"Notch Filter" Question for Those RF Experts - RF Cafe Forums
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" :-;
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.
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?
there a way the input of the chain can "see" the output?
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
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
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.
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 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).