a variable capacitance circuit comprising a capacitor array , associated switching elements and transient impedance varying circuits . the capacitor array comprises a plurality of capacitor elements connected to a common node coupled to a crystal oscillator in a crystal oscillator portion and each capacitor element includes a connected switching element that controls activation of selected capacitor elements that are selectively placed in operation as load capacitance with the crystal oscillator to change and adjust its frequency . further , circuits are provided in a temperature compensation portion to selectively control the activation of the switching elements based upon decoded compensating values provided in memory , such as based upon sensed oscillator temperature conditions . the transient impedance varying circuits comprise multi - level voltage generating circuits for changing the continuity impedance of activated switching elements , among switching elements being switched , to another impedance level or one of a plurality of additional impedance levels . as a result , an intermediate impedance value can be created between the on and off states of the switching elements so that there is no rapid change in the total equivalent capacitance imposed by the capacitor array on the oscillator resulting in smooth capacitance switching which corresponds to smoother frequency adjustment of oscillator output .