Correspondence: Testing for spurious response of receivers applied to measurement of transmitter harmonics
Alfred Eckersley · IRE Transactions on Radio Frequency Interference · 1961
A common problem in measurements of harmonic content of transmitter outputs is the one of ensuring that the measurement is not erroneous, due to the presence of sufficient fundamental-frequency voltage input to generate a spurious response in the measuring instrument. Particularly there is the case of excitation of an instrument, tuned to fo, by a strong enough component of fo/2 to give square-law action.