Electronics World articles Popular Electronics articles QST articles Radio & TV News articles Radio-Craft articles Radio-Electronics articles Short Wave Craft articles Wireless World articles Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs LinkedIn Crosswords Engineering Humor Kirt's Cogitations RF Engineering Quizzes Notable Quotes Calculators Education Engineering Magazine Articles Engineering software RF Cafe Archives RF Cascade Workbook 2018 RF Symbols for Visio - Word Advertising RF Cafe Forums Magazine Sponsor RF Cafe RF Electronics Symbols for Visio RF Electronics Symbols for Office Word RF Electronics Stencils for Visio Sponsor Links Saturday Evening Post NEETS EW Radar Handbook Microwave Museum About RF Cafe Aegis Power Systems Anritsu Alliance Test Equipment Amplifier Solutions Anatech Electronics Axiom Test Equipment Berkeley Nucleonics Bittele Centric RF Conduct RF Copper Mountain Technologies Empower RF everything RF Exodus Advanced Communications Innovative Power Products ISOTEC KR Filters Lotus Systems PCB Directory Rigol RF Superstore San Francisco Circuits Reactel RFCT TotalTemp Technologies Triad RF Systems Windfreak Technologies Withwave LadyBug Technologies Wireless Telecom Group Sponsorship Rates RF Cafe Software Resources Vintage Magazines Thank you for visiting RF Cafe!

LC Low Pass Filter refuses to work properly - RF Cafe Forums

RF Cafe Forums closed its virtual doors in 2010 mainly due to other social media platforms dominating public commenting venues. RF Cafe Forums began sometime around August of 2003 and was quite well-attended for many years. By 2010, Facebook and Twitter were overwhelmingly dominating online personal interaction, and RF Cafe Forums activity dropped off precipitously. Regardless, there are still lots of great posts in the archive that ware worth looking at. Below are the old forum threads, including responses to the original posts.

NOTICE: The original RF Cafe Forum is available again for reading, and the new RF Cafe Blog is an active board.

-- Amateur Radio

-- Anecdotes, Gripes, & Humor

-- Antennas

-- CAE, CAD, & Software

-- Circuits & Components

-- Employment & Interviews

-- Miscellany

-- Swap Shop

-- Systems

-- Test & Measurement

-- Webmaster

RF Burns

Post subject: LC Low Pass Filter refuses to work properly

Unread postPosted: Thu Apr 07, 2005 2:47 pm

Dear all,

I'm very perplexed. I designed a 5th order LC lowpass (Chebyshev) filter to have a cut-off frequency of 108 MHz (the upper frequency of the FM radio band) and have a minimum attenuation of 40 dB at 216 MHz. I went through all the usual procedures like normalising the design and getting the frequency scaling factor etc. The component values I obtained and the circuit layout is as follows: The input is connected between C1 (38 pF) and L1 (115 nH), the other leg of C1 goes the ground. Then the second leg of L1 is connected between C2 (66 pF) and L2 (115 nH), the second leg of C2 also goes to ground. Now, the second leg of L2 is connected between the output and C3 (38 pF), the second leg of C3 also goes to ground. Sorry I can't draw a picture here but its just a basic layout. I chose to build an LC lowpass circuit as I thought there would be too much power dissipation across the resistor in an RC lowpass design? The trouble is that when I built my lowpass filter it behaves the same as a Notch filter. That is, the signal gets attenuated at (approx.) the designed cut-off frequency, but slightly beyond this frequency the signal level shoots up to its original (unattenuated) level. I have used proper components (trimmer capacitors and tuneable coils) and their values are very close to the ideal values listed above. I kept my leads as short as possible, I screened the two inductors in case they couple with each other, I etched some copper away from the ground plate under the inductors in case this might affect the coil Q. None of these steps seem to make any improvement - my LC lowpass filter is still behaving like a notch filter! I have simulated my design on ADS and it works almost exactly as designed. It should be so simple but I just don't know why the thing isn't working in real life! You can probably tell that I'm quite frustrated by my filter (or lack of filter!) so any helpful suggestions will be most welcome.



Post subject: Filter respons

Unread postPosted: Fri Apr 08, 2005 12:03 pm

There are two parts to the operation of a filter:

1. The filter circuit itself, including parasitics, and

2. The "context" - the circuitry surrounding the filter. Since most filters work by reflecting the power that is to be rejected, in the stopband the input VSWR gets really bad.

That means that you generally don't want lots of transmission line between filters and whatever drives them - cable length can affect filter response in odd ways. So: How are you testing this filter?

Good Luck


RF Burns

Post subject:

Unread postPosted: Fri Apr 08, 2005 2:46 pm


Thank you for your reply. I am testing the filter on a Network Analyser. I have also looked at it (my filter) on the spectrum analyser. That is, I input a known signal from the signal generator and sweep through the frequency range and no matter what way I look at it or no matter what I do, it still behaves as a Notch filter. I have built it and re-built it, I have tried different components, I have reduced the length of my leads etc. Now I am going to try an build it one more time. I'll keep you informed.



Post subject:

Unread postPosted: Fri Apr 08, 2005 4:26 pm


I really don't underatand why you use tuneable coils? Why not use fixed value wire wound coils, like those manufactured by Coilcraft.

From the description of your problem sounds like you have some sort of resonance frequency in your filter. Could be that one of your coils is working at its SRF (Self-Resonance Frequency). Use fixed value components with high Q as possible for the inductors (as their Q is the dominant).

What is the BW of your filter is it 20MHz (88-108MHz)?

Build your filter on a board with good ground plane. Use vias to connect between the GND nodes to the ground plane. You should also take into consideration the Er and thickness of the substrate (even tough this is a relatively low frequency, this might affect the filter's performance), the Er of the filter will also dictate the 50 ohm trace width.



Post subject:

Unread postPosted: Sat Apr 09, 2005 9:57 am

I agree with the gentlemen above. At this low frequency the simulator should be "dead nuts" on the response. Most likely, the tuned coil is not behaving as you think it is, as suggetsed. I bet if you replace the tunable components with fixed components, like coilcraft coils and ATC caps, your filter will behave as expected.


RF Burns

Post subject:

Unread postPosted: Tue Apr 19, 2005 12:09 pm

I'm happy to report that I eventually managed to get my LC lowpass filters working, thanks in no small part to the suggestions made in response to my question. I took the advice of using wire wound ceramic chip inductors rather than tuneable coils. This change, along with keeping copper/wire lengths to a minimum and establishing very good grounds, helped to make the filter function properly. Once again, many thanks to those who contributed.

Respectully yours

RF Burns



Post subject:

Unread postPosted: Wed Apr 20, 2005 6:20 am

You welcome. Nothing compares to experience!

Posted  11/12/2012

Amplifier Solutions Corporation (ASC) - RF Cafe
TotalTemp Technologies (Thermal Platforms) - RF Cafe
Lotus Communication Systems Modular RF Component Building Blocks - RF Cafe
Windfreak Technologies Frequency Synthesizers - RF Cafe
RF Superstore (RF Components) - RF Cafe

Please Support RF Cafe by purchasing my  ridiculously low−priced products, all of which I created.

These Are Available for Free


About RF Cafe

Kirt Blattenberger - RF Cafe Webmaster

Copyright: 1996 - 2024


    Kirt Blattenberger,


RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website: