SNAA434 March   2025 LMX2820

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Creating Multiple Copies of the Input Signal
    1. 2.1 Skew and Slew Rate Considerations
    2. 2.2 Buffers vs. Resistive Splitters
    3. 2.3 Phase Noise Considerations With Buffers
  6. 3Considerations with Combining Outputs
    1. 3.1 Isolation Between Sources
    2. 3.2 Single-Ended vs. Differential Outputs
    3. 3.3 Losses Due to Combining
  7. 4Resistive Method for Combining Multiple Signals
    1. 4.1 General Case Where Source Output Impedance can be Different Than Load Impedance
    2. 4.2 Special Case Where Source and Load Impedance are the Same
    3. 4.3 Increasing R1 to Improve Isolation
  8. 5Impedance Matching With Reactive Circuit
  9. 6Loss Due to Phase Error
  10. 7Phase Noise Improvement by Combining Multiple Signals
    1. 7.1 Theoretical Improvement for Multiple Signals Designed for in Phase
    2. 7.2 Combining Multiple Signals With a Phase Error
  11. 8Summary
  12. 9References
  13.   A Appendix: Calculations for Resistive Matching Network
  14.   B Appendix: Calculations for Reactive Matching Network
  15.   C Appendix: Calculation of Loss Due to Phase Error

1 Introduction

When phase noise performance is critical and one PLL synthesizer does not work, one approach is to combine multiple synthesizers. Combining Two LMX2820 Synthesizer Outputs for Improved Phase Noise gives an good analysis of combining two signals for better phase noise and demonstrates the theoretical and measured 3dB benefit as well as shows the impact if the signals are not in phase. Also, combining N synthesizers can yield a 10×log(N) theoretical benefit, but there are questions of how to buffer or split the input signal, how to combine multiple outputs, and how much phase error is tolerable (for more than two devices).

General Method of Combining N Frequency Synthesizers Figure 1-1 General Method of Combining N Frequency Synthesizers

Figure 1-1 shows the general method of combing multiple synthesizers for better phase noise. The assumption is that one starts with one ultra-clean source that is lower phase noise than the synthesizes and buffers. For the purpose of this article, the assumption is to be noiseless. Forward from this reference, there is a need to distribute this reference to the synthesizers, and combing the synthesizers to a single output to the load. There are considerations of noise, skew, and matching that are discussed in this article. Especially at higher frequencies, there is importance in knowing how much phase error is tolerable and how this varies with the number of synthesizers. Although calibration routines and programmable delays can adjust the phases, these require calibration and can add noise.