1. A method for use in design of a system, the system to include a circuit and a plurality of sources contributing to a variable system effect, wherein the plurality of sources is provided by time-dependent current flow in the circuit, and wherein the variable system effect is frequency-dependent electromagnetic radiation, the method comprising: determining a plurality of functional units corresponding to a plurality of circuit blocks to form the system, each functional unit including a separate subset of sources in the plurality of sources; obtaining a plurality of constant functional unit source information, wherein each constant functional unit source information is associated with a respective functional unit in the plurality of functional units; determining at least one variable quantity; associating each functional unit with one of the at least one variable quantity; for each functional unit in the plurality of functional units, obtaining variable functional unit source information by combining the constant functional unit source information with the variable quantity associated with the functional unit; and deriving the variable system effect based on combining the variable functional unit source information.