Dual-pfd feedback delay generation circuit

Abstract

A dual-PFD circuit with delay feedback generated by a dual-modulus prescaler based on mode control from a feedback delay generation circuit. The PFD circuit can be used with a PLL feedback divider to divide a VCO clock signal VCO_clk and generate FB and FB_DLY signals. The PLL feedback divider includes a dual modulus prescaler to selectively divide the VCO_clk by either M or M+1 (such as 4/5) based on a divide mode control input to generate a prescaled divide signal, and a programmed counter/divider (N counter/1/N divider) to selectively divide the prescaled divide signal to generate the FB signal, and a delay generation circuit to selectively delay the FB signal by a pre-defined delay to generate the FB_DLY signal. The prescaler is responsive to the pre-defined delay from the delay generation circuit to change divide modes. The dual PFD circuit response to the FB and FB_DLY signals in relation to a reference signal to generate a phase comparison signal. the dual-PFD circuit can be used with a charge-pump coupled to the dual PFD circuit, and responsive the phase comparison signal to generate a frequency tuning voltage, for input to a VCO for generating the VCO clock signal. The dual PFD circuit, charge pump and VCO can be used in a PLL frequency synthesizer.