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Butler Matrix: In the manual what is the meaning of the statement "For clarity, the 0 is relative to that column only"?

  • Butler Matrix
    • An additional item of equipment used with GNSS positioning simulators
    • Most common application is CRPA antenna testing
  • This statement can be found in DGP00586AAA (design note) and an equivalent reference is also in DCS0021J (acceptance test procedure).
  • The full wording (taken from DGP00586AAA) is as follows:-
        "For clarity, the 0 is relative to that column only, there exists a phase difference between the ‘0’ of each column [as shown in the table below]."
  • The statement is intended to explain that you should not expect to see zero phase shift at the "0" output port when you change between input ports
  • You will instead see some phase difference between each input port, with the approximate phase difference provided in a table in DGP00586AAA
    • The above statement shows reference to the table in square brackets on because the table has not been included in this article.
  • For example
    • You are connected to the "0" input
    • There will be a small, non-zero phase difference at the "0" output (though within the phase accuracy defined by the product specification)
    • However, this should be treated as your zero phase, i.e. this is your master reference point.
    • You change to the "-45" input
    • There will be a different non-zero phase shift
    • The DIFFERENCE in phase shift between the two inputs should agree with the tabulated value (in this case ~90o)
  • In no way does this invalidate the purpose of the Butler Matrix
    • What really matters is the phase shift between each output port for any one chosen input port and NOT the phase shift between input ports.
  • So for the operating purposes of the unit you simply treat the "0" output as providing the 'no phase shift' signal for the chosen input port
  • All other output ports are the required phase shift relative to the "0" output
    • e.g. you choose the "-45" input and the relative phase shift between the "0" output and the "2" output = 2 x -45 = -90deg; which is what you really want to control.

Product : Positioning,Antenna