Variable Filter Circuit Design. - RF Cafe Forums
Post subject: Variable filter circuit design. Posted:
Sat Feb 25, 2006 1:40 pm
Joined: Sat Feb
25, 2006 1:39 pm
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
looking forward to your reply.
Post subject: Posted: Sat Feb 25, 2006
Joined: Mon Jun 27, 2005 2:02
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
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 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,
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.