BLK_ChargePumpLoopFilter : A loop filter for charge-pump PLL

A loop filter in a phase-locked loop (PLL) adjusts the control input to the oscillator based on the phase error information provided by the phase-frequency detector (PFD). In particular, a charge-pump loop filter performs this operation by employing a charge pump which charges or discharges a capacitor for a duration proportional to the phase error. Since this operation is essentially an integration, or a pole located at DC, it can steer the control input to the value that makes both the frequency and phase errors of the PLL zero. It is the primary reason why this type of loop filter was so widely used before the digital-type loop filter was introduced.

This charge-pump loop filter model consists of a pair of switched current sources and a passive RC-network, which is in turn composed of a series combination of a resistor Rs and a capacitor Cs and a shunt capacitor Cp. This implementation yields an effective linear transfer function that has two poles and one zero.

Input/Output Terminals

Name	I/O	Type	Description
out	input	xreal	output
dn	input	xbit	dn input
up	input	xbit	up input

Parameters

Name	Type	Default	Description
Iup	real	20e-6	charge-pump up current
Idn	real	20e-6	charge-pump down current
Vinit	real	0.5	initial control voltage
Rs	real	20e3	loop-filter series resistance
Cs	real	0.5e-12	loop-filter series capacitance
Cp	real	50e-15	loop-filter shunt capacitance