Looking to modify this circuit for 30Hz operation
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Posted by on July 18, 2008, 10:55 am
Please log in for more thread options ganged 200K dual pot - I should be able to get pretty close now. | |||||||||||||
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Posted by Phil Allison on July 19, 2008, 4:33 am
Please log in for more thread options ** You will have a large range of frequencies available now. From about 10 Hz up to 200 Hz or more. ..... Phil | |||||||||||||
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Posted by Bob Eld on July 18, 2008, 10:51 pm
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> I was given the parts to build this sine wave generator. As far as I
> can tell it's a Wein Bridge with the two LEDs serving the function > originally reserved for the incandescent lamp. > > http://www.stephenhobley.com/sinewave.gif > > Unfortunately it's fixed frequency, the 5K variable only trims one > "side" of the wave, not selecting the fundamental as I'd hoped. It > looks liket the 68K resistors and 0.047uF caps determine the period of > oscillation (one pair for each half). > > Does anyone know the mathmatical relationship between the values of R5/ > R6 and C1/C2 and the frequency of oscillation? I'd like to get this > working in the 30Hz range. > > Thanks, > Steve The frequency of oscillation of a Wien Bridge is: f = 1/(2*pi*(R5*(R6 + R9)*C1*C2)^1/2). The R's and C's are the same sub numbers as your schematic. There is no requirement that R5 = R6 or C1 = C2. That's just a convenience. When those components are equal, the above formula simplifies as can be seen and was posted. When the components are equal, the gain must equal exactly 3. The purpose of the feedback network with the LED is to allow the gain to automatically adjust to the right value for oscillation. If the R and C values are not equal, a different gain than 3 will balance the bridge for stable oscillation. The gain is critical, too much and the output slams into the amplifier limits and too little and the oscillation quickly dies out. The LED's help stabilize the gain by lowering the gain when the drive gets too high at which point the LED's conduct more. Because the LED's conduct at the top (and bottom) of the sine cycle, this method of stabilization adds some distortion to the sine wave which may be ok depending what you intend to do with the signal. | |||||||||||||
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> ganged 200K dual pot - I should be able to get pretty close now.