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

7.1 Theoretical Improvement for Multiple Signals Designed for in Phase

Consider first the case of combining two synthesizers that are designed for in phase and the same amplitude. In this case, the voltage can double, increasing the output power by 6dB. However, the noise of these two synthesizers is not correlated, so that the noise power increases only 3dB. This leads to a theoretical improvement in phase noise is 3dB. Even if there are losses in the combining, such as in the case of a resistive circuit, this applies to the output power and noise power equally, so the phase noise improvement is still 3dB. One consideration is that the input reference clock needs to be sufficiently clean. As the noise of this reference clock is common to both synthesizers, this actually increases 6dB with the noise power. Also, this method improves the noise due to the synthesizer, but not the reference clock. To generalize this result, when N signals in phase are combined, the theoretical improvement in phase noise is 10×log(N)