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pullup
Joined: 19 Jul 2004
Posts: 15
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 11 Oct 2004 3:11 Problem:bandpass filter design |
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I built a Biauad bandpass filter with TL084,the f0 is expected to be 220KHz,but when I finished the circuit, I found that it became an oscillator!
No matter how I change the value of the resistors,the only thing changed is the frequecy of the oscillator.
Can anybody tell me what is the problem and how can I avoid the oscillator?
Thank you for help.
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ljkong
Joined: 18 Jul 2002
Posts: 128
Helped: 1
Location: P.R.C
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| 11 Oct 2004 4:04 Re: Problem:bandpass filter design |
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do you have simulated your circuits?
if your circuit show oscillating, it means there is feedbacking. you must cut the feedbacking route.
then you can get what you want.
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pullup
Joined: 19 Jul 2004
Posts: 15
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| 11 Oct 2004 7:24 Problem:bandpass filter design |
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| I simulated my circuit in Multisim, it worked as my expected,but after I built it with real components,it ascilated.
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 11 Oct 2004 7:36 Problem:bandpass filter design |
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Of course it will become an oscillator with TL084 (jfet input opamps) I think TI warn people in datasheet.
Post the schematic to see what is wrong. Most likey you do not have any balance resistor/capacitor. It is not necessary in schematics but in real application we need them.
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pullup
Joined: 19 Jul 2004
Posts: 15
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| 11 Oct 2004 9:16 Problem:bandpass filter design |
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I built the circuit according to <<Rapid Practical Designs of Active Filters>> written by DAVID E. JOHNSON,there are no balance resistors or capacitors.
since TL084 will arouse oscillating,what kind of op amps should I use?
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 11 Oct 2004 14:25 Problem:bandpass filter design |
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| A schematic? Trust me bandpass filter schematics are public domain and are free for everyone to use.
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Kevin Weddle
Joined: 10 Jul 2004
Posts: 101
Location: San Antonio, TX
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| 11 Oct 2004 15:41 Re: Problem:bandpass filter design |
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| Many things can cause the appearance of an oscillation. My circuit oscillates without power. Apply a signal and don't worry about it. Do you get the desired results. The power supply also has ripple and it would be hard to tell if the circuit really oscillates on it's own. A good start would be to check the frequency you are getting, if it's too high then that is a normal oscillation. The next step is to check the input to the opamp for a signal. If it's in the mV range and the output is in the V range then you definitely have an oscillation. If the output is in the mV range then there is not an oscillation.
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pullup
Joined: 19 Jul 2004
Posts: 15
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| 12 Oct 2004 2:13 Re: Problem:bandpass filter design |
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To Kevin Weddle:
The output of my circuit is in the V range, and the frequency is about 130KHz,at the meantime,the input is in the mv range, so I can sure it is a oscillator.Thanks for your help!
To djalli:
Here is the schematic drawn in P*otel 2004:
according to the book
C=C1=C2
G=R4/R3
B=1/CR4
4*pi*pi*f0*f0=1/(R7*R8*C*C)
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strabush
Joined: 10 Apr 2002
Posts: 83
Helped: 6
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| 12 Oct 2004 6:42 Re: Problem:bandpass filter design |
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Do you short the input when you test it without signal?
The ACTIVE band pass filter depends on a zero (or very low) impedance source to drive it, so if you dont connect to it a signal you have to short the input to ground.
Does it still oscillates after shorting input?
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 12 Oct 2004 7:14 Problem:bandpass filter design |
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| pullup can you put a picture. I can not open it because I do not have P*otel.
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Regnum
Joined: 17 Jun 2004
Posts: 264
Helped: 12
Location: Hurlingham
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| 12 Oct 2004 7:52 Re: Problem:bandpass filter design |
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You said you have it simulated and the results were OK....
...so check if you have Vdd and Vss pins right decoupled, and obvious thinks like this.
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pullup
Joined: 19 Jul 2004
Posts: 15
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| 12 Oct 2004 8:19 Problem:bandpass filter design |
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I short the input to ground, and add decoupling capacitor between Vdd and Vss,but it still oscillating..
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pullup
Joined: 19 Jul 2004
Posts: 15
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| 12 Oct 2004 8:21 Re: Problem:bandpass filter design |
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The schematic in bmp format
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xbtxbt
Joined: 12 Oct 2004
Posts: 44
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| 12 Oct 2004 8:45 Problem:bandpass filter design |
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| yes,you should have simulated your circuit.Maybe you didn't use your software well enough
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 12 Oct 2004 17:45 Re: Problem:bandpass filter design |
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First graph is transfer function, bode plot. Bandwidth is marked as -3dB point for the circuit.
Second graph is response of circuit which I do not like to call it bandpass filter is due to an exitation from 1Khz to 400Khz
This also can serve as active lowpass filter for 5.6kHz bandwidth but too expensive one because you can build such one with fewer components.
the circuit response may look as lowpass filter but also a very bad one, plus also a horrible response as oscillator.
pullup must redo it again.
Please pullup post something for the filter you want, bandwidth, corner frequencies, Q quality, method of implementation, gain of the filter, any load for the circuit and either me or someone else here will be able to help.
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flatulent
Joined: 19 Jul 2002
Posts: 4856
Helped: 292
Location: Middle Earth
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| 12 Oct 2004 18:11 Re: Problem:bandpass filter design |
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Did your simulation use the models for the op amp you used or was it an idealized model? You are operating the circuit within a factor of 10 of the amp cutoff frequency. It will have a lot of phase shift at your frequency that may be the cause of the instability.
Try substituting an amp with a much higher bandwidth in your breadboard and see if the oscillations go away.
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 12 Oct 2004 19:45 Problem:bandpass filter design |
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| I simulated his/her (pullup) circuit using 741 opamp. My guess is that will not be a vastly big difference I did not use TL081 or TL084.
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Borber
Joined: 01 Jan 1970
Posts: 1485
Helped: 111
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| 12 Oct 2004 20:45 Re: Problem:bandpass filter design |
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My simulation shows that filter will oscillate if R8 is less than 5kohm. It is stable for greater resistances. Resistor value is not readable. If R8=500ohm filter oscillate at about 100kHz. Your filter is definitively bandpass. Peaking frequency is about 70kHz for R8=5kohm and 20kHz for R8=50k. So check R8, inpropper value is causing oscillations because of phase shift.
Verify that you have no capacitive load on output (there should be no long coax only oscilloscope tip) because these OPAMP's are known to oscillate at capacitive load. In this case you should connect load via 50ohm resistor.
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pullup
Joined: 19 Jul 2004
Posts: 15
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| 13 Oct 2004 2:08 Problem:bandpass filter design |
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Yes, I simulate the circuit with an idea model of op amps.Now I realize it is not a good way:(
To Borber: I change the value of R8 as you mentioned,the oscillation disappear! It is amzing!
How do you make it and what software you used to simulate?
I am a boy green to every aspect.
The mission asigned by my teacher is to pick up a 220KHz signal from an environment with plentiful noise,so he suggest me to use a bandpass filter.It is my first time to design a filter, and can't get any help from my teacher,for he is too busy:(
Thanks everyone who gave me help! all of you are really nice!
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 13 Oct 2004 2:59 Re: Problem:bandpass filter design |
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| pullup wrote: |
| The mission asigned by my teacher is to pick up a 220KHz signal from an environment with plentiful noise |
220kHz ±1% 10% 5% ?
Active or Passive?
Order of filter?
Filter type: Butterworth,Chebuchev, etc?
Filter implementation: Thomas, Akerberg,Frog..etc?
Gain: ?
Q quality: ?
Passband attenuation: ?
Stop band attentuation: ?
Delay: ?
Phase shift: ?
was anything mentioned?
Complete some of these or all of them and someone will help.
How do you want filter method you want it to be designed, of how you achieve to those numbers? You sure are not looking just for a pretty schematic?
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pullup
Joined: 19 Jul 2004
Posts: 15
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| 13 Oct 2004 4:16 Problem:bandpass filter design |
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220KH ±10%
Active
2 order
Filter implementation:As my try is Biauad bandpass filter, there are still some other types
unit gain
Q is about 30
the rest is not seriously restricted.
All above is my expect,not my teacher's demand,he just give me a work and say nothing,how can I say about it?
Of course I don't just want a pretty schematic,I need to know how and why,and the effect of components and so on,but what I can refer to is too limited.
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 13 Oct 2004 5:36 Problem:bandpass filter design |
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| "Biauad" term never heard it. What is it?
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pullup
Joined: 19 Jul 2004
Posts: 15
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| 13 Oct 2004 7:36 Problem:bandpass filter design |
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| Sorry,It is "Biquad".
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 13 Oct 2004 7:41 Re: Problem:bandpass filter design |
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Got it, it took me while to recognize "Biauad" you were actually talking about "Biquads". Analysis and you build these filters from analysis from pole-zero plot rather than Bode plot done for other active filters. You can build an 8th order filter this way just using a single opamp.
Attached is an article of how to design single second order amplifier biquad bandpass active filter. It is pretty would be as much as a detailed tutorial for you.
To check your work, download NuHertz Filter Solution software. This software allows building them. http://www.filter-solutions.com/
good luck!
djalli
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pullup
Joined: 19 Jul 2004
Posts: 15
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| 13 Oct 2004 8:59 Problem:bandpass filter design |
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To djalli
You gave me so much help that I don't know how to express my appreciation to you.
Any way,thank you very much!
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Borber
Joined: 01 Jan 1970
Posts: 1485
Helped: 111
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| 13 Oct 2004 10:59 Re: Problem:bandpass filter design |
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| These two filters from djalli are dificult for realisation because of .4pF and 409Mohm in first circuit and a need for precise capacitors in second (can be solved using trimers) but 409Mohm seems impossible. I think Biquad is simpler and realizable.
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pullup
Joined: 19 Jul 2004
Posts: 15
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| 13 Oct 2004 15:18 Problem:bandpass filter design |
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| Borber is right,It is hard to implement such two filters.
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Borber
Joined: 01 Jan 1970
Posts: 1485
Helped: 111
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| 13 Oct 2004 16:01 Re: Problem:bandpass filter design |
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Simple solution could be a modification of Butherworth filter. Demands like Q=30 are hard to realize with Biquad. This topology is simple to tune appropriately change R1 or R2 for gain adjustement and R4 for center frequency. Elemenths can be 10%. I am not shure this will satisfy your needs if you want exact values.
Last edited by Borber on 13 Oct 2004 17:56; edited 4 times in total |
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djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
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| 13 Oct 2004 16:05 Problem:bandpass filter design |
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Do not let that 40MOhm resistor scare you. There are 20, 30, 40Mohms resistors easily available.
2nd order positive sab. again NuHertz calculated values. It lowered Q to 23 only
and a second one more easily to implement. untitled2abc
Last edited by djalli on 13 Oct 2004 17:15; edited 2 times in total |
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flatulent
Joined: 19 Jul 2002
Posts: 4856
Helped: 292
Location: Middle Earth
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| 13 Oct 2004 16:09 Re: Problem:bandpass filter design |
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Another approach is to use a PLL. This will lock onto your signal. The lock detector output will indicate the presence of the signal. You may have to sweep the frequency to acquire lock.
Another approach is the superheterodyne method. Down convert the frequency range to the audio range where filtering is easier.
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