×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# 120 mhz bandpass filter

## 120 mhz bandpass filter

(OP)
I'm working in a 120 mhz bandpass filter, i've designed it through a high-pass after a low-pass filter.The problem is that although ive simulated it with and without the pads and microstrip lines, whenever i make the PCB, i never obtain the desired results.Ive tried to imulate it with RFSIM99, LIBRA and MICROWAVE-OFFICEE.The specification for my filter are:

BW1dB=14 MHz
BW3dB=16 MHz
bandpass s21>-2dB

I'm using SMD lumped elements on a miscrostrip structure PCB.
Can anybody help my?any idea?what am i doing wrong?What can i do to obtain more or less the same resaults as in the simulations?

### RE: 120 mhz bandpass filter

Hi!

Analysing of microstrip structure in software like LIBRA or others has always a little error connetcted to the analytical model of analysed structer.
To have more accurate results You should use some 2.5D simulators like MOMENTUM, SONNET etc. And also You have to remember about very detailed and accurate defining of dielectric material. In most cases it is the reson of all errors. Good luck.

### RE: 120 mhz bandpass filter

It is generally best to do an actual bandpass, than a lp/hp combination, unless we are talking about huge passbands.  Usually the filter departs from expected performance because 1) there is excess capacitance in the board layout acting in parallel with the lumped capacitors, 2) there is extra via hole or ground loop (you did use a good ground plane, didn't you) that changes the apparent series inductance used, 3) The component tolerances for values are inadequate 4) the component Q is too poor to achieve the desired roll offs.

### RE: 120 mhz bandpass filter

I made a series of these using fixed inductors and tunable capacitors for Cable TV. Without the tuning capability of capacitors, it's difficult to achieve perfect results.
We used Touchstone for analysis, results were pretty close to the Touchstone outputs.
kch
PS:biff44 is correct, do an actual bandpass design, fewer parts and better results.

### RE: 120 mhz bandpass filter

Theoretically, a bandpass filter has twice the roll-off as a lowpass or highpass filter.  So you can get away with fewer filter poles.

### RE: 120 mhz bandpass filter

Hi,

Hipass-Lowpass approach is used when you have a very broadband or steep rejection on both skirts of the filter.

For the spec that is given, your approach is correct. If you use a bandpass design, then the ratio between L&C in series will be very hard to realize (e.g. 1pf/1000nH).

Use Lowpass & hipass with one finite transmission zero (finite pole) on each side of the BPF will solve the problem.

Krytar

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

#### Resources

Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a partâ€™s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!