Re: Looking to modify this circuit for 30Hz operation


<stephen@xxxxxxxxxxxxxxxxx> wrote in message
news:ff5ad5b2-0602-43a7-a7da-96c4693943f0@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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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