Determining both where to place the local oscillators (LOs) and through which IF filter path to send the signal of interest is not trivial. Below is both a graphical description of the signal conversion which occurs in taking the signal of interest down the 1.5 GHz and a description of the algorithm used by the telescope control gui to choose which IF/LO set-up to use for an observation.

The graphic below shows an example of the down conversion path used in the 1-2GHz IFLO path:

To choose which IF path to take as well as where to place the LO (high or low side), we've developed an algorithm for the telescope control gui to follow.

Definitions:

_{center,highest} = the highest band central frequency
_{center,lowest} = the lowest band central frequency
_center = (_{center,highest} + _{center,lowest}) / 2
= the correlated bandwidth
_synth = the synthesizer frequency for a subcorrelator

The Algotrithm:

• If (_{center,highest} + /2) - (_{center,lowest} -/2) < 500 MHz then use the 500MHz set-up, with all local oscillators (LOs) in the high side of the band.

• If (_{center,highest} + /2) - (_{center,lowest} -/2) 500 MHz then the signal must be split into different filters.

• For each of the central subcorrelator frequencies (_SBC with correlated bandwidths _SBC:
  • If _SBC < _center then
    If _center - (_SBC - _SBC/2) > 230 MHz set subcorrelator into filter 1.
    else, set subcorrelator into filter 2

  • If _SBC > _center then
    If (_SBC +_SBC) - _center > 230 MHz set subcorrelator into filter 3
    else, set subcorrelator into filter 2

• To choose the LO side:
  • For the lowest frequency SBC, always choose the low side LO.

  • For the highest frequency SBC, always choose the high side LO.

  • The middle bands have two options:
    • Low option:
      _synth = 1500 + (_SBC - _center) - 260 MHz

    • High option:
      _synth = 1500 + (_SBC - _center) + 260 MHz
    
    

  Check to see if the high option gives an LO frequency which lies within another subcorrelator band above the subcorrelator of interest. If it does not, then use the high option.

  If the high option does give an LO frequency which lies within another subcorrelator band above the subcorrelator of interest, then check if the low option gives an LO frequency which lies within another subcorrelator band below the subcorrelator of interest. If it does not, then use the low option.

  If neither choice is any good, pick the option which keeps the LO the farthest from the center band of any other subcorrelator.