microstrip low pass filter design issues with sonnet - Page 1
Forum section: electronics / rf-microwave

author: promach
author_rank: Frequent Contributor
timestamp: June 22, 2019, 07:35:03 am
content: I am trying to replicate the microstrip low-pass filter in SonnetLite from the book byJia-Sheng Hong - Microstrip Filters for RF Microwave Applications, 2nd Edition (Wiley Series in Microwave and Optical Engineering) (2011, Wiley)However, the S12 graph shows that it is not a low pass filter at all. Could anyone advise why ?box resonance warning analysis logsCode:Run 1: Sat Jun 22 00:27:07 2019. SL291418.107194.Em version 15.53-Lite (32-bit Windows) on DESKTOP-DG59KC2 local.Project: C:\Users\kevin\Documents\sonnet\15.53-Lite\three-pole microstrip lowpass filter using open-circuited stubs\three-pole microstrip lowpass filter using open-circuited stubs.son.Frequency: 0.001 GHZSonnet Warning EG2680:Circuit has potential box resonances.Circuit: primary structureThe estimated box resonance frequencies are listed below.The calculations assume lossless materials, with a box filled withonly the specified dielectric stackup materials.4.140 GHZ TE Mode 1,0,14.673 GHZ TE Mode 1,0,2Only lowest order modes considered.See the chapter on package resonances in the Sonnet User's Guide.Date: Sat Jun 22 00:27:07 2019Sonnet Warning EG2680:Circuit has potential box resonances.Circuit: left box wall SOC magnetic wall standardThe estimated box resonance frequencies are listed below.The calculations assume lossless materials, with a box filled withonly the specified dielectric stackup materials.4.167 GHZ TE Mode 0,1,14.700 GHZ TE Mode 0,1,2Only lowest order modes considered.See the chapter on package resonances in the Sonnet User's Guide.Date: Sat Jun 22 00:27:07 2019Post-Analysis:Errors detected: 0 Warnings detected: 2.Analysis completed Sat Jun 22 00:27:10 2019.

author: promach
author_rank: Frequent Contributor
timestamp: June 22, 2019, 11:20:36 am
content: Someone helped me to solve most of the problems, and I have a working low pass filter now.But I am still very concerned about thecorrectness of the dielectric layer configuration. Any advise ?I have attached the modified SonnetLite design file in this current post

author: chrisl
author_rank: Regular Contributor
timestamp: June 22, 2019, 05:25:48 pm
content: The original design uses the 1.27 mm thick substrate with the DK of 10.8. You are using the FR4 with the DK of 4.4 but the thickness is 2mm??It should go other way.EDIT: decrease the thickness of the FR4 substrate to ~0.55mm and fine tune the design.

author: promach
author_rank: Frequent Contributor
timestamp: June 23, 2019, 03:50:25 am
content: ok, now I wish to re-design the microstrip low pass filter at lower cutoff frequency at 100MHz, so that I could fabricate the PCB and do the actual PCB testing.Any advice ?The microstrip filter design equations are so complicated.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 23, 2019, 04:44:50 am
content: Remember, a microstrip filter is just using segments of microstrip to approximate an idealized lumped element filter. So go back to your standard LC filter design rules. To make a 1.5 GHz filter into a 100 MHz filter, you need to increase the capacitance and inductances by 15x. So increase the area of the capacitor plates / low impedance microstrip by 15x. increase the length of the inductors / high impedance microstrips by 15x.If that is impractically big, consider changing your design. There isn't usually a need to make 100 MHz filters out of microstrip. Lumped elements are usually quite adequate.Because of the distributed nature of the microstrips, their approximation to idealized L and C values is only valid at low frequency. At high enough frequency it will stop following the desired rolloff, and typically you will have narrow resonances of high transmission. Whether or not that is a problem for you is something you will have to decide.

author: promach
author_rank: Frequent Contributor
timestamp: June 23, 2019, 04:07:13 pm
content: How do I tell if the textbook example is using T configuration or Π configuration ?I have also attached the latest low pass filter SonnetLite design file

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 24, 2019, 11:52:30 am
content: This is a T. The big chunk of copper in the middle is a capacitor, the two thin traces are inductors.

author: promach
author_rank: Frequent Contributor
timestamp: August 23, 2019, 10:36:01 am
content: For the two ports (which are allocated two rectangle patches), I know I can use PCB calculator, but how do I decide the input and output impedance of the microstrip transmission line ?Is VNA always 50 ohm ?So, do I need any impedance transformer microstrip in this low-pass filter design ?

author: promach
author_rank: Frequent Contributor
timestamp: September 05, 2019, 01:15:12 am
content: What is the difference between open-circuited stub and stepped-impedance ?QuoteSee2GHz Microstrip Low Pass Filter Design with Open-Circuited StubI am trying to understand why open-circuited stub has a better S21 response compared to stepped impedance according to theresult at figure 5.4 below:this microstrip equationfor Zo does not depend on length.Could anyone point me to a microstrip equation that describes the relationship between stub length (in this case, 5.78mm) and λ/4 at 5.6 GHz and substrate properties (dielectric constant and thickness) ?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: September 06, 2019, 07:37:58 am
content: promach, this has also been explained to you on reddit. I really think you should try and look up some basic transmission line theory lectures on youtube, or read through the first few chapters of pozar. You are having issues with having some of the root concepts of these transmission lines 'click'. This makes sense - some of these concepts are hard to get around in the beginning, esp. so when you are very used to not thinking about wave behavior.