> connection to an external power source, which would enable the system to be
> powered off a car battery, but this point is a matter of opinion more than
> anything else.

This is a good question. I think we decided that it doesn't matter
much will the power dissipate at our board or at external converter
when converting 12V to 6V. But we're open to discussion about this, as
I believe it's an important point.

Alexander Chemeris.

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