SNAA361A april   2022  – may 2023 LMX2820

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Reference Frequency
  6. 3Standard LMX2820 Phase Noise Performance
  7. 4 Pre-multiplier Stage Performance
    1. 4.1 External VCO Loop Filter Design
    2. 4.2 Active Loop Filter Implementation
    3. 4.3 External VCO Performance Results
  8. 5External PFD Topology
    1. 5.1 External PFD Loop Filter Modification
    2. 5.2 External PFD Measurement Results
  9. 6Putting it Together in the Real World
    1. 6.1 Real-World Performance Results
    2. 6.2 Why use the Dual Approach?
  10. 7Conclusion
  11. 8Revision History

Abstract

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.