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Spirent KB Article
Doc ID: FAQ15381
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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"?
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.
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 ~90
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