# help:microstrip interdigital filter

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#### godsound

##### Newbie level 6
Hi all

I am a freshman in interdigital designing. and now I need to design a microstrip interdigital bandpass filter. I chose symmetric structure with tapped-line input.

I have got even- and odd- mode impedance, but am in trouble obtaining the physical dimensions- width, spacing, length. I have got a bunch of equations from literatures, but I could make them in a clear order. Would any one please give me a design example or tell me how to obtain its physical structure step by step?

Thanks.

#### armsgroup

why you don't use the design wizard in any S/W like genesys or MWO that will be very easier aslo you can prepare the draft design and run the optimizer to get better dim.

#### godsound

##### Newbie level 6
I could obtain the physical dimensions directly by ansoft deisgner now. Besides, by using parameter sweep in sonnet, I also can calculate them step by step now.

But I still don't quite undetstand without software's assitance how I could analytically calculate Spacing for an interdigital filter with equal linewidth and tapped-input lines.

Any one who can explain some details to me? Thanks.

#### biff44

Good for you for questioning what some dubious black box computer programs are telling you!

Basically, an interdigital filter is made up of transmision line resonators. The clasic way of thinking of this is that each resonator has a static capacitance, and that there are mutual capacitances (or coupling factors) between the resonators. By chosing the static and mutual capacitances to coincide with a lowpass filter prototype design, you can get the desired response.

There is a pretty good description of this in section 10.06 of Matthaei, Young, and Jones' Microwave filters, impedance-matching networks, and coupling structures.

#### flyhigh

Hi,

your problem is called transmission line synthesis (start from electrical parameters to get the geometrical ones, the other way is analysis). You can use any transmission line an/sin program available in ADS (Linecalc), MWO (TxLine) or Ansoft Designer (?) to do the synthesis of edge coupled microstrips or striplines. Typically you will have to specify one parameter (w or s) the other is calculated based on Ze and Zo and the substrate deffinition you supplied. TxLine is available for free from AWR site.

flyhigh

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