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Low pass filter, cut off 500 Hz, short response time...

V

Viram

Jan 1, 1970
0
I'm trying to design a LP-filter with a cut off at around 500 Hz. My
problem is getting a short response time and low/no resonance peak. What
order should the filter be? I'm currently trying with a simple
LC-filter...
 
L

lloyd

Jan 1, 1970
0
Viram escribió:
I'm trying to design a LP-filter with a cut off at around 500 Hz. My
problem is getting a short response time and low/no resonance peak. What
order should the filter be? I'm currently trying with a simple
LC-filter...
When you design a LP filter, actually you are limiting the response
time. For 500Hz the time constant is about 0.3 msec.
If you want to limit the resonance peac, you can share the capacitor in
some smaller parallel units and put one resistor in series with one of
them.
 
D

Don Pearce

Jan 1, 1970
0
Viram escribió:
When you design a LP filter, actually you are limiting the response
time. For 500Hz the time constant is about 0.3 msec.
If you want to limit the resonance peac, you can share the capacitor in
some smaller parallel units and put one resistor in series with one of
them.

No, the resonance peak is down to the style of filter. Generally a
Chebyshev will be bad, a Butterworth somewhat better and a Gaussian
has no peak. You pay a price for the lack of resonance in a droopier
response around the corner frequency.

d

Pearce Consulting
http://www.pearce.uk.com
 
Viram said:
I'm trying to design a LP-filter with a cut off at around 500 Hz. My
problem is getting a short response time and low/no resonance peak. What
order should the filter be? I'm currently trying with a simple
LC-filter...

You need to get hold of a decent text on filter design - I use the
"Electronic Filter Design Handbook" by Arthur B. Williams and Fred J.
Taylor - ISBN 0-07-070434-1. Since then there has been a third edition
(ISBN 0-07-070430-9) which is supposed to be even better, but it seems
to be out of print - www.amazon.com offers three second hand copies for
about $130. You should be able to find it - or something very like it -
in a university library.

The usual choice for what you seem to want to be doing is a
"linear-phase" or "Bessel filter", though a "synchronously tuned"
filter (consisting of identical multiple poles) actually gives fastest
settling for a given number of poles, with the step getting to better
than 99% after five time constants. Adding more poles makes the step
steeper at the 50% point, and pushes the 50% point back from around one
time constant at one pole to closer to three at ten poles.

A simple LCR filter is two poles ...
 
D

Don Lancaster

Jan 1, 1970
0
You need to get hold of a decent text on filter design - I use the
"Electronic Filter Design Handbook" by Arthur B. Williams and Fred J.
Taylor - ISBN 0-07-070434-1. Since then there has been a third edition
(ISBN 0-07-070430-9) which is supposed to be even better, but it seems
to be out of print - www.amazon.com offers three second hand copies for
about $130. You should be able to find it - or something very like it -
in a university library.

The usual choice for what you seem to want to be doing is a
"linear-phase" or "Bessel filter", though a "synchronously tuned"
filter (consisting of identical multiple poles) actually gives fastest
settling for a given number of poles, with the step getting to better
than 99% after five time constants. Adding more poles makes the step
steeper at the 50% point, and pushes the 50% point back from around one
time constant at one pole to closer to three at ten poles.

A simple LCR filter is two poles ...

Do it digitally.
Distortionless digital filters are superb because they can look
backwards as well as forwards in time.

More on my website.

But if you must go analog, use my Active Filter Cookbook.

--
Many thanks,

Don Lancaster
Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
voice: (928)428-4073 email: [email protected]

Please visit my GURU's LAIR web site at http://www.tinaja.com
 
T

Tam/WB2TT

Jan 1, 1970
0
Viram said:
I'm trying to design a LP-filter with a cut off at around 500 Hz. My
problem is getting a short response time and low/no resonance peak. What
order should the filter be? I'm currently trying with a simple
LC-filter...
Unless you like inductors the size of tuna cans, use an active filter.
(Except if you are building a speaker crossover filter).

Tam
 
Don said:
Do it digitally.
Distortionless digital filters are superb because they can look
backwards as well as forwards in time.

More on my website.

But if you must go analog, use my Active Filter Cookbook.
Please visit my GURU's LAIR web site at http://www.tinaja.com

Don Lancaster's Active Filter Cookbook has many fans. Williams and
Taylor cover a wider range of filters and filter types.
 
G

gwhite

Jan 1, 1970
0
Don Lancaster's Active Filter Cookbook has many fans. Williams and
Taylor cover a wider range of filters and filter types.

If you want the bible of filters...:

http://www.amazon.com/exec/obidos/tg/detail/-/0471986801/

The time plots--which is what the OP cares about--are all there.

I don't have the following, but it too has focus on the time domain and is less
expensive:

http://www.noblepub.com/shopexd.asp?id=6

Don's book works great where the needs are straightfoward and the frequency is
not very high. It is a fast path to getting done, and that says something.
 
J

Joel Kolstad

Jan 1, 1970
0
gwhite said:
If you want the bible of filters...:
http://www.amazon.com/exec/obidos/tg/detail/-/0471986801/
The time plots--which is what the OP cares about--are all there.

Zverev is a great book, certainly, but it's really not a good 'introductory'
tome on the subject. I think the poster would be better off with Don
Lancaster's old book, or perhaps "Analog and Digital Filter Design" by
Winder.
I don't have the following, but it too has focus on the time domain and is
less
expensive:

http://www.noblepub.com/shopexd.asp?id=6

Noble Publishing is also the home of Randy Rhea, whose book "HF Filter
Design and Computer Simulation" is quite good as well. Rhea's a very
practical kind of guy... (although if you look at the price of his
publications, he wants to make a pretty darned good living at it too!)

---Joel
 
gwhite said:
If you want the bible of filters...:

http://www.amazon.com/exec/obidos/tg/detail/-/0471986801/

The time plots--which is what the OP cares about--are all there.

Williams and Taylor cite Zverev very frequently, and include time plots
for every filter type - both step response and impulse response. I
imagine that Zverev presents the theory better - Williams and Taylor
just direct you to the relevant bit of the literature - but Williams
and Taylor is very much a book for working enegineers.
I don't have the following, but it too has focus on the time domain and is less
expensive:

http://www.noblepub.com/shopexd.asp?id=6

Don's book works great where the needs are straightfoward and the frequency is
not very high. It is a fast path to getting done, and that says
something.

Williams and Taylor is handy when the situation gets more complicated -
I once ran into a situation where we needed a fast-settling filter, but
couldn't start the filter chain with a high-Q stage because of the risk
of clipping, and was able to find a "linear phase with equiripple
error" filter which did the job.
The electron microscope involved ended up being loaded on the truck at
eleven in the evening under the eye of the managing director, so I
scored a few brownie points on that one.
 
R

Rob Gaddi

Jan 1, 1970
0
Tam/WB2TT said:
Unless you like inductors the size of tuna cans, use an active filter.
(Except if you are building a speaker crossover filter).

Tam
I'll second that one. If you head over to TI's site and check their
analog knowledge base for a program called FilterPro it'll design RC
active filters for you up to 10 poles with either MFB or Sallen-Key
topologies. Though in my opinion they're far too conservative on the
gain-bandwith requirements of the op-amps.
 
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