I want to simulate in Simulink a frequency synthesizer PLL in the region of 77GHz.
Due to the confinements of the technology used in the VCO (the vco is simulated in cadence), the voltage input of the VCO must be from 0 to 1.2 Volts.
My problem is specific: the PLL locks only when the charge pump is implemented as shown in the attachment, that is only when the dc sources connected in the switches of the charge pump are 1 volts and -1 volts (for inserting and extracting current respectively in the loop filter) .
If i have 0 volts instead of -1 volt, the system doesn't lock if the destination frequency is smaller than the current one.
Any suggestions on how to simulate the charge pump so as the input of the vco is from 0 to 1 volt and not from -1 to 1 volt ?

Thank you very much for your time