频合常见问题答疑Ask the Applications Engineer―30 by Adrian Fox [adrian.fox@analog.com] PLL SYNTHESIZERS Q. What is a PLL Synthesizer? A. A frequency synthesizer allows the designer to generate a variety of output frequencies as multiples of a single reference frequency. The main application is in generating local oscillator (LO) signals for the up- and down-conversion of RF signals. The synthesizer works in a phase-locked loop (PLL), where a phase/frequency detector (PFD) compares a fed back frequency with a divided-down version of the reference frequency (Figure 1). The PFD’s output current pulses are filtered and integrated to generate a voltage. This voltage drives an external voltage-controlled oscillator (VCO) to increase or decrease the output frequency so as to drive the PFD’s average output toward……