A study is presented on the variation of the circuit impedance at the input port where the active IMPATT device is placed in a resonant cap X-band oscillator. The real and imaginary parts of the impedance seen by the diode ie at the device plane have been studied with respect to the effective cap radius, cap height, contact post radius and cap thickness. This study is expected to provide useful information for impedance matching and impedance transformation between the circuit and the active device considering device circuit interaction. It further shows that the real part of the impedance is slightly dependent on effective cap radius and strongly dependent on cap height while the imaginary part of the impedance is strongly dependent both on effective cap radius and cap height of the resonant cap cavity. The effects of thin and thick post and cap upon the circuit impedance have also been studied.