# In ADS, how can I change the electrical length on a transmission line to depend on the frequency sweep?

I'm setting up an S-Parameter test on a transmission line, but I want the electrical length to change and depend on the frequency. How can I go about that? The variables that I can put in are all constant-I can't figure out how to make non-constant variables. I also tried putting in a sweep plan, but I can't figure out how to use it as a variable in the transmission line.

## 1 Answer

This effect is called dispersion, and ADS is able to model it. However, you might have trouble with it. The way that dispersion works is limited by the laws of physics, and the ADS simulator attempts to obey these laws. The consequence is that electrical length versus frequency cannot change in an arbitrary way. Using ADS for this type of analysis requires an understanding of dispersion and how it is constrained. This is an advanced topic. Start with the Kirschning and Jansen formula.

• Thanks for the response. Since the electrical length is $\beta*l$, and $\beta =2\pi/\lambda$ all I'm expecting for is beta to change and thus the electrical length. I wouldn't expect it to be that hard/advanced a topic. – Goldname Feb 19 '17 at 5:13
• Do you still know how to go about doing this in ADS? I read through the paper but it doesn't say. – Goldname Feb 19 '17 at 5:15
• In that case, just specify the electrical length in degrees at a particular frequency, and the simulator will adjust it at other frequencies. All the transmission line models do this sort of thing automatically. I was thinking you wanted to model, for example, change in dielectric constant versus frequency. – Tom Anderson Feb 19 '17 at 5:16
• Oh, no I'm just looking to change the electrical length. How would you specify the electrical length at a particular frequency, and how does the simulator know how to adjust it automatically at different frequencies? All I see is one option in the electrical length where I can enter an expression with constants or a number. – Goldname Feb 19 '17 at 5:22
• Assuming that you mean the model TLIN, it has three parameters, Z, E, and F. E is electrical length in degrees at frequency F. The simulator uses the corresponding time delay for other frequencies. If you have problems with the simulation, ask another question and post the schematic, the result that you got, and how it didn't match the result that you expected. – Tom Anderson Feb 19 '17 at 6:44