User Tools

Site Tools


difx:difx2.0looff

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

Both sides previous revision Previous revision
Next revision
Previous revision
difx:difx2.0looff [2009/10/29 04:33]
adamdeller
difx:difx2.0looff [2015/10/21 10:08] (current)
Line 1: Line 1:
 +
 ====== Local Oscillator (LO) offsets in DiFX2.0 ====== ====== Local Oscillator (LO) offsets in DiFX2.0 ======
  
Line 7: Line 8:
 The LO offset is corrected by adding (or subtracting) a constant amount of phase from every channel **after** the FFT.  This is much less computationally intensive than doing it beforehand. ​ But it does mean that the maximum LO offset that can be handled without significant decorrelation is ~kHz for a typical continuum observation,​ as shown in the figure below. ​ This is because the intrinsic duration of the FFT is of order a few 10s of microseconds,​ so a constant approximation becomes bad for very high rates (large LO offsets). ​ But if the LO offset is really very large, it can be handled by a combination of LO offset and [[difx2.0specselect|spectral selection]]. The LO offset is corrected by adding (or subtracting) a constant amount of phase from every channel **after** the FFT.  This is much less computationally intensive than doing it beforehand. ​ But it does mean that the maximum LO offset that can be handled without significant decorrelation is ~kHz for a typical continuum observation,​ as shown in the figure below. ​ This is because the intrinsic duration of the FFT is of order a few 10s of microseconds,​ so a constant approximation becomes bad for very high rates (large LO offsets). ​ But if the LO offset is really very large, it can be handled by a combination of LO offset and [[difx2.0specselect|spectral selection]].
  
-{{lo_offset.png?​500}}+{{lo_offset.png?​800}}
difx/difx2.0looff.txt · Last modified: 2015/10/21 10:08 (external edit)