The dual loop PLL architecture provides the lowest jitter performance over a wide range of output frequencies and phase noise integration bandwidths. The first stage PLL (PLL1) is driven by an external reference clock and uses an external VCXO to provide a frequency accurate, low phase noise reference clock for the second stage frequency multiplication PLL (PLL2).

PLL1 typically uses a narrow loop bandwidth (typically 10 Hz to 200 Hz) to retain the frequency accuracy of the reference clock input signal while at the same time suppressing the higher offset frequency phase noise that the reference clock may have accumulated along its path or from other circuits. This cleaned reference clock provides the reference input to PLL2.

The low phase noise reference provided to PLL2 allows PLL2 to operate with a wide loop bandwidth (typically 50 kHz to 200 kHz). The loop bandwidth for PLL2 is chosen to take advantage of the superior high offset frequency phase noise profile of the internal VCO and the good low offset frequency phase noise of the reference VCXO.

Ultra-low jitter is achieved by allowing the phase noise of the external VCXO to dominate the final output phase noise at low offset frequencies and the phase noise of the internal VCO to dominate the final output phase noise at high offset frequencies. This results in best overall phase noise and jitter performance.