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"Notch filter" question for those RF experts - RF Cafe Forums

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Post subject: "Notch filter" question for those RF experts

Unread postPosted: Sun Jun 26, 2005 3:03 pm

Hi, I have never designed a notch filter before. Now I have come into a problem where positive feedback occurs within my amp chain. I wonder if a notch filter can attenuate that osc frequency.

So if you can point me to any info such as "notch filter for RF:101" :-;

I would appreciate very much,





Post subject: Oscillation & Notch Filter

Unread postPosted: Mon Jun 27, 2005 12:46 am

You would do better to understand the cause of the oscillation.

Unwanted feedback is obviously happening - but adding a notch filter will in general only move the frequency of the oscillation around.


1. Are all the stages in the chain stable independently

2. Have you neutralized or unilateralized every stage?

3. Is there a way the input of the chain can "see" the output?

4. Does the simulation oscillate? What sort of margin do you expecct?

Good Luck!



Post subject:

Unread postPosted: Mon Jun 27, 2005 8:13 pm

Yes. To find out why the amplifier is oscillating is a good idea.

If you are sure that some harmonic on the output is causing oscillations by transmition from the output to the input of the amplifier, than notch filter can help. But it is not the best solution. Better solution is to make your amplifier stable even if this unwanted harmonic is present.

Notch filter can be explained as a simple serial LC resonator (tuned on your unwanted signal) put in paralel with the transmission line... Search the internet for more details, it is not a problem.




Post subject: Notch Filter Design Software

Unread postPosted: Wed Jul 13, 2005 1:06 pm


You can download free Student Version (SV) of ansoft designer, link attached:


Click on "downloads" from the top tap, then choice "Download Ansoft Designer SV". Once running on your PC, Select "Project", then "Insert Filter Design". Then you have all features including "Bandstop" which is Notch Filter. Good Luck.




Post subject:

Unread postPosted: Thu Jul 14, 2005 10:27 am


Site Admin

User avatar

Joined: Mon Jun 27, 2005 2:02 pm

Posts: 373

Location: Germany

Hello Tom,

The purpose of Notch Filter is not to eliminate positive feedback in gain chains, its purpose is to act as frequency trap for unwanted particular frequencies (As it is a notch and therfore narrowband).

You should find the cause of the oscillations in your amplifiers chain. You can do it either by simulation or by measuring each amplifier in your stage and solve the problem by elimination. The problem might be that your amplifier(s) are unstable at first place and your source/load matching networks are located in the unstable region on the stability circles. You should use a simulator and see to which reason you are allowed to match the amplifier(s).


Best regards,

- IR

Posted  11/12/2012

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