Hi rockyou,
Are you sure, that this is not an effect of your digitizer pls?
Can you check it wit an analog scope?
Did you checked with higher sampling rate too?
K.
Added after 1 minutes:
hmm,
Sorry; I checked yet, that is an output of simulation...
I don't know if it is important for you, but your circuit has some disadvantages:
Amplitude/frequency of the produced signal depend on power supply voltages and the opamp type.