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

5 Impedance Matching With Reactive Circuit

The resistive matching circuit is simple and provides a good match. However, the circuit sacrifices output power. By using an inductor and capacitor, a match can be provided that gives theoretically 6dB higher output power. However, at high frequencies, one needs to be aware of non-designed for behaviors of inductors and capacitors.

Reactive Matching Circuit Figure 5-1 Reactive Matching Circuit

The value of L and C can be calculated (Appendix B) as follows:

Equation 12. L   =   j × Q × R S o u r c e N 2 π × f × j     =   R S o u r c e × Q 2 π × f × N  
Equation 13. C =   - j 2 π × f × X C     = Q R L o a d ×   2 π × f  

These work provided the following restriction is satisfied.

Equation 14. R S o u r c e N     < R L o a d

Directly shorting the outputs together can create some concerns about isolation between the sources. To alleviate this concern, one can add a series resistance and recalculate RSource including this resistance. However, this approach does sacrifice some power.