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Frequently Asked Question

How do I choose between active and passive filter in PLL loop?

Applications requiring ultra low jitter performance use passive filters vs active filters as passive filters only contribute thermal noise from its components. In an active filter, input referred thermal noise and 1/f noise from transistors is increased by the gain of the circuit. This translates into more noise on the oscillator's voltage control pin which randomly frequency modulates the output signal, resulting in more phase noise.

Active filters are typically used when the oscillator's(VCO/VCXO/TCXO) control voltage range is outside the range of the charge pump supply. In these cases, the charge pump current cannot drive the oscillator's output over its full frequency range. The active filter provides gain or a level shift to exploit the oscillator's full control voltage range.

The AD9510/11 allows the use of a 5V separate Vcp power supply for VCOs requiring a 5V tuning range.