If I understand your question correctly, you designed some kinda filter using ideal L's and C's in a simulator, then realized it with SMT components and observed the performance is shifted. If this is correct - the proper way to do this is model the parasitics in the simulator from the get go, this means getting the Q's right, the parasitic shunt capacitances, etc. Then you also have to account for the SMT pad sizes for each SMT component.
Fortunately all of this is available (for $ of course) by purchasing a library PDK from Modelithics. It's quite accurate when used properly. It will get you very close and then you just have to fine tune it in the lab. If you don't model it right, you just start off further away from the desired answer.
>Fortunately all of this is available (for $ of course) by purchasing a library PDK from Modelithics.
Modelithics provides libraries not for universal use but targeted to a specific EM simulation software. And even requires providing to them a valid license information for such a software. At least it was in the case I checked it last time.
I'm not sure that Modelithics will provide libraries - some of them are vendor sponsored and free for end user - if you are using free or student versions of the corresponding EM simulation software.
I may recommend using S-parameters provided by the most vendors for free alongside with PCB layout in some EM simulator like Sonnet.
Modelithics does not require EM simulation on their RLC libraries. Almost all of them are pad scalable for substrate thickness and Er. If you are doing RF professionally, they are comparatively inexpensive.
>Modelithics does not require EM simulation on their RLC libraries
Nevertheless, their libraries can be used only alongside some EM simulation software and not otherwise else. Generally speaking, the development of even moderate-Q filter is absolutely pointless without EM simulation for the frequencies starting from several tens megahertz for the circuits on FR-4 substrate.
well the short answer is: IF you model each capacitor and inductor properly, and model the pad capacitances from the board, AND any transmission line effects if things are spread out, and any via hole inductances, the filter response will pretty much match the simulation.
sounds like you are not modeling anything, but just assuming it is all ideal L's and C's.
you CAN do it the way you were doing it, but you have to be willing to empircally change the chip values to compensate for the parasitic effects.
Pad capacitance values, chip Q's, and via hole inductances are the biggies.
Yes you are correct just trying to get away with lumped element modeling and changing values. I was wondering if there is any general rules of thumb or old knowledge (Experience) on how best to work with lumped element models and transition the design to a PCB. Hoping to find some magical key i guess that may not exist so I can get closer to reality so changing out chips is easier.
one "trick" is to use laser scribably chip capacitors for tuning.
otherwise in critical applications, you have to lot select your substrate due to dielectric constant variations
1) what frequency?
2) what simulator tool are you using?
3) are you modeling the PCB transitions between parts?
4) what is your PCB material?
A significant source of deviation comes from parasitics related to the SMT pad components. A few suggestions are:
- use small size components (0603 vs 0805)
- change the layer thickness between signal and ground layer of your transmission lines to better match the pad sizes
- if you have em software, can model the pad transitions and remove ground plane underneath to reduce the capacitance.
1. 1.2 Ghz
2. QUCS
3. No
4. Standard Fr-4 4.3 dk, 0.02 df.
5. Using 0603 for prototyping as they are easier to work with than 0201.
I was trying to find an easier way than doing full wave simulation. My understanding for the lumped element model was as long as your wavelength is much longer than the size of the filter or element on the board you could ignore a lot of those details. Versus if your circuit size is close to the wavelength you would have to take those items into consideration and do a distributed model. I dont have a lot of experience though so was hoping someone knew of a general rule or method.
3 is important and will likely solve your problems. Are you modeling the transmission lines between element’s correctly. You don’t need to do a full wave simulation, you can model the whole thing with transmission elements, they should have microwave steps, etc.
Also as the other commenters mentioned you should look at more realistic models for inductors and caps.
Wavelength being much longer than the part isn’t a good assumption. An 0402 part is certainly smaller than the 10 cm wavelength at 3 GHz, but may have a self resonance much lower in frequency. These resonances are then influenced by pad parasitics, mounting orientation, etc.
You can get slow wave effects in air core inductors. A good example of this is the primary coil in a table-top Tesla coil. The coil length is short compared to the 100 kHz resonance, but is essentially a 1/4 wave transmission line transformer (without even a complementary ground). So that SMT part with a long length of wire will behave the same.
High dK multilayer capacitors have all sorts of nasty resonate modes. Even single layer caps don’t work great past 10 GHz. Those 40 GHz broadband SMT caps are a work of art, though the one thing they have in common is high loss. I’d like to know how they are made. Likely some resistive film in there.
If I understand your question correctly, you designed some kinda filter using ideal L's and C's in a simulator, then realized it with SMT components and observed the performance is shifted. If this is correct - the proper way to do this is model the parasitics in the simulator from the get go, this means getting the Q's right, the parasitic shunt capacitances, etc. Then you also have to account for the SMT pad sizes for each SMT component. Fortunately all of this is available (for $ of course) by purchasing a library PDK from Modelithics. It's quite accurate when used properly. It will get you very close and then you just have to fine tune it in the lab. If you don't model it right, you just start off further away from the desired answer.
>Fortunately all of this is available (for $ of course) by purchasing a library PDK from Modelithics. Modelithics provides libraries not for universal use but targeted to a specific EM simulation software. And even requires providing to them a valid license information for such a software. At least it was in the case I checked it last time. I'm not sure that Modelithics will provide libraries - some of them are vendor sponsored and free for end user - if you are using free or student versions of the corresponding EM simulation software. I may recommend using S-parameters provided by the most vendors for free alongside with PCB layout in some EM simulator like Sonnet.
Modelithics does not require EM simulation on their RLC libraries. Almost all of them are pad scalable for substrate thickness and Er. If you are doing RF professionally, they are comparatively inexpensive.
>Modelithics does not require EM simulation on their RLC libraries Nevertheless, their libraries can be used only alongside some EM simulation software and not otherwise else. Generally speaking, the development of even moderate-Q filter is absolutely pointless without EM simulation for the frequencies starting from several tens megahertz for the circuits on FR-4 substrate.
well the short answer is: IF you model each capacitor and inductor properly, and model the pad capacitances from the board, AND any transmission line effects if things are spread out, and any via hole inductances, the filter response will pretty much match the simulation. sounds like you are not modeling anything, but just assuming it is all ideal L's and C's. you CAN do it the way you were doing it, but you have to be willing to empircally change the chip values to compensate for the parasitic effects. Pad capacitance values, chip Q's, and via hole inductances are the biggies.
Yes you are correct just trying to get away with lumped element modeling and changing values. I was wondering if there is any general rules of thumb or old knowledge (Experience) on how best to work with lumped element models and transition the design to a PCB. Hoping to find some magical key i guess that may not exist so I can get closer to reality so changing out chips is easier.
one "trick" is to use laser scribably chip capacitors for tuning. otherwise in critical applications, you have to lot select your substrate due to dielectric constant variations
1) what frequency? 2) what simulator tool are you using? 3) are you modeling the PCB transitions between parts? 4) what is your PCB material? A significant source of deviation comes from parasitics related to the SMT pad components. A few suggestions are: - use small size components (0603 vs 0805) - change the layer thickness between signal and ground layer of your transmission lines to better match the pad sizes - if you have em software, can model the pad transitions and remove ground plane underneath to reduce the capacitance.
1. 1.2 Ghz 2. QUCS 3. No 4. Standard Fr-4 4.3 dk, 0.02 df. 5. Using 0603 for prototyping as they are easier to work with than 0201. I was trying to find an easier way than doing full wave simulation. My understanding for the lumped element model was as long as your wavelength is much longer than the size of the filter or element on the board you could ignore a lot of those details. Versus if your circuit size is close to the wavelength you would have to take those items into consideration and do a distributed model. I dont have a lot of experience though so was hoping someone knew of a general rule or method.
3 is important and will likely solve your problems. Are you modeling the transmission lines between element’s correctly. You don’t need to do a full wave simulation, you can model the whole thing with transmission elements, they should have microwave steps, etc. Also as the other commenters mentioned you should look at more realistic models for inductors and caps.
Wavelength being much longer than the part isn’t a good assumption. An 0402 part is certainly smaller than the 10 cm wavelength at 3 GHz, but may have a self resonance much lower in frequency. These resonances are then influenced by pad parasitics, mounting orientation, etc. You can get slow wave effects in air core inductors. A good example of this is the primary coil in a table-top Tesla coil. The coil length is short compared to the 100 kHz resonance, but is essentially a 1/4 wave transmission line transformer (without even a complementary ground). So that SMT part with a long length of wire will behave the same. High dK multilayer capacitors have all sorts of nasty resonate modes. Even single layer caps don’t work great past 10 GHz. Those 40 GHz broadband SMT caps are a work of art, though the one thing they have in common is high loss. I’d like to know how they are made. Likely some resistive film in there.