Default rules for ALMA LO modulation
(i.e. Walsh phase switching and/or LO offsetting)

  • This wikipage was written as part of the Observing Modes Workshop (January 2011)
  • Adapted from Peter Napier's memo: 2007-04-04 SYSE- on ALMA EDM (local PDF)
    • It notes that 180 deg Walsh switching does not work well to eliminate spurious signals in high spectral resolution modes
    • See also: his 2006 memo
  • For a discussion of LO offsetting and Walsh switching, see ALMA Memo 287 by D'Addario. The interval is 30.517578125 kHz, and the integer multiple corresponds to the antenna number as follows (as shown in the TMCDB, and at this AIV wikipage):
    • DV01-DV25 1-25
    • DA41-DA65 26-50
    • CM01-CM12 51-62
    • PM01-PM04 63-66
  • Table notes:
    • "TWO_LOS" means to modulate LO1 and LO2
    • "THREE_LOS" means to also modulate the TFB digital LOs (and/or apply a spectrum shift in the ACA)
    • Note that the TFB digital LOs are more accurately termed LO4, not LO3. The digitizer clock frequency of 4.0 GHz is often referred to as LO3.

FDM (TFBs enabled)
FDM (TFBs enabled)
Receiver Band all spectral windows placed in one sideband some spectral windows placed in lower sideband, and some spectral windows placed in upper sideband (in any of the 4 basebands) only one sideband desired per spectral window (i.e. always reject the image sideband) both sidebands desired in one or more spectral windows (in any of the 4 basebands)
3-8 (2SB)
180 deg Walsh switching and TWO_LOS offsetting
THREE_LOS offsetting
9-10 (DSB)
180 deg Walsh switching and TWO_LOS offsetting
180 and 90 deg Walsh switching and no LO offset #
THREE_LOS offsetting
90 deg Walsh switching and no LO offsetting *

* provides no rejection of spurious response (birdies), but includes the maximum instantaneous bandwidth mode (16 GHz)
# if 90 deg Walsh switching is not yet implemented, use 180 deg Walsh and TWO_LOS offsetting

Observing tool screenshot showing how the selection will be made :

OT phase2.png

-- ToddHunter - 2011-01-28
Topic attachments
I Attachment Action Size Date Who Comment
845217.pdfpdf 845217.pdf manage 706 K 2013-01-23 - 00:01 ToddHunter SPIE conference proceeding on the two ALMA correlators
Detailed_LO_Offset_Commands.docdoc Detailed_LO_Offset_Commands.doc manage 29 K 2011-02-01 - 10:57 ToddHunter written by Peter Napier for Rodrigo Amestica
OT_phase2.pngpng OT_phase2.png manage 49 K 2011-01-28 - 12:01 ToddHunter  
SYSE- SYSE- manage 257 K 2018-01-15 - 15:31 ToddHunter SYSE- by Napier, P.
Topic revision: r19 - 2021-05-10, ToddHunter
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