Copyright: 1996 - 2024
BSEE - KB3UON
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:
"Notch filter" question for those RF experts - RF Cafe Forums
RF Cafe Forums closed its virtual doors in 2012 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. If the folks at
phpBB would release a version with integrated
sign-in from the major social media platforms, I would resurrect the RF Cafe Forums,
but until then it is probably not worth the effort. 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.
|-- Amateur Radio
Gripes & Humor
-- CAE, CAD, &
Test & Measurement
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).