Re: [Openbts-discuss] Problem with OpenBTS 5.0.0 and B210

[Tom T. <to...@ts...>]

On Mon, Aug 4, 2014 at 3:57 PM, Robert Light <rob...@gm...> wrote:
> B200/B210 is a piece of !$&§"%& when used with OpenBTS. (sorry for !$&§"%&)
> I had about 20 of them and none of them worked out of the box with OpenBTS. USRP1 was more reliable.
> The first thing- 10MHz external reference is a MUST and not "optional" or "recommended" like some websites say.

The clocking stability has always been a complicated issue going back
to the early days of GSM. Full spec for 3GPP 05.10 is 50 ppb, which is
in the realm of GPSDO and OCXO devices. Deployed BTS installations
should and generally do meet this requirement, though, many users find
more relaxed requirements to be sufficient for lab or experimental
purposes. The on-board clocks of USRP devices target the latter use
case with the option of reaching specification-level stability through
the external reference. That said, there is an inherent YMMV aspect
when operating outside of spec.

Furthermore, part of the confusion may be that B2xx seems to be
testing with higher variance on measured clock stability - though
within hardware spec - compared to the previous B100 designs. USRP1
can't be directly compared, since it requires an external 52 MHz for
OpenBTS use to start with.

Balint does provide a suitable approach for B2xx without external
referencing, which is something to consider.

> B2x0 has poorly designed reference clock circuitry and many other mistakes in the RF shared with already ancient USRP1 design.
> The clock circuitry can be reworked, but you will loose warranty. If you do not want that to happen, use external ref clk.

Since you haven't mentioned other clocking issues (e.g. phase error),
I assume the issues you are seeing are mainly stability related with
the on-board clock. Or are you referring specifically to something
else?

More generally, I assume you're already familiar with GSM clocking,
but other readers may look to the OpenBTS clocking page for more
information on the topic.

http://openbts.org/w/index.php/Clocks

  -TT