The LMX2820 is a low phase noise RF synthesizer. The device offers additional features for cases where improved phase noise performance is needed. This note characterizes the inherent phase noise performance of the LMX2820 with varying PFD frequencies and reference sources. This analysis is used as a baseline to compare against the improvement topologies. The first option uses an external VCO instead of the internal one. This facilitates a higher-Q, lower phase noise VCO at the expense of tuning range. The second option uses an external mixer to bypass the internal N-divider. This eliminates the PLL phase noise degradation that the internal divider introduces. Combining both techniques with a superior reference yields the ultimate realizable phase noise performance.
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The LMX2820 RF synthesizer inherently provides excellent phase noise performance and operates up to 22 GHz; however, there are cases that need improved phase noise performance. The LMX2820 offers a couple of different topologies to improve synthesizer phase noise. One technique bypasses the internal voltage-controlled oscillator (VCO) and uses an external one. This introduces a superior low phase noise VCO while maintaining the programming features of the PLL device. The second technique uses an external mixer to down-convert the RF frequency to the PFD (Phase Frequency Detector) frequency. This bypasses the internal feedback dividers which are one of the sources for degraded PLL phase noise.