These formulas require that the signals and be simultaneously collected by a computer. In modern instruments it is possible to add and substract instantaneous signals and . The correlation between two signals must be obtained on special correlation meters. Since true-RMS voltmeters are commonly available, the cross-correlation term can also be determined as
provided the two terms on the right hand side are not very close to each other.
and 
be simultaneously collected by a computer. In modern instruments it is possible to add and substract instantaneous signals 
and 
. The correlation between two signals must be obtained on special correlation meters. Since true-RMS voltmeters are commonly available, the cross-correlation term can also be determined as 
