How to design multilayer bandpass filter - RF Cafe Forums

 Post subject: How to design multilayer bandpass filter

Posted: Thu Dec 03, 2009 6:15 am 

helloo, im still beginner in RF design, i need some ideas related to the multilayer substrate,fabrication process and what type of substrate that compatible for multilayer design

Normally in previous design,resonator was used in the middle of the layer while the grounding part was located at the bottom and top of prototype.i need some information especially formula to design the resonator

 Post subject: Re: How to design multilayer bandpass filter

Posted: Fri Jan 22, 2010 11:36 pm 

The technology you'd use varies depending on what frequency range you need to cover.

Filter design is a specialized field - you might find the books "Theory and Design of Microwave Filters" by Ian Hunter, and "Microstrip Filters for RF/Microwave Applications" by Hong & Lancaster to be good guides.

For microstrip in general, the books by Ramesh Garg are good, and widely available at university libraries and technical bookstores.

If those are too technical, then you have a much longer learning curve ahead of you than you expect.

Good Luck!

 Post subject: Re: How to design multilayer bandpass filter

Posted: Mon Jan 25, 2010 12:40 pm 

Hi
Whilst Fred is right, there may bea short cut if you are in a hurry, plus you did not reveal too much about how far into this arcane art you have already explored.
I have used ANSOFT student version to design a band pass filter like this but not yet built and tested it.
This tool needs just chosen type, system impedance, Chebychev ripple, and bandwidth, and gives Z00 and Z0e values (or coupling coefficients) which can then be used to compute strip widths and spacings.
This can be downloaded in the SV for free.

_________________
At bottom, life is all about
Sucking in and blowing out.

 Post subject: Re: How to design multilayer bandpass filter

Posted: Mon Jan 25, 2010 1:03 pm 

Nubbage,

Thanks for the suggestion of Ansoft - some of the synthesis programs have gotten good, and so Mohdariff may get lucky with that approach. Using it will at least help bound the problem.

-Fred

 Post subject: Re: How to design multilayer bandpass filter

Posted: Wed Jan 27, 2010 12:35 am 

thank you for all information. I'm very pleased with your cooperation. actually i have been develop my own design and its around 80% to complete.Our problem is how to setup the SMA connector on multilayer design because the input and output port located in the between of 4 stack up layer. did you have any suggestion or idea to setup the connector. for your information our design consists of 4 layer.the first and last layer are the grounding while the resonator in the middle.the whole body of the connector also grounding accept the pins, so how to ensure that the grounding part on the design and connector body interact each other

 Post subject: Re: How to design multilayer bandpass filter

Posted: Wed Jan 27, 2010 3:20 am 

The standard approach is to create a rectangular frame by silver-soldering copper bars together to form a shallow box, deep enough to accommodate the stack of 4 layers. On the top and the bottom is a copper plate, and if needed, a soft copper foil gasket that picks up and grounds any vias installed. The frame also carries the input and output SMA sockets. These have a tongue of copper foil soldered to the center pin such that when the sandwich is clamped together, the foil makes good contact with the grounded matching element (the top and bottom layers). The entire assembly can be clamped together by screws that pass through from the top face of the frame through to the bottom, either clamped by nuts or by tapping the lower bars to receive the screws.
All the above is one-off labour intensive and not suitable for mass production. I have seen a medium-volume production equivalent produced in a computer-machined aluminum die-cast box with a single top lid. There was no special surface treatment of the aluminum, which I found surprising, but it worked fine at 3GHz.

_________________
At bottom, life is all about
Sucking in and blowing out.

 Post subject: Re: How to design multilayer bandpass filter

Posted: Fri Jan 29, 2010 8:50 am 

what is the function via hole (plated through hole) on multilayer design.for example our filter consists of 4 layer,the top and bottom is a ground plate while the resonator located at the core of the substrate that is FR4.so, should I create the plated through hole with a pad on the outer layer to interface it.

 Post subject: Re: How to design multilayer bandpass filter

Posted: Fri Jan 29, 2010 10:05 am 

There is a strong risk of creating spurious cavity-mode resonances when you enclose the filter in a box. The vias will minimize radiation beyond the confines of the traces. Ideally there should be a zone created around the traces on all layers of the sandwich structure, with solid grounding being done outside of that zone, and just the resonator traces being left inside the zone. All grounds should then be tied together with short low inductance via bonds.
This will minimize the EM field value beyond the resonator structure, and thus the launching of "box resonator" waveguide-type modes.

_________________
At bottom, life is all about
Sucking in and blowing out.

 Post subject: Re: How to design multilayer bandpass filter

Posted: Fri Jan 29, 2010 10:20 am 

"short low inductance via bonds". how to identify and measure that

 Post subject: Re: How to design multilayer bandpass filter

Posted: Sun Apr 04, 2010 7:26 pm 

I wouldn't use FR4. Its too lossy. Also the FR4 can vary quite a lot in dielectric constant between one manufacturer and another or even between different batches from the same manufacturer.

You might want to think about airline/solid rod type combline and interdigital filters which are not difficult. There is a simple to understand article on combline and ID filter design, and also a spreadsheet on:
http://www.rfshop.co.uk/blog/Filters/