a 0.1 rmada length open microstrip serve as an capacitor ?

[andrew_luo]

a 0.1 rmada length open microstrip serve as an capacitor ? while if the ground of the microstrip is taken, it work as an inductance? am i right ,what is your opioion? and the width of the microstrip is nearly 100 mil.

 

[gecky]

Re: a 0.1 rmada length open microstrip serve as an capacitor

Use Line Cal in ADS or AppCAD by hp/agilent. The software automatically calculates the equivalent impedence based on operating freq etc....

For X lamda open cct stub, look at Smith chart. Rotate "twd source" from the infinite impedence pt by X lamda. For 0.1 lamda, u get -j1.42 as the impedence. If shorted stub then u start from the sero impedence pt.

Correct me if i'm wrong

 

[DJ]

Re: a 0.1 rmada length open microstrip serve as an capacitor

Open stub Lamda/4 length at F0 presents short impedance. If the length is shorten than Lamda/4 then it acts as a capacitor, and for L> Lamda/4 as an inductor.
The capacitor equivalent circuit value is a function of the Characteristic impedance (Width of the line for a known substarte). If you tune the length you can see on Smith chart it changes from capacitor(X<0) to an Inductor (X>0) and at Lamda/4 it resonants.

 

[andrew_luo]

Re: a 0.1 rmada length open microstrip serve as an capacitor

thanks for answering,
we can use some simulate tools to caculate the impedence,ADS,hfss,etc.it is easy to simulate the microstrip,however,if the line is just single layer copper, what is the characteristic of the line?i think is not the same as a microstrip. when the length is far shorter than lamdar,it is a inductance,but i dont know the case while the length is near or longer than lamdar.

 

[djalli]

Re: a 0.1 rmada length open microstrip serve as an capacitor

thanks for answering,
we can use some simulate tools to caculate the impedence,@DS,hfss,etc.it is easy to simulate the microstrip,however,if the line is just single layer copper, what is the characteristic of the line?i think is not the same as a microstrip. when the length is far shorter than lamdar,it is a inductance,but i dont know the case while the length is near or longer than lamdar.

Do you have any exact data for this microstrip?

Substrate height?
Substrate width?
Substatre loss tangent?
Substrate permeability?
etc known?

afer ADS LineCalc I found is a good tool then!

from there things are much easier and based on frequency number or range you can find what you want or approximate real good them. Of course after you find out from above the Z0 characteristic impedance then you know periodicity locations/lenghts where it behaves like cap, or inductor.

djalli

 

[DJ]

Re: a 0.1 rmada length open microstrip serve as an capacitor

andrew_luo,
How do you implement the open stub? If it is printed on a substrate and you moved the ground where is the ground now? If you moved it farther then the characteristic impedance is higher now. It is still capacitor and not inductor.
If instead the open stub is such as an open wire then again it has a capacitor equivalent circuit. There are several software to solve it but can you explain exactlly the way it is implemented?

 

[andrew_luo]

Re: a 0.1 rmada length open microstrip serve as an capacitor

How do you implement the open stub? If it is printed on a substrate and you moved the ground where is the ground now? If you moved it farther then the characteristic impedance is higher now. It is still capacitor and not inductor.
If instead the open stub is such as an open wire then again it has a capacitor equivalent circuit. There are several software to solve it but can you explain exactlly the way it is implemented?

the way that i implement open stup is simple, just remove the copper on the bottom of substrate, now the ground is not very far since the whole board is nearly 18*22mm,

why the characteristic impledance is higer if the ground is far away?
why it is still a capacitor instead of incuctor?
can you tell me the tool that can simulate this problem

 

[andrew_luo]

Re: a 0.1 rmada length open microstrip serve as an capacitor

DJ and anyone who knows the above questions,plz come in. This is easy but critical issue.

 

[DJ]

Re: a 0.1 rmada length open microstrip serve as an capacitor

A TR line has aquivalent LC model (LPF with high cut off freq). As you move farther the ground then the aquivalent capacitor goes down (such as two capacitor plats). The characteristic impedance is Z0=(L/C)^0.5.
Means as C goes down Z0 goes up.
For a wide band capacitor you might use open stub with low Z0. For an inductor use high Z0 line.
Now, any microwave line has somewhen its ground (Thing the location on the ground that the RF current has 180 deg compared to the main line for a specific location Z0 impedance (you might have to define), and if it is less than Lamda/4 lond it acts as a capacitor (Use a Smith Chart to see). If you have a crazy dimention (a very wide line)it even might resonant such as Antena.
You might use momentum EM simulation to get the S parameter of the circuit and if it is more complicated use an HFSS instead.