Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
you would like to post something on RF Cafe's
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
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
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
3. Is there a way the input of the chain can "see" the
4. Does the simulation oscillate? What sort of margin do
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
Unread postPosted: Wed Jul 13, 2005 1:06 pm
You can download free Student Version (SV) of ansoft designer, link
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.
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 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).