regarding port impedance values in HFSS
I am facing some peculiar problems in HFSS while simulating microstrip and CPW structures (perfectly symmetric structures) between 10 GHz and 50 GHz. I am using Driven Modal setup with wave ports.
1. The values of port characteristic impedance obtained after simulation ARE NOT SAME at both ports even though both ports have been IDENTICALLY set up and the structure is perfectly SYMMETRIC (Just a simple line designed for 50 Ohms char. impedance). For example, I get 49.2 Ohms at one port and 50.1 Ohms at the other port (for a perticular frequency)
WHY the difference? Which is the correct value? How do I know it is exactly matching or 50 Ohms?
2. These port impedance values and their difference vary with the "Port Field Accuracy" settings in the Analysis options. However HFSS manuals advise not to tamper with this value which by default is set at 2%. Should I stop tampering with this?
3. The port impedance values obtained for a "PORTS ONLY" solution DO NOT MATCH with the values for a "FULL FREQUENCY SWEEP" for the same structure.
For example I got Zport1 = 49.5 Ohms and Zport2 = 50.1 Ohms for a "ports only" solution and Zport1 = 47.9 Ohms and Zport 2 = 48.5 Ohms for "full sweep" (this trend continues for all frequencies)!!
Why this difference? What should I assume as the more correct approximation for the actual port impedance?
I am using recommended port sizes for both microstrip (5×signal line Width by 4×height) and CPW lines (3×(twice gap + central conductor width) by 4× height)
A few more questions:
4. What should be the value of "frequency of setup" during simulation? Should it be IN BETWEEN the frequency range of interest or the maximum frequency value?
5. In many manuals, the microstrip and CPW structures have been designed by taking a "radiation" boundary envelop on touching all sides of the structure/ substrate except the top surface (that space has air or vacuum). However others suggest to take a large airbox on all sides of the structure (with the boundary being at least λ/4 times the wavelength corresponding to the lowest frequency of interest from all surfaces) Which method should I follow?
HFSS gives an error if I define a wave port inside the airbox, and gives a warning if the ports are in the same plane as the airbox radiation boundary.
Any help regarding the aforesaid topics will be greatly appreciated.
thanks
regards
Ayan
well, no problem for that warning. define the prots on a rectagle attached to the radiation surface (same face). It will be OK.
and for different Zin, what do you mean by two Zin for microstrip? you mean one in each side? so one is incoming port and the other is outgoing port? which one is excited? the whole thing is little wiered, could you post the model here? it will be better to see the model first, and then discuss.
.m
Use any good planar tool and you won't have these problems.
Hi Ayan,
I changed the port accuracy to 0.1% and I ended up with Z0 values which differed minimally. The question is why should they differ at all. The only reason I can think of is that meshing is not the same at both the ports. I don't know why else it would be different. Next question would be as to why the meshing is not the same. Awaiting experts' comments on this.
PS: What James says on the planar tools is absolutely right. In my experience, full 3D solvers like HFSS do not seem to handle planar circuits as accurately as MoM based planar solvers like Sonnet and IE3D.
-svarun
1. It is normal. The mesh might be symmetric in one direction but not in others. Try to make the port smaller and increase the port accuracy. To get 50 ohm line, you can try with optimetrics.
2. Try to make it 0.1%
3. Try to use Zpv for the CPW.
Hi roy83,
(1) If the impedance of two ports are very close to 50 (for your example, one is 49.2 and the other 50.1) and the field distribution of the port is correct, the difference has little effect on your results. The difference comes from numeric method and it is impossible that the impedance of two ports are the same (50). In order to get the experimental results consistent to simulation results, you should normalize the port impedance to 50 because the port impedance of testing system is 50.
(2)" frequency of setup" is the frequency of adaptive meshing adaptive. If the results at both sides of this frequency are important to you (for passband filter), you should set " frequency of setup" to the middle of frequency band, else than set " frequency of setup" to the high frequency of frequency band.
(3) For open structure (the microstrip,CPW et al.), you should draw a air box surrounding the structure with at least λ/4 times the wavelength corresponding to the lowest frequency of interest from structures , and then define all of the faces of the airbox to radiation boundary.
(4) Waveport can be defined only outside the structure. If you want to define a port inside the structure, you should use the lump port.
Best Regards.
Ayan, one way to define wave ports in HFSS is to use a cap at the end of the wave port plane. for example: you feed your printed patch antenna with a coax. at the end of the coaxial, you draw a tiny cylinder with the same radius of the coax. You define this cap perfect E. and the bottom surface of the coax. as a wave port.
"A wave port must have power flow in only one direction normal to the face"