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Use of non-50ohm impedances in RF Budget Sims

Question asked by MLYNCH on Mar 9, 2005

I have some questions on how to use non-50ohm impedances in RF budget sims.

I have looked at the Budget_Transformer design in the tutorial/RF_Budget_Examples_prj project.  

In that design, it uses a transformer to go from a 50 to a 75ohm impedance system.  It uses another transformer at the end to go back to 50-ohms.

The notes say that budget measurements at internal nodes are done with 50ohm loads, so signal levels may be different than expected.  

Q1) Does this mean that only the measurements done at the final node are valid?  (ie. measurements at internal nodes cannot be used as reported.)  Which measurements are valid and which are not?

Q2) If we want an accurate measurement at an internal node, can we insert two transformers back to back at that location that transform from Xohms to 50 ohms, and then from 50 ohms back to Xohms?

Q3) Can the RF budget analysis still be used when the Z1 and Z2 values for a component are different?  

For example, let say my system has the following components:

LNA: Z1 = 50    Z2 = 200
MXR: Zref = 200
FIL: Z1 = 200   Z2 = 200
VGA: Z1 = 200   Z2 = 1000

Can I still get reliable results...

...from end-to-end if I insert a 20:1 transformer at the output?

.. for intermediate nodes (ie. after the FIL) if I insert a 4:1 transformer and then a 1:4 transformer?

Thanks in advance,