Re: [Openbts-discuss] OpenBTS Clock

[Thomas T. <tt...@vt...>]

On Mon, Mar 26, 2012 at 2:25 PM, Samuel Liverpool <sam...@ho...> wrote:
> UHD Warning:
> The hardware does not support the requested RX sample rate:
> Target sample rate: 0.270833 MSps
> Actual sample rate: 0.271739 MSps
> kal: Scanning for GSM-900 base stations.
> chan: 40 (943.0MHz + 3.422kHz) power: 139281.38
> chan: 59 (946.8MHz + 3.410kHz) power: 16115.09
> chan: 122 (959.4MHz + 3.528kHz) power: 12133.84

These offsets indicate that something is not working with the GPSDO
reference. These numbers are roughly comparable in performance to the
better side of an unreferenced USRP.

I don't recall from the previous emails, but do you have the recent
Kal commit that updated to the UHD 003.004.000 interface?

> Is it necessary to have the GPS antenna connected and locked to a GPS signal
> for the reference clock to work with OpenBTS?
> Does the ClockTamer clock work well with the N210 and if so which one is
> better?

I don't know enough about the GPSDO unit to say how well it should
perform in the absence of a GPS signal. But, the GPSDO unit includes a
stable OXCO, which I imagine should provide a relatively good
reference on it's own given suitable warmup. Perhaps you could ask
that question on the usrp-users list?

The ClockTamer uses a TCXO, which will be less accurate than a GPSDO
locked unit, but it can be software tuned with single Hz resolution.
Generally, the ClockTamer is used on the USRP1 at 52 MHz, but could
also work at 10 MHz. I will leave it to Alexander for further comment.

  Thomas