# problem evaluating S-parameters

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#### satishgra

##### Member level 3
Hi,

I am having issues with matching the S-parameters for Prob 4.28a from pozar that ADS throws up and the ones that i ended up from my manual calculations.
I am using a micro strip with 50ohm char impedance

manual calculations say that S11 is 1/3
But, ADS says it is 0.0002

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Hi

Regards

#### satishgra

##### Member level 3
Hi,

I am attaching the files in .rar format.
The ckt is to basic..look at the link...

Indeed I am getting the same results as what the tutorial says. But, I am not able to come up with the same results through manual calculations

Regards,
satish

#### satishgra

##### Member level 3
for the sake of clarity, i had simulated the same microstrip using HFSS. That throws me a altogether different result. please look at the attached file

#### satishgra

##### Member level 3
Hi,

I had forgot too include the AIRBOX in HFSS. That was the reason for me to see different results Now, The results are matching.

Can somebody let me know how to determine these results manually.

Regards,
satish

#### tyassin

Hi

Looking at your ADS file. You have two 50 ohm ports. In between you have a 50 ohm transmission line. That will give you a VERY low mag(s11) and a VERY high return loss. Just like you get in your ADS simulation.

I do not know this problem 4.28a from Pozar. But if it is to put a 50 ohm transmission line in a 50 ohm system and then get a reflection coefficient og 1/3 it is incorrect.

Regards

#### satishgra

##### Member level 3
buddy,

Can you let me know how to determine the S11 value for a circuit manually. Is there a way we can do it.

Regards,
satish

#### tyassin

Hi

I do not know if you study..... but take a look in the above mentioned book by Pozar. In the very simple question you have you usually calculate the reflection coefficient at the load(port 2) and then transfer it down the transmission line.
I hope you are familiar with the basic formula to calculate the reflection coefficient: $\Gamma=\frac{Z_{l}-Z_{0}}{Z_{l}+Z_{0}}$

Hope it helped.

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#### satishgra

##### Member level 3
I am not able to calculate them manually. I do have enough knowledge of the basic formula's. pls find the attachd file.

If i try to calculate the value of the element "Z" from the graphs, i see that it varies greatly with frequency. I dont know how can calculate these values from the basic formula

Can you help me out with this.

Regards,
satish

#### tyassin

Hi

I think you have mixed up a lumped element "Z" and the distributed nature of a transmission line Zo. So when you simulate, it is not a transmission line between the ports but a lumped element, capacitor, inductor or resistor.

#### satishgra

##### Member level 3
no buddy.....I am still simulating it as a distributed element. I am using a microstrip. just for clarification, if i use those 4 values that i had mentioned in the previous attachment, I am seeing that S11 is indeed correct.

So, the problem is how can i replicate the behaviour of S11 using Z as a microstri[p of Z0 = 50 ohm on paper

#### tyassin

Hi

I think maybee I am confused about the problem. But if you want to calculate with a 50 ohm microstrip, you use the ABCD parameters for that.

What exactly will you calculate using a microstrip?

Regards

#### satishgra

##### Member level 3
I am indeed using ABCD parameters to determine S11. It turns out to be that S11 is equal to

S11 = (Z) / (Z + 2Z0)
The modelling of the value of "Z" is where i am having issues.

From the graphs, i need this to be equal to

From S11 @ 1G -> Z = 0.2 + 0.7i
From S11 @ 5G -> Z = 2.25 + 0.29i
From S12 @ 1G -> Z = 4.13 + 28.4i
From S12 @ 5G -> Z = 87.5 + 99.4i

for various conditions (assuming Z0 = 50 ohm)

#### tyassin

Hi

OK so you are simulating a 50-ohm transmission line. This is shown in your attachment....all the pictures.

Then you calculate S11 and S21 using a series Z-element, with ABCD parameters.

OK.....so the results for the simulation of the 50 ohm transmission line at various frequencies, you want those to fit with your formula?

But I think you get a formula for a Z-element and then you try to fit it with an transmission line results.

Hope I understand you correct.

#### satishgra

##### Member level 3
Finally solved the issue...It was my approach that was wrong. I was equating Zin results to Z; Rather, I should have this equated to Z + Zl

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