2008/11/9 Bruce Griffiths <[EMAIL PROTECTED]>: > When one is measuring the beat frequency between an offset standard and > the DUT, the sound card timebase doesn't have to be more accurate than > the the DUT as it is only measuring the error in the small offset > between the DUT and the offset standard. > However the local standard against which the DUT is being compared does.
So as this is just measuring a beat, there is no requirement for absolute accuracy in the measuring system, it just needs the reference frequency to be as accurate as the desired measure of absolute accuracy. Yes, I can see that. > For example with a 10MHz DUT and a 10MHz local standard offset by 100Hz > from 10MHz, the sound card only has to measure the 100Hz offset > frequency ( between the DUT and the offset standard) to an accuracy of > 1E-7 in order to determine the frequency of the DUT to an accuracy of > 1E-12. So 100Hz offset from 10MHz is 1E-5 and if I can measure the 100Hz to an accuracy of 1E-7 that would give an overall measurement of 1E-12. So that would mean that the sound card device would somehow have to sample with at least 1E-7 accuracy. That would mean taking enough samples of sufficient accuracy to determine this 1E-7 accuracy. A sound card has a sample frequency of approx 44KHz but to see an offset of 1E-7 would that not take a fairly long sample time? How would this affect the ability of such a system to determine ADEV for small tau? > When one uses a dual mixer system to compare 2 non offset 10MHz signals, > most of the error contributions from the offset source and ADC sampling > clock are common to both channels and tend to cancel on subtraction. I can see that, assuming both channels are sampled at the same time. Thanks Bruce. 73, Steve -- Steve Rooke - ZL3TUV & G8KVD Omnium finis imminet _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
