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 <[email protected]> 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 <[email protected]> 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 >> >> >
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