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November 22, 2017

# Using a hammer instead of pliers

2 weeks 6 days ago - 2 weeks 6 days ago #868
Microwaves101 created the topic: Using a hammer instead of pliers
Have done some work on MMIC and MMIC RF transitions off-chip, trying to simulate using HFSS. It seems like HFSS is not capable of giving an unambiguous answer for microstrip port impedance. If you draw a 70um wide line on 100um GaAs, you know by heart it is fifty ohms. HFSS does not agree...

Does anyone have advice on how to make sure HFSS is not spitting out incorrect results when using microstrip ports? That would make a great article..

I guess the lesson is to use Momentum, Sonnet or Axiem for everything on a MMIC. But I think we still need a 3D tool for looking at RF transitions.

Steve
Last Edit: 2 weeks 6 days ago by Microwaves101.

2 weeks 4 days ago #870 by Fahmi
Fahmi replied the topic: Using a hammer instead of pliers
In order to create a good microstrip port in HFSS, please follow the following steps:
1- If you have the actual dimensions of the air cavity above the substrate then use it otherwise create a cavity that is at least 4-5 times the substrate thickness
2- Create a rectangle that is centered at the trace (width wise) and that extends from the ground plane up to either the top edge of the actual cavity or to 4-5 times the substrate thickness. The width of that rectangle should be either the actual width of the actual substrate , or at least 5 times the width of the metallic trace.
3-Assign a wave port to that rectangle , and make sure to define an integration line from the bottom of the trace to the ground plane, this way you are telling HFSS to calculate the port impedance along that integration line which happens to have the maximum E-filed.
I ran the example with the dimensions you provided and the port impedance was calculated as 46.45 Ohms, in 9-11 GHz range.
I will have to dig an old presentation by Ansoft (the previous owner of HFSS ) which details that, if I find it I will post it to the thread.

2 weeks 20 hours ago #871
Microwaves101 replied the topic: Using a hammer instead of pliers
Fahmi

Thanks for your thoughts! Can you tell me why HFSS comes up with 46.5 ohms for a line that is known to be 50 ohms?

I know very little about HFSS... but could a "lumped port" be useful in this case?

Steve

1 week 5 days ago #872 by thadrien
Steve,

The lower impedance you get compared to what it should be is a telltale sign your port box is both 1/ too small and 2/ electrically shielded.

It is because the simulation method puts a box around the port, which is connected to ground, which reduces the impedance.

About the other tools: Momentum is cool for standard microstrip circuits but is awfully complex to use when you want to do fully custom parametrized layouts, both Sonnet and Momentum are slow when your metal filling factor is high (e.g. ground planes) due to the simulation method. Don't know Axiem.

1 week 5 days ago #873 by Fahmi
Fahmi replied the topic: Using a hammer instead of pliers
Steve,
I played a bit with the model, and gave it a bit more thought, the following factors may be at play here
1- the trace thickness, I used 10 um , changing that will affect the impedance
2- the air box I used was a bit small increasing its height will increase the impedance,
I worked a lot with 15 mil alumina so 4 or 5 times that is about 75 mils much more than 5 times the
100 um GaAs substrate, which makes the box small as Hadrian pointed out, given that in air the
wavelength is the same, so my previous statementabout the box height need to be revised.
3-I also did not use radiation boundaries, again due to custom, the structures I
worked with were always housed in cavities.
As for lumped ports, I did not use them before, coming from the waveguide world, I always used wave ports,.
what I can tell you is that the simulations using wave ports, gave very good results (both in waveguide and microstrp) compared to measured results for filters which are finicky to start with .

1 week 5 days ago #874 by thadrien

Fahmi wrote: which makes the box small as Hadrian pointed out, given that in air the
wavelength is the same, so my previous statementabout the box height need to be revised.

When I spoke about box size, it is not compared to the wavelength but to the substrate thickness. The conclusion would be the same for lower frequencies.

Fahmi wrote: 3-I also did not use radiation boundaries, again due to custom, the structures I
worked with were always housed in cavities.

For people using radiation boundaries, be careful: most often the simulator will use electric (metallic) or magnetic boundaries for the port calculation, even if radiation boundary conditions are used. And radiation boundaries don't like to be too close to the structure.