An amplitude flatness and phase linearity calibration method for an RF source across a wide frequency bandwidth uses a simple square law diode detector and at least a pair of equal amplitude frequency tones. A baseband generator for the RF source generates the tones, which are applied in series to a correction filter and an up-converter to produce an output RF signal. The tones are stepped across a specified frequency bandwidth, and at each average frequency for the tones a magnitude and group delay is measured as well as a phase for the beat frequency between the tones. The resulting measurements are used to calibrate filter coefficients for the correction filter to assure amplitude flatness and phase linearity across the specified frequency bandwidth.