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Variable filter circuit design. - RF Cafe Forums
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hawk7820
Post subject: Variable filter circuit design. Posted: Sat
Feb 25, 2006 1:40 pm
Lieutenant
Joined: Sat Feb 25, 2006
1:39 pm
Posts: 1
Location: LONDON
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.
cheers.
Top
IR
Post subject: Posted: Sat Feb 25, 2006 5:06 pm
Site Admin
Joined: Mon Jun 27, 2005 2:02 pm
Posts:
373
Location: Germany
Hello hawk7820,
Welcome to RF Cafe
Circuits Forum!
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.
Good
luck!
_________________
Best regards,
- IR
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fred47
Post subject: Variable FilterPosted: Wed
Mar 01, 2006 1:28 pm
General
Joined: Wed Feb 22,
2006 3:51 pm
Posts: 104
Hi!
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
to observe?
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
limited,
Over what range must the filter be tuned? A 1% variation
is a lot easier than a 200% variation, especially at RF frequencies.
Good Luck!
Is filtering even the correct approach? Maybe
spectrum analysis is better, given your needs.
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nubbage
Post subject: Variable Filter Circuit DesignPosted:
Thu Mar 02, 2006 5:01 am
General
Joined: Fri Feb
17, 2006 12:07 pm
Posts: 218
Location: London UK
I agree with
Fred47.
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.
Posted 11/12/2012
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