Electronically tunable low-pass filter in PLL frequency synthesizer

This paper presents a low-pass filter circuit configuration with changeable time constants for a phase locked loop (PLL) frequency synthesizer in multiprogrammable divider (PD) method. The low-pass filter employs operational transconductance amplifiers (OTAs) and grounded capacitor as the basic building units. The time constants can be electronically changed through adjusting the bias currents of OTAs. The PLL frequency synthesizer with the proposed filter has higher speed lock-up time and quality than the conventional one.