Copyright: 1996 - 2024
BSEE - KB3UON
RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling
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Variable filter circuit design. - RF Cafe Forums
RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media
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Forums activity dropped off precipitously. If the folks at
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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.
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Post subject: Variable filter circuit design. Posted: Sat
Feb 25, 2006 1:40 pm
Joined: Sat Feb 25, 2006
hello guys, I am new here.
glad to find such a good forum.
I am working on a project "optical
listening device" whicn could be able to monitor the status of chmical
pipes. we use a photodiode to detect the reflected laser beam by the
mirror which is attached to the pipe. Then we gonna to analyze the siganl
detected in terms of frequency.(Any fluid has a certain frequncy when
it is flowing, any abstacle in the pipe will slow down the fluid and
then change the freqency.)
our proposal is that we use a band
pass filter to get rid of the noise and only focus on the spectrum that
indicates the normal status of the pipes. This filter also acts as a
refernce, which could distinguish the normal and abnormal(say the pipe
is blocked) status in the pipe.And then we have to biuld a variable
filter to detect the rest of the spectrum.
but now i am not
clear how the variable filter works and how to compare the signal received
and the reference signal, and then trigger an alarm if any abstacle
in the pipes slow down the fluid too much or completely block the pipe.
looking forward to your reply.
Post subject: Posted: Sat Feb 25, 2006 5:06 pm
Joined: Mon Jun 27, 2005 2:02 pm
Welcome to RF Cafe
To answer your question the technology of implementing
a filter depends pretty much on the frequency of operation, required
bandwidth (BW) and other paramaters. It doesn´t necessarily has to be
a classic variable RF Filter - most common one that comes to mind is
by using Varactor Diodes. You can implement a filter by DSP (FIR, IIR)
or an active filter with switched capacitors... in order to help you
more please provide more information about the frequencis you work with
the BW of operation etc..
You can compare between the detected
and reference signals by different magnitudes: If the detected signal
points over a blocked pipe, it will be out of the filter's pass region
and therefore attenuated. The amount of attenuation points on how much
the pipe is blocked..You have to tune the filter for that.
Post subject: Variable FilterPosted: Wed
Mar 01, 2006 1:28 pm
Joined: Wed Feb 22,
2006 3:51 pm
Here's another approach, based
on defining your requirements:
You mention eliminating undesired
noise. Have you categorized and described that noise? Is it constant
or does it change? Is the noise correlated to the change you're trying
Do you know the frequency and bandwidth of the signal
you're trying to observe? DSP techniques work well at lower RF frequencies,
and adaptive filtering is quite advanced in that area, but it is sample-rate
Over what range must the filter be tuned? A 1% variation
is a lot easier than a 200% variation, especially at RF frequencies.
Is filtering even the correct approach? Maybe
spectrum analysis is better, given your needs.
Post subject: Variable Filter Circuit DesignPosted:
Thu Mar 02, 2006 5:01 am
Joined: Fri Feb
17, 2006 12:07 pm
Location: London UK
I agree with
This looks more like a job exactly right for Fast Fourier
Transform processing, or DSP, but a final choice would depend on the
essential parameters already mentioned.
With such a software
suite running on an industrial grade rugged PC, most of the electronic
design is done, leaving you to concentrate on a roofing filter and low
noise pre-processing front-end.