Practical Filter Design

Bob Simpson

Presentation Outline

• What does “practical” filter design mean?

• Picking a substrate, easy, right?

• How big was that?

• Exploring a design example, an “RF Brick”

• Designing the filters, do simulation tools really work?

• Steps to completion

• Verifying your design

• Wasn’t that easy?

• Demonstrations & Questions?

What does “practical” filter design mean?

• Select the proper substrate (application driven)

• Select the correct packaging (waveguide effects)

• Select the proper simulation tools (not all are created equal)

• Choose the best PCB shop (some are better than others)

• Design for manufacturing (Monte Carlo Er and dimensions)

Picking A Substrate

• Many substrates to choose from, here is a link to a large list:

http://www.pcbmaterialworld.com/pdf_downloads/PCBMWStocklist.pdf

The selection process usually means choosing these items

• Dielectric Constant

• Dimensional Stability

• Thickness

• Loss Tangent

• Application Dependencies (microstrip, stripline, etc)

How big was that?

• Most designs are constrained by legacy packaging

• Usually integrated with other circuitry

• Etch-a-Sketch, Tape Cad or 3D drafting?

The “RF Brick”Design Example

Design Example Requirements

• Design an RF Band Pass Filter for a Ku Band Down Converter

• Center Frequency 11,100MHz

• 800MHz System Bandwidth Minimum

• 50dB LO Rejection at 9750MHz

• 60dB Image Rejection at 8800MHz

• 5dB or less insertion loss

• Legacy hardware driven

The RF Input Filter goes here. The size is 1.160” by 0.360” by 0.010”

The Legacy Hardware , the “RF Brick”

Top cover touches the PCB where ever gold is exposed

The “RF Brick” side view

The “RF Brick” Cover

The lightest color grey is the metal that contacts the gold on the PCB

The “RF Brick” PCB with cover in place

This illustrates the wave guide beyond cut off of the filter shields

Waveguide Beyond Cutoff?

• Circuit shielding should include waveguide below cutoff when possible to minimize undesired propagation

E-plane

H-Plane

Bad locations to split waveguide

Good location to split waveguide

Designing the filters Do simulators work?

• Easier to originate filter designs in linear simulator

• Optimize performance in EM simulator

• Be aware that the results are often different

• Linear Simulator typically does not include circuit parasitic’s - uses formulas and calculates the results based on dimensions - some parasitic’s can be added manually

• EM Simulator typically does include parasitics - fields from adjacent components are considered in analysis - metal thickness and losses are included • Shield walls are included in EM simulations, not in Linear simulations - waveguide beyond cutoff is included in analysis

Do Simulation Tools Work?

Linear Simulator Comparison

Comparison of several linear simulations for the same wideband filter design

Simulation vs. Measured

Measured results indicate that some simulators are very close

Steps to

Completion

Designing the RF Filter

• Assumption is that substrate and filter topology have been chosen

• First do a linear simulation to optimize the filter design

• Export a DXF file and import it into the EM simulator

• Set up EM simulator to include shield walls and cover

• Do the EM simulator, time to results dependent on set up

• Compare Linear to EM simulation results

• Modify filter artwork in the EM simulator if required

• Do Monte Carlo on dimensions in EM for etching tolerance and substrate

• Export EM DXF file for PCB layout processing

Band Pass Filter Design

This is the most dimensionally accurate filter representation and can be imported into most PCB design tools to get Gerber files needed for PCB process by a typical board shop.

This is the DXF output from Agilent’s Genesys for the Ku Band filter in our example. This DXF file will be imported into Sonnet Pro for final analysis.

Analysis of the Genesys DXF output file in Sonnet Pro

Band Pass Filter Design

It can be seen that the Genesys simulation file is high in frequency

After optimization of the Genesys DXF output file in Sonnet Pro

Band Pass Filter Design

Measured Results

Filter Notes:The filter required the addition of 3 mils to each open stub, the 1st pass simulation has all open stubs increased in length by 3 mils. The frequency was correct. However, the BW is borderline narrow. For 2nd pass, decrease the coupled line spacing by 2 mils (only the center, or widest spaced pairs, not the end pair as the Return loss looks good) and re-sweep. Closing the gap improved performance but needs more. The 3rd pass will narrow the coupled line gaps on the inner wide gapped lines by another 2 mils. The 3rd pass is still not quite right. The 4th pass will move the two outer coupled line gaps 1 mil closer. THis still is not right. Slightly low in frequency and not enough bandwidth. The 5th pass will move all coupled lines closer by 1 mil and remove 1 mil from all open stubs.. let's get radical! The 5th pass looks good!

Band Pass Filter Design

I like to write down the various steps and changes I make during both the linear simulation as well as the EM simulation. This helps to remember what “tweaks” you’ve made as you step through the various iterations optimizing the filter. This is a part of those notes from the original filter design work. Although I have not, one could make the dimensional changes to the linear simulator file based on the EM simulation results for a reverse comparison.

EM Simulation Notes

The Sonnet simulations included the metal thickness of 1 mil (1/4 oz. copper with nickel/gold, no extra copper for vias). Grid set to 1 mil. Etching tolerance 0.5 mil. 20 mil thick Rogers R5880 as mentioned previously. Typical simulation time for each pass is approximately 1 hour when using metal thickness and a 1 mil grid and setting the mesh density to mid-point. I tried maximum mesh density, it required approximately 1200MB of memory, after 2 hours there had been no progress in the simulation so it was stopped and the mesh density reduced. I also used the memory saver function. It should be noted again that there is always a dimensional difference between the exported DXF (from Genesys) and the Sonnet analysis layout. With a grid of 1 mil this error is about 0.25 mil worse case. Use the Sonnet generated DXF that has been “tweaked” as the PCB artwork given to your PCB designer. This should insure that what Sonnet analyzed will be what the PCB is made with. Additionally, using a grid of 0.5 mil is a slight improvement in accuracy between the Genesys DXF and the Sonnet simulation artwork, but at the expense of a large increase in simulation time! EM simulation using Sonnet taxes the capabilities of a PC on which it is run! These simulations were run on a PC which is an Athlon 64 X2 Dual Core running at 1900MHz with 4GB Ram running at 600MHz and a 300GB HD. This is probably a minimal system for this kind of work!

Verifying

Your Design

The Test Fixture

This is the filter test fixture which can be used to measure your filter performance. It requires a “thru” board to measure the insertion loss of the fixture and it’s two 3 dB pads.

The Test Fixture

Measurement results of the filter test fixture with the “thru” board installed.

The Test Fixture

The filter test fixture with a test filter installed.

Filter Measurement Results

Insertion Loss is approximately 3dB after subtracting the fixture insertion loss

In Summary, Wasn’t that easy?

• Designs are often driven by the mechanical design

• Chose your substrate based on application and performance trade offs

• Do your simulations in a linear simulator first to optimize overall performance

• Finalize your simulation in an EM simulator so parasitics are included

• Make any performance corrections to artwork in the EM simulator

• Use exported DXF artwork output to PCB designer to retain accuracy

• Build test boards and fixture for first pass designs

• Use calibrated VNA to measure your test boards to confirm performance

• Integrate your final filter design into your overall design

Demonstrations

&

Questions?