In this paper, we present methods to improve output flatness of Ultra-wideband(f1~80f1) Frequency Generator (UFG) with variable output function. That include optimal structure and output ripple calibration algorithm to realize good flatness. The UFG was manufactured by our methods, then experiments were performed. The flatness of band 1~3 and band 4 are within ${pm}2dB$, ${pm}4dB$ respectively. These results are very excellent in view of ultra-wideband including millimeter wave band. From this results, we can confirm the validity of our methods.