Set or query the source or source A of algebraic operation/functional operation/the outer
layer operation of compound operation.
 For algebraic operations, this command is used to set source A.
 For functional operations, only this command is used to set the source.
 For compound operations, this command is used to set source A of the outer layer
operation when the outer layer operation is algeriac operation and the range of
<src> is {CHANnel1|CHANnel2|CHANnel3|CHANnel4}; this command is used to set
the source of the outer layer operation when the outer layer operation is functional
operation and <src> can only be FX.
Note: When the outer layer operation of compound operation is algebraic operation, at least
one of source A and source B of the outer layer operation should be set to FX.
 When "FX" is selected, you can send
the :MATH:OPTion:FX:SOURce1, :MATH:OPTion:FX:SOURce2,
and :MATH:OPTion:FX:OPERator commands to set the sources and operator of the
inner layer operation.
The query returns CHAN1, CHAN2, CHAN3, CHAN4, or FX.
:MATH:SOURce1 CHANnel3 /*Set source A of algebraic operation to CH3*/
:MATH:SOURce1? /*The query returns CHAN3*/