The XFEL/SPring-8 project is in progress. SASE in X-ray region requires a high peak current of 3 kA, which will be achieved with an extremely stable rf accelerating field. In most severe case, the acceptable error in an rf phase is extremely small, such as 0.01 degree in root mean square. To make a stable rf field, we developed a high-speed DAC/ADC and an IQ modulator/demodulator to control/detect the low level rf signals. Modules with high setting/detecting accuracy are one precondition to achieve good stability. But the developed modules could have offset and gain errors because of the requirement of the high-speed operation. So we made the calibration method using a network analyzer to correct the errors. By using this method we could reduce the phase error of the C-band IQ modulator from 0.5 degree p-p to 0.1 degree p-p, for example. In this paper the detail of the calibration method and its performance will be shown.