SDR-IQ clock drift is quite visible on 524288-bin FFT in SpectrumLab (just zoom into WWV's carrier and leave it open). it also appears quite visible on NetSDR http://www.hfunderpants.com/?p=328 http://www.hfunderpants.com/?p=350 between Rubidium clocks or Thunderbolt GPS 10mhz sources, there are solutions..