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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