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Strange problems with microstrip filter

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DimaA

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I've designed a microstrip bandpass filter in range 3.2-3.9 GHz using ADS. The substrate is FR4 (Er=4.7,tand=0.01,h=1.6mm). The simulation run ok and everythinng seemed to be ok. But when I built the filter and started to measure I discovered few strange things.
First, the whole filter response was shifted about 300MHz right: center frequency is now 3.8 GHz instead of 3.5GHz, and the bandwith was about 300MHz instead of 700MHz.
Can you explain this and what can be done to predict these things when designing?
 

Most likely the substrate Er is NOT what you thought it was.

You need to get some material, measure its Er (some use a simple circuit, like a 1 pole resonator), and when you are happy with the Er then make the full circuit with the SAME LOT NUMBER material.


It is possible that you did not properly model the open circuit end and stepped width discontinuity effects.

If you are desperate to ship some product, you might be able to paint thin layers of varnish or conformal coating or lay a thin dielectric sheet over the entire filter to move the frequency down. The varnish acts to raise the effective Er of the substrate, thereby lowering the frequency of the measured passband.
 

Thanx for reply.
I've actually thought about Er, but I probably didn' realize that could be a serious problem. What about change in bandwith? Simulation of lower Er actually moved the response to right, but it didn't change the bandwith. Is there an explanation of this?
 

FR4 is not a proper substarte for your frequency
also checkif the width and gaps between lines of your circuit are the same as your design
good luck
 

The Er of of this material is not well controlled and also will be very lossy at this frequency.
 

try Er = 4.3 or 4.29 ... for ur frequency...or use better materials like teflon
 

If you need low cost material, there is one called GTEK that is slightly better than FR4 in the loss area and costs about the same. The good feature is that the er is tightly controlled.
 

Just one more comment on FR4. The loss factor at these ferqs are about 0.025 - 0.035 range. It may also drastically change the design. And perhaps the Er may be down to 3.8 - 4 range too.
g579
 

I need to k now something more about your

method of testing filter. When did you find

errors ? in which step of the project ?
 

Also, get it under a proper measuring microscope and see if it is actually etched per your design. Without proper callouts (and often even with them) board houses can be off by +/- 2-3 mils in line and space. Overetching would lead to higher center frequency and lower BW.

Lastly, models are not perfect. If you have access to HFSS, Momentum, or even Sonnet it is worth running your design through those before fab. My filters on RO4350 usually end up a little narrower and a little off frequency. I end up having to first do an iteration or two to get the CF and BW right, then another couple iterations to tune in the match (I usually only have to adjust the tap point of my first resonator 5-10 mils to get a decent match).
 

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