<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><head><meta http-equiv="Content-Type" content="text/html charset=big5"></head><div class="">:</div><div class="">The "FFTSpecRes = 0.00390625¡¨ (e.g., the duration for FFT is 2.56x10^{-4} sec) was determined in the .v2d file, so the pointer to examine the fractional sample error for 3 baselines are:</div><div class="">CARMA-SMA: 0.61</div><div class="">CARMA-SMTO: 0.10</div><div class="">SMA-SMTO: 0.71</div><div class=""><br class=""></div><div class="">With a delta fractional sample delay of 0.1 samples, at the high end of the band there will be sin(0.1)/0.1 ~ 99.8% correlation still. But with 0.71, you have only 91.8% correlation. That is not so far from what the plots seem to show. So I suspect if you reduce the spectral resolution by at least a factor of 8 you'll see matters improve greatly.</div><div class=""><br class=""></div><div class="">For normal continuum observations I would expect that a channel width of ~0.5 MHz would suffice. That leads to an FFT length of 2 microseconds. With a geometric rate of up to 1 microsec/s (maximum due to Earth rotation), then you have a delta geometric delay of 2e-12 seconds from one FFT to the next. Since the sampling time for a 2 GHz band is 2.5e-10s, you're then looking at ~0.01 sample shifts per FFT, which is back within the range of negligible impact.</div><div class=""><br class=""></div><div class="">It is good to remember that the wider the input band to DiFX, the coarser the best attainable spectral resolution without running into this problem. With a 2 GHz wide input band, you can't go much better than 62.5 kHz channels (at least on 8,000 km baselines) without accepting some decorrelation. Whereas with a more typical band, like 128 MHz, you can go 16x narrower. That is one of several moderately annoying limitations with very wide band inputs in DiFX (the others being mostly logistical, relating to chunk sizes, timekeeping etc).</div><div class=""> </div><div class="">¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X¡X</div><div class=""><font color="#942193" class="">Examination on the problem:</font></div><div class=""><br class=""></div><div class="">Here I put the revised correlated results here (LresFFT_2048M_Hz.pdf). </div></body></html>