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What Do Clocks, Carriers, Local Oscillators, and FM Have in Common? (.PDF Download)

Oct. 17, 2017
What Do Clocks, Carriers, Local Oscillators, and FM Have in Common? (.PDF Download)

The master timing control circuit for most electronic applications is a precision signal source. For digital applications, it’s an accurate clock. In RF applications, it’s a carrier source for a transmitter or a local oscillator (LO) for a receiver. And if frequency modulation (FM) is involved, a modulator and demodulator are needed along with a carrier source.

In all of these cases, a phase-locked-loop (PLL) frequency synthesizer is an excellent choice. It provides not only the precision and accuracy needed, but also a flexible way to change frequency.

PLL 101

Just as a review, a PLL synthesizer is a closed-loop controller with feedback (Fig. 1). A voltage-controlled oscillator (VCO) generates the output signal. Frequency is determined by an LC resonant network, but controlled by a dc input voltage derived from a phase detector and a low-pass loop filter. 

1. The basic configuration of a PLL features a VCO output, controlling circuits such as the phase detector, and feedback dividers.

The input to the phase detector is usually a stable reference signal from a crystal oscillator. The second input to the phase detector is a signal derived from the VCO output that’s usually divided down by a variable-frequency divider. The two inputs to the phase detector must be the same frequency.