Re: [Discuss-gnuradio] external clock issue with B100 + WBX

[Guy Holtzman]

OK, I discovered what was the problem.

it turns out I did not generate a carrier, but a just constant 0 (instead of constant 0.7)

when generating a carrier, the Rx and Tx are perfectly synced

sorry for all the trouble, and thanks for the help

Guy

On Tue, Jun 4, 2013 at 8:57 PM, Guy Holtzman <address@hidden> wrote:

on the Rx side, I use uhd.tune_request(915000000,10e6)

then I down-sample and display a FFT

the amount of down-sampling is enough that I can see  how much frequency offset I get.

I am testing it with a carrier generated from a second B100, using uhd.tune_request(915000000)

even if the reference had an accuracy problem. deriving the clock would produce the same error for all devices connected to it, given the reference signal is within spec

under the scope, the reference signal seems to be a perfect sine with an rms within spec.

currently, for now, I will have to understand more what the tuning does, and why.
since the freq offset I see is constant, I can live with it, for now.

Regards, Guy

On Tue, Jun 4, 2013 at 7:55 PM, Marcus D. Leech <address@hidden> wrote:

Hi,
I am using the same clock reference to both b100 devices.
the clock reference is XL microwave model 500, it is rubidium based, and it has multiple outputs.
since I am connecting it to both B100s (the actual same clock reference) I was expecting that the Rx and Tx frequencies will be exactly the same. but I still got 570Hz difference.
when I did not invoke the tune request command, the Rx was distorted (the Tx was fine, I tested it with a spectrum analyzer)

Regards, Guy

Define "exactly the same".   Even a Rb reference oscillator has some imprecision in it, although not usually as high as 600ppb, unless the vapour
  tube is nearing the end of its life.

How did you measure the frequency offset?  The measuring device has to have a precision much better than the thing you're trying to measure,
  or your results will just be nonsense.

If you don't use offset tuning (using tune_request with an LO offset), then the DC anomaly will be right in the center of your passband.  If you're using
  a narrowband carrier for testing, that DC offset will interfere with your measurements.

--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org