Measuring VSWR in a Simulation?

[Stutzer]

Hi,
I would like to measure the VSWR (or the small signal output impedance)
of a circuit I simulate (using PSpice A/D by Cadence).
What would a circuit look like, that indicates the VSWR of the circuit
loaded with a given impedance, or how would I have to calculate it using
measurable values?

I don't konw if the concept of standing waves applies to infinitely
small lenghts of the "transmission-lines" resp. wires (what is probabely
assumed for simulation).
My actual goal is to get a good matching of the circuits output and the
load.

So far thanks for reading, I hope you can help me.
Regards
D. Stutzer

 

[Mantra]

Hi,
I would like to measure the VSWR (or the small signal output impedance)
of a circuit I simulate (using PSpice A/D by Cadence).
What would a circuit look like, that indicates the VSWR of the circuit
loaded with a given impedance, or how would I have to calculate it using
measurable values?

I don't konw if the concept of standing waves applies to infinitely
small lenghts of the "transmission-lines" resp. wires (what is probabely
assumed for simulation).
My actual goal is to get a good matching of the circuits output and the
load.

So far thanks for reading, I hope you can help me.
Regards
D. Stutzer

SPICE's generally can't do it explicitly because SPICE is a
lumped-equivalent-based simulator. Some harmonic balance simulators
can because they are distributed-model-based. It's probably easiest
to just using the formulae and/or a Smith chart.

rho = (Zload - Zo) / (Zload + Zo)

VSWR = (1 + |rho|) / (1 - |rho|)

or the load is purely real (no reactance):

VSWR = Rload < Zo ? Rload/Zo : Zo/Rload (in C)

and if you have a slotted line to measure:

VSWR = Vmax/Vmin anywhere along the transmission line

Standing waves are relevant any place 1) you can *not* used the lumped
equivalence model, that is, you must use the distributed impedance
model, 2) where you have any significant impedance discontinuity, and
3) where the reflected power is big enough to "matter". Pretty much
all transmission lines and antennae qualify #1. #2 depends on your
equipment and cables. #3 depends on equipment specs and your choice
to worry about it.


MM

 

[Stutzer]

Hi,
Thanks for the information.
Actually, the goal is to match an antenna to the output of a (audio-)
FM-Oscillator. So what I need is the output impedance of the circuit (at
its resonance freq.).

Thanks again and best regards,
D.Stutzer

 

[Stutzer]

Hi,
I'm sorry but I did not post this message really correctly. Therefore
there is the same discussion going on at sci.electronics.cad.
Please take a look at it as well if you read my article.
Thanks and please pardon my mistake.