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Wien Bridge Osc. wrong FREQUENCY !!!

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Templar

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wien osc

Hi all,
Okay, this is my first post here. I need some help. As for my problem, i guess it's really strange and is driving me crazy.
The relation the gives the frequency for a wien bridge is as follows (if it is wrong, please correct it)
f = 1÷{2*pi*(C1*C2*R1*R2)} as in the schematic.

In the hardware implementation, i made R1=R2=1KΩ and C1=C2=4.7nF .

If the value for the frequency is calculated ==> 1÷(2*pi*R1*C1). isn't it?
It gives me exactly 33.862 KHz. On the Oscilloscope, it gives me about 2.5 MHz !!!!!!!!

I also noticed that when i move or hover my hand over the circuit, the frequency decreases a little bit and it's amplitude varies, which means, i guess, my hand's stray capacitance with the circuit is comparable with the used 4.7nF caps...

Please do you have any ideas why is the frequency is about 60X times the expected one?

PS. the colors of the resistors that determine the frequency is (in order) : brown black red
and the caps WRITTEN value is : 4n7
(i.e. i've checked everthing in the circuit, the oscilloscope scale, VIRTUALLY, EVERYTHING IS CORRECT EXCEPT THE FREQ.)


 

flatulent

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lm318 wien bridge circuit

Try reducing the gain of the op amp part of the circuit.circuit. You have parasitic oscillations not controlled by the RC networks.
 

v_c

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You said that f = 1÷{2*pi*(C1*C2*R1*R2)}.
if C1=C2=C and R1=R2=R that would make f=1/(2 pi C^2 R^2).
So I don't think the above formula for the oscillation frequency is correct.
But I don't think that is the problem anyway, I am just pointing it out to
make sure you check the source of the above expression.

With R1=R2=1K? and C1=C2=4.7nF, oscillation frequency should be
33.862 kHz.

1. One piece of information you did not provide is what opamp
you are using
. The opamp must have a gain-bandwidth product of > 43 times the oscillation frequency. In your case that is approximately 1.5MHz.

2. Is your signal a clean sinewave or is there distortion present?
What are your supply voltages?
 

Templar

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For v_c, sorry, i forgot to put the square root :) , i.e.
f = 1÷{2*pi*√(C1*C2*R1*R2)}. [/size]
I'm using LM318, and in its datasheet, what is only mentioned is its small signal BW which is 15 MHz, and nothing is mentioned about G.BW product.
The sine is a clean one.
And i'm using 12/-12 supply.



For flatulent, i think you're PERFECTLY RIGHT, because when i removed R1, C1 and C2, and only kept R2, i had a sine wave at the output!!!!
Is this as you said due to gain? or it is due to the usage of a breadBoard (not pcb, in other words).
I have a potentiometer to control the gain. If i reduce the gain below 2 (for the condition of oscillation to be satisfied), the circuit won't function.
Anyway, i'll try it, but please, if you have any comments, i might need them...
 

Borber

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Maybe, problem is because of breadboard. You have to place capacitors 100nF from +Vcc and -Vcc to ground. Try to make as short leads as possible.
 

v_c

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Take a look at this app note by TI. It covers using opamps for Wien Bridge Oscillators. It points out the characteristics that the opamp must have to insure a good oscillator.

**broken link removed**
 

Templar

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Still very high harmonic signal

I've increased the gain of the opamp and the expected sine wave appeared, but still superimposed over it, the 2.5 MHz sine.

What shall i do???
 

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