Abstract: A direct measurement technique is disclosed for determining baseband frequency response of an FM transmitter without using response-altering test devices otherwise injecting frequency dependent characteristics, such as pre-emphasis networks and demodulators. The technique enables absolute, universal standardization. Variations in a sideband amplitude in the output spectrum from the FM transmitter unit under test are monitored while changing the frequency and controlling the amplitude of the modulating signal so as to maintain a constant modulation index B. A signal generator supplies the modulating signal directly to the FM transmitter without frequency dependent pre-emphasis. A spectrum analyzer directly examines the output spectrum of the FM transmitter without demodulation.