<html><head><meta http-equiv="Content-Type" content="text/html charset=big5"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div class=""><span style="color: rgb(255, 38, 0);" class="">Problem:</span></div><div class=""><br class=""></div><div class="">In a simple test to correlate with the 2 GHz simulated data (4-sec duration in this example) of the flux in 2 Jy for CARMA (SEFD = 5), SMA (SEFD =10) and SMTO (SEFD =20), a strange trend can be found from the obtained result as shown in ¡§HresFFT_2048MHz.pdf¡¨.</div><div class=""><br class=""></div><div class="">In the correlation obtained for CARMA-SMA and SMA-SMTO, an obvious coherence lost following the increasing of the frequency can be seen on the figure.</div><div class="">But this trend does not appear on the correlation for CARMA-SMTO.</div><div class="">Such a problem may be related to the simulator (e.g., the computation with the earth rotation since this problem occurs on the correlation to the long baseline).</div><div class="">What¡¦s the origin and how to fix it?</div><div class=""><br 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="#0433ff" class="">Adam¡¦s response:</font></div><div class=""><br class=""></div><div class="">If you have a very large FFT and a high fringe rate, then the assumption that the fractional sample error remains constant for the duration of the FFT becomes incorrect. This means that the frequency-dependent correction that is applied is also not exactly right, with the error being zero at the reference band edge and maximal at the other band edge.</div><div class=""><br class=""></div><div class="">One can figure it out: work out the duration of an FFT in seconds, multiply that by the fringe rate in ns/s, and divide the result by the sampling time in ns. If that number is large then you're starting to get substantial decorrelation at the far end of the band.</div><div class=""><br class=""></div><div class=""><br class=""></div><div class="">In the high-resolution FFT plot for correlation (e.g., HresFFT_2048MHz.pdf)</div></body></html>