Re: [Discuss-gnuradio] B100 clock with UHD

[Tom Tsou]

On Tue, Jan 21, 2014 at 5:45 AM, Robert Light <address@hidden> wrote:
> I thought OpenBTS would use Transceiver52MHz to communicate with B100 and to
> configure the AD9522 to output 52MHz sampling clock. However, with OpenBTS
> working with B100 and WBX I measure 64MHz sampling clock. And I am confused
> here. I didn't compile OpenBTS with resampling option. So where does the
> resampling happen? Or, am I wrong thinking that OpenBTS uses
> Transceiver52MHz ? Where is the UHD in this chain ?

Please post subsequent questions to the OpenBTS mailing list.

Note there is no longer any user configurable resampling option.
Device specific rate configuration (clocking and sampling) is handled
by the code internally.

http://wush.net/trac/rangepublic/wiki/BuildInstallRun#AllOtherDevicesexceptforUSRP1

OpenBTS and B100 runs at 64 MHz. This was changed from 52 MHz because
of significantly better performance and stability.

https://wush.net/trac/rangepublic/wiki/USRP

The host transceiver operates internally at 1.0833 Msps transmit and
270.833 ksps receive. These sample rates are converted and matched to
device specific values on the host side in between the GSM processing
and device I/O. For B100 the conversion rate is 65/96, which is
selected to keep both halfband filters in place on the FPGA minimizing
pass band distortion.

The name 'Transceiver52M' is historical, which is admittedly
confusing. The name originates from the initial public OpenBTS
release, which only supported the USRP1. Back then, there were
completely separate implementations for 64 MHz and 52 MHz for this
single device. Eventually, all USRP code was merged into the 52 MHz
code that is the active version remaining today.

  -TT