how do you proceed if your measurments are different than simulation

[inass57]

Hi guys! inexperienced engineer here!!

so i have made a bunch of circuits (Lna, Filter, couplers ...) , they all had good results while doing the simulation on ADS, i also did EM simulation and co simulation for ciruits with SMC for all the circuit.
Now i started measuring my circuits and almost all the results are different from those i had in my simulations. even the adaptation on my coupler for exemple is not good (-8dB).

Now i'm a little stuck on how to proceed afterwards? i know that on almost all the circuit my main problem is adaptation, but how do i work on that?
I don't know if this is relevant but i'm using SMA connectors that works at my working frequency (9.4GHz)

Thank you for your time, any insight would help immensely.

 

[KlausST]

hi,

review your post.
How do you think we can give any advice as long as we don´t see what you have done, how you tested it and what results you´ve got?

Klaus

 

[BigBoss]

Even-tough the measurements won't be %100 consistent, the difference should be quite small.
So, you do some mistakes while simulating the system.
-Consider package parasitics
-Pay attention EM simulation and Port places/types
-Use s-parameters for small signal simulations ( s-parameters are measured ones and they are very approximate to reality-take care biasing conditions are exactly same !!)
-Use realistic passive component models ( use also s-parameters if applicable)
so on..

 

[mtwieg]

Simplify things until you get agreeable results. Start with a transmission line, then a hybrid coupler, etc... things you don't really even need simulation to design adequately.

If you can't even get simple stuff to work out, then you need to take a good hard look at your measurement methods, and simulation methods, since at least one is lying to you.

 

[dick_freebird]

Everything matters at >GHz.

One good thing to do, is to get some "golden units" of
known pedigree and attributes, that are similar to what
you are working on (LNA for LNA, mixer for mixer, filter
for filter, ...) and determine whether you can get the
known right answer from your bench. Buy top quality
SMA cables that purport to be super low loss well past
where you think you care - if measuring distortion and
intercept points, you have to be good for the harmonics
of interest, not the carrier frequency alone.

Then work your way down the lineup measuring your
and golden units, see where results really start to
diverge and there's where to spend your time on "why?".

That's one of the real tragedies of university education,
hardly ever are you taught what to do when things go
wrong - only theories about how it's be if everything
went right. I think everyone ought to have a "gap year"
where they work as a lab technician in their major focus.
But then, who needs a worthless technician who's going
to leave just before they became useful?

 

[biff44]

back in the day, we would use "lumped element equivalent" discontinuity models, compute the values of those models based on test results, and update them for a specific circuit and frequency band, then respin the board and you would probably hit your performance goals.

One of the sad things of modern microwave analysis programs is that you can no longer access all those closed form discontinuity models, but must blindly accept the "proprietary" model that is used. We do have the benefit of Emag simulation software now, but sometimes Emag simulators lie too.

I guess the best thing you can still do is understand what the closed form Discontinuity models are....use the proprietary ones...but add in lumped element corrections to get the whole mess to match your measured results. Such as...add a capacitance at the end of a microstrip open circuit, and adjust the value until your simulation matches the measured results (and sometimes that capacitance needs to be a negative one), and so on.

- - - Updated - - -

The other thing to remember....closed form discontinuity models for microstrip ASSUME the next discontinuity is electrically "far" away. That is because if you have two discontinuities close to each other, the evanescent modes have not died off yet. In that case, the values in your closed form discontinuity model have to be changed.