Loop filter with active discrete-level loop filter capacitor in a voltage controlled oscillator

A loop filter with an active discrete-level loop filter capacitor can be used in a VCO (such as for CDR). A loop filter capacitor function is simulated by sensing input loop filter current (such as with a current mirror and source follower in the input leg), and forcing back a loop filter (VCO) control voltage. Loop filter voltage control is provided using a VDAC with a discrete-level VDAC feedback voltage, incremented/decremented based on the sensed loop filter current. In one embodiment, the VDAC voltage is provided as the non-inverting input to an amplifier, with the inverting input providing the control voltage, forced to the VDAC feedback voltage. The VDAC feedback voltage can be provided by increment/decrement comparators based on a voltage deviation on a C2 capacitor (from a reference voltage) that receives the sensed loop filter current (effectively multiplying the C2 capacitance to provide a simulated loop filter capacitance).