I find a circuit that generate sine and cos wave with exact and stable
frequecy.but i don't understand this circuit,please help me.
http://xydarcher.51.net/circuit.JPG
It's an odd circuit, and I can't help but wonder if wherever you found it,
they might have copied it wrong.
The left hand op-amp is clearly a a low pass filter. The second stage is
clearly an integrator. There is enough shift through those two stages so
bringing the output of the second stage to the input stage makes the thing
oscillate. The zener diodes limit the output of the second stage, may even
square up the signal (in one direction, one zener is merely a forward biased
diode with about .7v drop and the other zener is acting like a zener,
conducting when voltage across it reaches the voltage where it "zeners",
then when polarity is reversed, the roles of the diodes reverse).
But I can't say I've seen this particular circuit before. It seems
a bit odd.
Usually, when a quadrature oscillator is needed, you see two integrators
in a similar circuit, each providing 90 phase shift (and so two stages
in series provide 180 degrees of shift, and that fed back to the input
makes the whole thing oscillate).
Michael
