PLL Frequency Synthesizers

PLL frequency synthesizers are electronic devices that produce frequencies coherent to a reference frequency. They use phase-locked loops (PLLs), electronic circuits that consist of a phase/frequency detector (PFD), low pass filter, and voltage-controlled oscillator (VCO). PLL frequency synthesizers use the PFD to compare a feedback frequency with a divided-down version of the reference frequency. Integral PLL frequency synthesizers divide the output frequency by an integer. Fractional PLL frequency synthesizers divide the output frequency by a non-integer. With both types of devices, the PFD’s output (current pulses) are filtered and integrated to generate a voltage. In turn, this voltage drives the VCO to increase or decrease the output frequency, moving the PFD’s average output towards zero. Sensitivity varies over the full VCO frequency range. Input reference counters are used to reduce the reference input frequency. Feedback counters are used to reduce the output frequency for comparison with the scaled reference frequency. Applications for PLL frequency synthesizers include wireless base stations, mobile hand sets, personal digital assistants (PDAs), broadband wireless access, satellite communications, and local area networks (LANs).