My First Filter Design - RF Cafe Forums

 Post subject: My First Filter Design

Posted: Tue Jun 28, 2005 8:16 am 

My first design task was to design a Quadraplexr which is a filter with single inpuut and 4 outputs (different frequencies). This filter had total of 42 components (capacitos and inductors), and was made out of 4 sections - each section was a filter of a different frequency. the lowest frequency was around 20MHz and the gighest was around 420MHz.

I designed this Quadraplexr by using an All-Pole topology in order to reduce the number of values for the capacitors and inductors. Once I was satisfied from the simulation results, I began to implement the filter. I used a copper board as GND plane and on top of it I soldered the capacitors and inductors in the same format as they were sketched in the schematic. Hence the filter was built "in the air" without the effect of the dielectric constant of the PCB, which formed the GND plane. The filter worked exactly like in the simulation. I was very happy because the design, which was rather not trivial worked at first shot. Then I asked another colleague in my team to design a PCB for it. He took this mission and designed a FR4 PCB with thickness of 30 mil.

I assembeld the componenets on the PCB and was shocked to find out that the output frequencies were compleltely different than before. The reason was of course the addition of capacitance by the FR4 substrate.

I modified the values of one filter within the Quadraplexr: Reduced the values of the shunt capacitors by 20% and the output characteritic retuend to be very similar to the when the filter was built "in the air". I did that to the other 3 sections.

From this day and on, I added the substrate properties to the simulation including via holes, and realized how much exact modeling is important to get accurate simulation results. I am doing that even in relatively low frequencies (below 100MHz) just to be careful and avoid "surprises"!!

[/quote]

_________________
Best regards,

- IR

 Post subject:

Posted: Tue Jun 28, 2005 8:28 am 

Why did you post this here instead of in the Circuits forum? Maybe it's a language thing. Kirt, you should move it out of here.

 Post subject:

Posted: Tue Jun 28, 2005 9:09 am 

I think that this story has nothing to do with Circuits forum. This is an experience I had at my work from which everybody can learn + it was a funny experience for me as a novice engineer. The Circuits forum is intended for people who have questions and raise problems they have regarding circuit design.

_________________
Best regards,

- IR

 Post subject:

Posted: Tue Jun 28, 2005 9:22 am 

You are right. I apologize for not reading it thoroughly. Kirt can you remove my stupid post?

 Post subject:

Posted: Fri Jul 15, 2005 2:09 am 

Where to? Kirt, do you have a place to move self-proclaimed-stupid-posts?

 Post subject:

Posted: Fri Jul 15, 2005 7:43 am 

Greetins Another Guest:

I do not have anywhere to store "self-proclaimed-stupid-posts;" about all I can do is delete them. I hesitate to delete just parts of a thread because then visitors will have reason to suspect me of selectively removing responses to manipulate the forums. In the past, I have locked entire threads, and I regularly go through and delete the spam ( ) posts. In a couple cases, I deleted content at a poster's request, and made a note in it's place stating so.

It appears maybe this case applies since the poster requested it. However, since the poster is just an anonymous "Guest," I will let it stand unless the ffended party, "IR," specifically requests that it be removed.

Is that confusing enough for you ???


- Kirt Blattenberger

 Post subject:

Posted: Fri Jul 15, 2005 8:24 am 

Greetings Kirt,

I am not offended by the thread of Guest, as he immideatly after recognized his mistake and even "punished" himself by naming his post stupid.

Have a nice day you all!!

_________________
Best regards,

- IR

 Post subject: Re: My First Filter Design

Posted: Tue Aug 30, 2005 9:09 am 

[/quote]

For this very reason I'm a big fan of coupled resonators since they readily absorb capacitance to ground. I usually begin a design by using the famous book written by Zverev (they have predistorted "k and q" tables that include finite Q resonators). Using this as my starting point I'll then optimize the implementation of the filter on it's substrate using a combination of HFSS and ADS. HFSS handles (models) all of the unwanted coupling and ADS can optimize very quickly. I've found thus method to work well.