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 |