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

4.3 Increasing R1 to Improve Isolation

To improve isolation, one approach is to visualize adding a series resistance, RISO to each source and then do calculations as before, but use an increased source resistance of (RSource+RISO) . The extreme case is to to make the maximum choice for RISO that satisfies Equation 3.

Equation 9. R I S O   =   N 2 2 × N - 1 × R L o a d   -   R S o u r c e

When this choice is made, the other components can be solved for:

Equation 10. R 1   = N × R L o a d   -   R S o u r c e
Equation 11. R 2   =   0

In the case that the source and load impedance are the same, this choice yields very similar power or perhaps a fraction of a dB higher. The concern can be though that the trace impedance needs to be matched to something much different than 50Ω. In Figure 4-4, the trace needs to technically be 66.7Ω between these resistances. Figure 4-5. In this case, the calculated power is about 0.7dB lower, but the isolation between sources is improved by about 8dB.

Combination of Two Sources With Added Isolation Figure 4-4 Combination of Two Sources With Added Isolation
Combination of Three Sources With Added Isolation Figure 4-5 Combination of Three Sources With Added Isolation

For the case of three sources, adding the isolation resistance theoretically delivers the exact same power, but improves isolation by 10dB. However, matching of trace impedance is something to take into consideration as this has changed the matching far from a 50Ω system.