100 Ohm Microstrip differential line design

Hello,
I am trying to design a 100 ohm microstrip differential line in ADS layout. I am a new user to ADS. My doubt is regarding defining ports. As we know theoretically there would be two same signals with 180 phase. So how to define these two opposite ports and how to see the results( differential impedance i.e. 100 ohm). As differential impedance is 2*Z_odd_mode, so how to see z_odd_mode in results.
Thank you

For purely differential port excitation, you just define one port that is connected between the two lines (no connection to ground). The port impedance must be 100 Ohm then.

A more complex setup would be to use an ideal center tapped tranformer as mode converter, and then have two ports: one for the differential signal and another for the common mode signal.

Thanks for your kind reply
I have attached the port defining snapshots. Am I doing right??
And could you please tell me how to see differential impedance from s parameters in results, I mean how could I be ensure it is a 100 ohm differential line?
Thanks in advance

And could you please tell me how to see differential impedance from s parameters in results

Click to expand...

I suggest S11 = 0 for purely differential setup.

In a real world, you may be interested to measure both differential and common mode parameters. Network analysers are often performing single-ended multiport measurements and calculate differential and common mode S-parameters.

akjyani said:

I have attached the port defining snapshots. Am I doing right??

Click to expand...

Your screenshot shows that "Ref impedance" is 50 ohm. Change it to 100 Ohm.

akjyani said:

And could you please tell me how to see differential impedance from s parameters in results, I mean how could I be ensure it is a 100 ohm differential line?

Click to expand...

You can check S11 (after setting ports to 100 Ohm), or do some math with Y and Z parameters to calculate the line impedance. Zline=sqrt(Z11/Y11)

Here is a picture to show the more complicated method that I described above: If you simulate the line as a 4-port in Momentum, and do the common/differential mode excitation at circuit level with a balun.

Thank you for your reply
Could you please tell me when I am using the differential ports with reference impedance 100 ohm in port editor and plotting the s parameters, does it plot it with respect to 50 ohm or 100 ohm. And one more doubt I am using stoz(s(1,1)) to plot impedance. I dont know this stoz(S(1,1)) is differential impedance or odd mode impedance and do i have to use z0=100 ohm to read impedance from smith chart??
thank you

akjyani said:

Could you please tell me when I am using the differential ports with reference impedance 100 ohm in port editor and plotting the s parameters, does it plot it with respect to 50 ohm or 100 ohm.

Click to expand...

If you have defined the port as 100 Ohm, and you plot that dataset, the Data Display will show 100 Ohm S-parameters. But you can always check if you plot the PortZ values.

akjyani said:

And one more doubt I am using stoz(s(1,1)) to plot impedance. I dont know this stoz(S(1,1)) is differential impedance or odd mode impedance

Click to expand...

It only depends how you have defined your port. If your port (+) and (-) is connected for differential mode, then it is differential.

akjyani said:

and do i have to use z0=100 ohm to read impedance from smith chart??

Click to expand...

The Smith chart center (1*Z0) is your reference impedance. It makes sense to use Z0=100Ohm for your 100 Ohm line, because that is easiest to interpret in the chart.

But of course, you could also use Z0=50Ohm, and then your 100 Ohm line would be at the 2*Z0 location in the Smith chart.

Thanks for rply
I have one more doubt, if u could just solve it.
If i design a simple 50 ohm microstrip line and simulate it.
no to verify that it is a 50 ohm microstrip line what should i check in output results of ads momentum?
If I check s11 in smith chart then at desired frequency, s11 should be at (1,0) point of smith chart and it shows z0(1+j0) impedance there.
and when I plot stoz(S11,50), it also gives 50 ohm at same frequency.

But when I use Z=stoz(S)
and then plot Z(1,1), it gives different result
or I can use conventional conversion formula
z11=(((1+S(1,1))*(1-S(2,2))+S(1,2)*S(2,1))*50)/((1-S(1,1))*(1-S(2,2))-S(1,2)*S(2,1))
which seems the correct method to calculate input impedance.
so Z(1,1) and z11 bothe are same but smith chart does not show it at (1,0) point

so to verify that microstrip is of 50 ohm, which formula should I go for , Z(1,1) or stoz(S11,0)
and if i use Z(1,1), why its not at (1,0) point at smith chart?

thank you!!!

Z11 is the input impedance of the line, not the line impedance Z0.
See my post #5 above.

Thaks a lot !!
Now I got it
when I am using Z11, it is showing me the input impedance of a open circuit line which should not be equal to the chars impedance of line.
And if I use stoz(S11,50), it will behave as a 1 port simulation and it will take the second port as 50 ohm load. So basically it will show the input impedance of a microstrip line terminated with a 50 ohm load. Hence, if stoz(S11,50) (i.e. input impedance) is 50 ohm, it means chars impedance of the line is also 50 ohm, since it is terminated with 50 ohm load, right??

akjyani said:

when I am using Z11, it is showing me the input impedance of a open circuit line which should not be equal to the chars impedance of line.

Click to expand...

Yes.

Z11 is the input impedance of a line with all other ports open.
Y11 is the input admittance of a line with all other ports shorted.

From this, you can use sqrt(Z11/Y11) to calculate the line impedance.
NOTE: The result is NOT valid where the line length is n*lambda/4.

akjyani said:

And if I use stoz(S11,50), it will behave as a 1 port simulation and it will take the second port as 50 ohm load. So basically it will show the input impedance of a microstrip line terminated with a 50 ohm load.

Click to expand...

No, you misunderstood that. stoz(S11,50) is the same as stoz(S11) and the same as Z11.

I only used the stoz(S) function because some datasets don't have Z-parameter results included, and we can calculate them from the S-parameters with stoz(S). Same for Y-parameters: if they don't exist, we can calculate them with function stoy(S)

akjyani said:

Hence, if stoz(S11,50) (i.e. input impedance) is 50 ohm, it means chars impedance of the line is also 50 ohm, since it is terminated with 50 ohm load, right??

Click to expand...

No, as explained above. Use sqrt(Z11/Y11) to get the line impedance. And don't forget to test your math with an ideal transmission line where you know the line impedance(schematic simulation).

Hi, Volker,
I setup a ADS file as attachment, but it seems Zdiff is independent on the trace width, and also Zoo and Zoe. Zdiff is always 1, whatever the trace width is 50mil or 5mil, and Zcommon is always 0.316, Zoe=0.632, Zoo=0.5.
Could you comment on this?
BR,//Tony_LTH

Tony, you have used the wrong terminations for your lines at schematic lines. It is not correct to short them.

Have a look at my example workspace (attached). The 7zip ADS archive is renamed to *.7z so that the forum attachedment upload accepts it.

Hi, Volker,
Thanks for your reply. I am still confused about how you get Z11 and Y11.
If they are from setting in the S_Param, but when I did it, there is always a warning message:
"The Y-parameter matrix does not exist because some or all of the Y-parameters are infinite. Setting all Y-parameters to zero."
So my real(Zdiff) is always 0. Could you help me on this?

- - - Updated - - -

I can get Y(1,1) and Z(1,1) from Layout file DDS, but it seems can NOT be used by SCH.
The layout database has discrete data and fitted data, is it the root cause?

- - - Updated - - -

I finally used S4P to solve the issue, but directly used layout symbol not ok, because ADS may confuse the discrete data and fitted data.