LynneyDee said:

hi everyone,

I hope there's somebody here who can help me. here's the schematic of

the 440Hz reference oscillator of a minimoog synth:

http://users.telenet.be/Lynneymoog/reference.bmp
I've simulated this circuit in Electronics Workbench and (believe it

or not) I got the same result : 440Hz. now I'd like to calculate the

theoretical central frequency. I used this formula: fo = 1/(2.Pi.R.C)

I've been working on this for a couple days now, but I just don't

come to 440Hz...

please, PLEASE, what am I doing wrong?

Look at the Wien Bridge circuit here

http://www.ecircuitcenter.com/Circuits/opwien/opwien.htm
R1, C1 are in parallel

R2, C2 are in series

Normally, R1=R2=R and C1=C2=C and the frequency is 1.0 / (2*pi*R*C)

The capacitors are equal in your circuit, but the resistances are not - so

we don't quite get the right answer using the standard formula. We need to

analyse the network. Relating the above symbols to component values in your

circuit:

C = 0.03uF || 0.6nF = 30.6e-9

R1 = R50 || R55 = 92k || 16.9K = 14.3e3

R2 = R17 + R68 + R71 = 10e3

Now we need to plot the transfer function of the network, and find the

frequency where the phase shift passes through 180 degrees. I use a maths

package called SCILAB for things like this. You can download it for free

from

http://www.scilab.org/
If you run the following code in SCILAB, you get a graph which shows phase

shift passing through 180 degrees at 440Hz

s = poly(0, 's');

R1 = 14.3e3;

R2 = 10e3;

C = 30.6e-9;

xp = 1 / (1/R1 + C*s);

f = xp / (R2 + 1/C/s + xp);

xbasc(0);

xselect();

bode(syslin('c', f), 10, 10e3, .01);

SCILAB has functions for calculating the exact frequency, so you don't have

to read it off the graph.

Or, you can get a very rough approximation of the frequency using the

standard formula with R=10k and C=30nF.