"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
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.
is obviously happening - but adding a notch filter will in general only move the frequency of the oscillation
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?
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.
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.
Unread postPosted: Thu Jul 14,
2005 10:27 am
Joined: Mon Jun 27, 2005 2:02 pm
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
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).