The LC VCO includes an LC oscillator module with first and second tank nodes and a control module with positive and negative input
voltage terminals. The control module includes four
voltage dependent capacitive elements which are configured to be biased for operation as
voltage dependent variable capacitances. The voltage dependent capacitive elements are interconnected such that the effect of a common mode input voltage is to increase the
capacitance of two of the voltage dependent capacitive elements, while simultaneously decreasing the
capacitance of two of the other voltage dependent capacitive elements by a substantially similar amount, such that a differential voltage applied across the positive and negative input voltage terminals is operable to change the
capacitance of the voltage dependent capacitive elements, and thereby the frequency of the LC oscillator module, while effects on the output frequency of the oscillator caused by a common mode voltage tend to cancel. Accordingly, a fully differential
signal path in phase locked loops employing LC oscillators, with improved
noise rejection, substantial suppression of
common mode noise, and a minimal increase in complexity over a single-ended control design can be achieved. Additional supply voltages need not be employed.