Thevenin and s-parameters...
 

Hello,
I'm have "little" doubt about s-parameter simulations. My
question is very simple but I do not know if the answer is the same.
The Thevenin theorem can be applied to a s-parameter simulation?
That is, if I do a simulation using a port P1 of 50 ohm where at valley
of the generator I've connected 2 impedences, one in parallel Z1 and
one in series Z2. Supposing I obtained S11 at frequency f1. Then I
incorporate the impedence Z2 (at frequency f1) in the port 1 impedence
(using Thevenin), can I obtain the same S11 parameters for f1 fequency?

Camelot

Thevenin and s-parameters...
 

"camelot" wrote in message
ups.com...
Hello,
I'm have "little" doubt about s-parameter simulations. My
question is very simple but I do not know if the answer is the same.
The Thevenin theorem can be applied to a s-parameter simulation?
That is, if I do a simulation using a port P1 of 50 ohm where at valley
of the generator I've connected 2 impedences, one in parallel Z1 and
one in series Z2. Supposing I obtained S11 at frequency f1. Then I
incorporate the impedence Z2 (at frequency f1) in the port 1 impedence
(using Thevenin), can I obtain the same S11 parameters for f1 fequency?

Camelot

I believe the thevenin theorem applies to s-parameter simulation provided
you assume the amplifier (generator) is perfect Class A and perfectly
stable. You need to be aware of where the load is located on the
transmission (1/4 wave? Not sure what you mean by 'valley'). See
http://www.sss-mag.com/pdf/arrl_circles.pdf

Thevenin and s-parameters...
 

camelot wrote:
Hello,
I'm have "little" doubt about s-parameter simulations. My
question is very simple but I do not know if the answer is the same.
The Thevenin theorem can be applied to a s-parameter simulation?
That is, if I do a simulation using a port P1 of 50 ohm where at valley
of the generator I've connected 2 impedences, one in parallel Z1 and
one in series Z2. Supposing I obtained S11 at frequency f1. Then I
incorporate the impedence Z2 (at frequency f1) in the port 1 impedence
(using Thevenin), can I obtain the same S11 parameters for f1 fequency?

From: "Fields and Waves ...", by Ramo and Whinnery:
"It must be emphasized, as in any Thevenin equivalent
circuit, that the equivalent circuit was derived to
to tell what happens in the *load* under different
*load* conditions, and significance cannot be automatically
attached to a calculation of power loss in the internal
impedance of the equivalent circuit."

Power loss in the internal impedance of the Thevenin
equivalent circuit is related to the s-parameters.
|s11|^2 is power reflected from the network input
divided by the power incident on the network input.
--
73, Cecil http://www.w5dxp.com

Thevenin and s-parameters...
 

Thank you for your comments,
I'm doing some calculations and simulations in order to verify if I
can bypass representation problem of s-parameters in other ways rather
than the Thevenin one.
There is one thing I'd like to submit to your attention, the known
formula
S11=(Zin-Z0)/(Zin+Z0),
where Z0 is the port impedance and Zin the load, does not works if you
consider Z0 not pure real (usually 50 ohm) but composed by a real and
an imaginary part i.e. Z0 = a+jb.
Are there other known formulas for S11 for Z0 real+imaginary?

Camelot

Thevenin and s-parameters...
 

"camelot" wrote in message
ups.com...
Thank you for your comments,
I'm doing some calculations and simulations in order to verify if I
can bypass representation problem of s-parameters in other ways rather
than the Thevenin one.
There is one thing I'd like to submit to your attention, the known
formula
S11=(Zin-Z0)/(Zin+Z0),
where Z0 is the port impedance and Zin the load, does not works if you
consider Z0 not pure real (usually 50 ohm) but composed by a real and
an imaginary part i.e. Z0 = a+jb.
Are there other known formulas for S11 for Z0 real+imaginary?

Camelot

http://www.rfcafe.com/references/electrical/s-h-y-z.htm

Also:

http://www.daycounter.com/Calculator...lculator.phtml

Regards,

Frank (VE6CB)

Thevenin and s-parameters...
 

On Jan 29, 2:12 am, "camelot" wrote:
Thank you for your comments,
I'm doing some calculations and simulations in order to verify if I
can bypass representation problem of s-parameters in other ways rather
than the Thevenin one.
There is one thing I'd like to submit to your attention, the known
formula
S11=(Zin-Z0)/(Zin+Z0),
where Z0 is the port impedance and Zin the load, does not works if you
consider Z0 not pure real (usually 50 ohm) but composed by a real and
an imaginary part i.e. Z0 = a+jb.
Are there other known formulas for S11 for Z0 real+imaginary?

Camelot

Frank has already provided you with links to (I presume)
transformations to other linear two-port representations.

But I'm curious. Why do you think that the formula you wrote above
doesn't work when Z0 is complex? In what way do you think it does not
work?

I'm also curious why you would pick a complex reference impedance for
S-parameter work, but that's really a different issue.

Cheers,
Tom

Thevenin and s-parameters...
 

"camelot" wrote in message
ups.com...
Thank you for your comments,
I'm doing some calculations and simulations in order to verify if I
can bypass representation problem of s-parameters in other ways rather
than the Thevenin one.
There is one thing I'd like to submit to your attention, the known
formula
S11=(Zin-Z0)/(Zin+Z0),
where Z0 is the port impedance and Zin the load, does not works if you
consider Z0 not pure real (usually 50 ohm) but composed by a real and
an imaginary part i.e. Z0 = a+jb.
Are there other known formulas for S11 for Z0 real+imaginary?

Actually, characteristic impedance Z0 is assumed to be 50 Ohm, 300 Ohm or 75
Ohm (etc.), based on the type of transmission line, under the assumption
that the line is "lossless", thus in theory only it only considers the
complex components.

For a lossless line, Z0 = sqrt(L/C)

In reality, the resistance (conductance) must be considered, whe

Z0 = sqrt((r+jwl)/(G+jwC))

Thevenin and s-parameters...
 

Hi Tom,
well, after few researches on several books, I found that the formula
I provided by me works only for real Z0. The general formula valid in
case Z0 is complex is the follow:

S11=(Zin-Z0*)/(Zin+Z0)

Where Z0* is the conjugate of Z0. Obviously, if Z0 is real, the
conjugate coincide with the real one.
However, thank you for your interest ;-)

Camelot


But I'm curious. Why do you think that the formula you wrote above
doesn't work when Z0 is complex? In what way do you think it does not
work?

I'm also curious why you would pick a complex reference impedance for
S-parameter work, but that's really a different issue.

Cheers,
Tom

Thevenin and s-parameters...
 

On Jan 30, 11:13 pm, "camelot" wrote:
Hi Tom,
well, after few researches on several books, I found that the formula
I provided by me works only for real Z0. The general formula valid in
case Z0 is complex is the follow:

S11=(Zin-Z0*)/(Zin+Z0)

Where Z0* is the conjugate of Z0. Obviously, if Z0 is real, the
conjugate coincide with the real one.
However, thank you for your interest ;-)

Camelot

But I'm curious. Why do you think that the formula you wrote above
doesn't work when Z0 is complex? In what way do you think it does not
work?

I'm also curious why you would pick a complex reference impedance for
S-parameter work, but that's really a different issue.

Cheers,
Tom


I'm not sure where you got it, but the formula with the complex
conjugate is NOT correct! The formula without complex conjugate is
correct, for complex Z and Z0, both.

That the formula using the complex conjugate is incorrect is trivial
to see: consider that a line terminated in a load equal to the line's
characteristic impedance (be it purely resistive, or complex) has no
reflection. That is, if load Z = Z0, there is no reflection. Then if
Z0 is complex and has a non-zero reactive component, your formula
yields S11 which is not zero, for a line which is terminated to have
no reflection. As far as I am concerned, that would be incorrect.
The original formula, without complex conjugate, yields the correct
answer for this case. Can you come up with a case where it is
incorrect?

Cheers,
Tom

Thevenin and s-parameters...
 

Hi Tom,
as far as I am concerned, your observations are rights only if we are
talking about real quantities. If you have a load that is complex then
you obtain perfect matching (there is no reflection) only if Z0 is not
equal to Zin but it is the conjugate of Zin. If you remember, this is
the same condition valid for the maximum power transferring when the
load in complex. Do you agree with these considerations?

Regards,

Camelot



I'm not sure where you got it, but the formula with the complex
conjugate is NOT correct! The formula without complex conjugate is
correct, for complex Z and Z0, both.

That the formula using the complex conjugate is incorrect is trivial
to see: consider that a line terminated in a load equal to the line's
characteristic impedance (be it purely resistive, or complex) has no
reflection. That is, if load Z = Z0, there is no reflection. Then if
Z0 is complex and has a non-zero reactive component, your formula
yields S11 which is not zero, for a line which is terminated to have
no reflection. As far as I am concerned, that would be incorrect.
The original formula, without complex conjugate, yields the correct
answer for this case. Can you come up with a case where it is
incorrect?

Cheers,
Tom- Hide quoted text -

- Show quoted text -