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'[EE]: Oscillatory behaviour in a BPF'
2003\05\07@102808 by Mike Hord

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It boggles the mind.

I have a program called FilterFree.  It purports to take the grunt work out
of designing low-order analog filters.  I'm trying to design a 1st order
Butterworth BPF, centered at 60 kHz with BW +/- 2kHz.  When I add in the 3rd
stage of the filter design (which looks okay to me) provided by FilterFree,
instead of a BPF, I get a BEAUTIFUL (I mean, you could market this thing) 57
kHz sine wave, regardless of the input to the filter.

My op-amps seem to be working (at least, I can implement simple voltage
follower and amplifier circuits with them), and I was wondering if anyone
else has ever encountered this problem, or has any opinion about FilterFree.

No, I'm not going to try to ASCII art the circuit.  It's a little more than
I want to attempt.  If anyone really wants to see it, let me know and I'll
post it on my website tomorrow.  It really is a good sine wave generator;
the sine wave it generates is actually a bit more stable than the one my
signal generator is coming up with.

Mike H.

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2003\05\07@112547 by Ofer Saferman

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Hi,

I have no experience with FilterFree, I myself use Filter Wiz Pro 3.2.
What is the gain-bandwidth product of the OpAmps you are using ? If it is high
the OpAmps tend to oscillate in all kinds of circumstances (even if the data sheet
says that they are unity gain stable). How many poles for your filter (or equivalenty
what are the passband and stopband attenuations) ? What type of filter topology
FilterFree suggests to you (Sallen-Key, MFB, etc.) ?
If you put up the circuit it will be easier to analyze and comment on.
If you specify all parameters like: center frequency, bandwidth, passband and stopband
attenuation, whether the filter should be inverting or non-inverting, I can supply you with
a suggested design by Filter Wiz Pro.

*********** REPLY SEPARATOR  ***********

On 07/05/2003 at 09:27 Mike Hord wrote:

{Quote hidden}

~~~~~~~~~~~~~~oOo~~~~~~~~~~~~~~~
Ofer Saferman, M.Sc.
Senior Design Engineer
Paragon Communications
6 Pal-Yam
Haifa, 33095
Israel
Phone: +972-4-8642775
Fax.:    +972-4-8642737
e-mail: spam_OUToferTakeThisOuTspamparagoncommtech.com

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2003\05\07@155340 by hard Prosser

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Mike,

Is there any possibility of feedback around your circuit? - i.e - are the
inputs & outputs well separated and grounds connected at a common point?
Another posibility is coupling through the power suppy - does adding more
capacitance to the supply rails change the frequency or anything?

Can you try alternative types of opamps? -

Can you find any node that is particularly sensitive to changing the
oscillation - I am thinking you may actually be viewing the result of an RF
instability that you may be able to kill by adding a few pf at a sensitive
spot.

Some unwanted oscillation can be amazingly stable - I had a mosfet RF amp
oscillating at exactly exactly 1GHz once. Easy to fix with a ferrite bead
in that case though!

Richard P




It boggles the mind.

I have a program called FilterFree.  It purports to take the grunt work out
of designing low-order analog filters.  I'm trying to design a 1st order
Butterworth BPF, centered at 60 kHz with BW +/- 2kHz.  When I add in the
3rd
stage of the filter design (which looks okay to me) provided by FilterFree,
instead of a BPF, I get a BEAUTIFUL (I mean, you could market this thing)
57
kHz sine wave, regardless of the input to the filter.

My op-amps seem to be working (at least, I can implement simple voltage
follower and amplifier circuits with them), and I was wondering if anyone
else has ever encountered this problem, or has any opinion about
FilterFree.

No, I'm not going to try to ASCII art the circuit.  It's a little more than
I want to attempt.  If anyone really wants to see it, let me know and I'll
post it on my website tomorrow.  It really is a good sine wave generator;
the sine wave it generates is actually a bit more stable than the one my
signal generator is coming up with.

Mike H.

_________________________________________________________________
Help STOP SPAM with the new MSN 8 and get 2 months FREE*
http://join.msn.com/?page=features/junkmail

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[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads

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2003\05\08@163747 by Peter L. Peres

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I have never used FilterFree but usually you need to care for the low and
highly reactive input impedance of the next stage. So add an opamp as
voltage repeater between the first and second stages and put a 100R
resistor in series with its output (between this buffer stage and the
input of the next butterworth).

A resistor PI or T attenuator will also do this job probably.

Peter

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