Filter network for phase-locked loop circuit

摘要

A filter network is presented for a phase-locked loop (PLL) circuit having a voltage controlled oscillator (VCO) with a control input and a bias input. The filter network includes a conventional filter circuit that provides a "pole" and a "zero" to the transfer function of the PLL circuit. The conventional filter circuit is coupled between the control input of the VCO and ground. An additional filter circuit is also coupled to the control input of the VCO, and to a bias input of the VCO. This additional filter circuit provides at least one additional "zero" to the PLL transfer function to extend the frequency range of the PLL circuit without impairing circuit stability. A transconductance amplifier is preferably employed within the additional filter circuit to facilitate tailoring of the open loop gain of the filter/oscillator circuitry.