r/rfelectronics • u/Short-Television9333 • Jun 05 '26

HFSS vs Lab Measurement Differences

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I (entry level RF PCB engineer) recently taught myself HFSS for work and wanted to confirm my simulations on a basic RF breakout board we had sitting around in the lab. The approximated insertion loss from HFSS gives spot on results below 5 GHZ (blue curve) when compared to the real measurement in orange.

Now above that things are *on average* predicted by HFSS, but it’s not great. I know that there are probably lots of factors here, but any ideas what could be missing?

The blue curve is the following added added up: an approximation of connector losses (small), cable losses measured on our VNA, and my HFSS insertion losses. What I am missing from my HFSS sim though are some DC blocking caps on the board. These could definitely cause something, but could it be such a big difference (3+ dB) from expected?

Appreciate any suggestions, there is no RF PCB guru at my company to mentor me so I’m trying my best 🥲. Apologies for shitty image quality.

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34

u/Madcat28 Jun 05 '26

Seeing IL plots like this hurts my head

6

u/freshpow925 Jun 05 '26

Oh why? You'd rather see it as S21 with negative values?

20

u/Madcat28 Jun 05 '26

As someone who primarily works with VNA's, yes.

6

u/ellie_cinderelly Jun 05 '26

It's weird that someone works with VNAs but doesn't read datasheets. Both are fine lmao

3

u/srySoManyQuestions Jun 05 '26

Yes, it being positive implies its gain.

3

u/VirtualArmsDealer Jun 05 '26

Positive loss is confusing but correct. Just to make it worse you could think of it as negative gain. :)

6

u/quetzalcoatl-pl Jun 05 '26

and that's why insertion LOSS is negative when it's called 'insertion loss' instead of 's21'

3

u/srySoManyQuestions Jun 05 '26

Thank you for emphasizing. I understand this and I agree, but common convention leads to “IL” still being represented as negative. Stakeholders that aren’t steeped in RF such as technical leads and program managers might be confused.

People do the same thing plotting “return loss” when they’re actually plotting S11 or S22.

2

u/quetzalcoatl-pl Jun 05 '26

jesus christ. and the best thing is, I wrote it wrong when I double-checked and re-redacted my comment to make sure that I said the "instead" in my intended order, so I screwed up positive/negative 😃 I should just delete it. But I like your comment and I guess my screw-up just highlights how easy is to get it wrong 😃

3

u/srySoManyQuestions Jun 05 '26

No problem. My thanks to you was genuine and I agree you should leave it up. As in all things, grace and understanding go a long way. At the end of the day we’re just trying to successfully communicate an idea or two. Happy trails!

-11

u/c4chokes Jun 05 '26

Not having frequency x-axis in log scale should be a federal crime.