Two tone test method for determining frequency domain transfer

A method has been developed and used to non-intrusively interrogate electronic systems in order to determine the system's characteristic response to electromagnetic energy. The method involves subjecting the system under test to two continuous wave signal sources of different frequencies fl and fh and measuring the difference frequency DELTA f (fh-fl) emitted from the system under test. The difference frequency, created by mixing action of nonlinear electronic components within the system, is a relative indication of the system's response to fh, fl. By incrementing the two source signals fl and fh by the same frequency step size, DELTA f remains constant and is measured for each fh,fl value. The compilation of amplitude measures of DELTA f, normalized by the corresponding amplitude measures of fh, fl, provides a relative measure of the frequency domain transfer function of the system under test. The present method is performed with the test equipment in total isolation from the system under test and may be used to quickly and cost effectively identify those frequencies to which the system under test may be susceptible.