First use a current source with I1 to charge a capacitor.
Second a current controlled sink with two times I1.
Third both conected to the same side of the capacitor, the other side to ground.
The current source always active, and charging the capacitor.
When the voltage in the capacitor reaches the level that you want or need, then activate the current sink and the capacitor will discharge with I1.
When the voltage in the capacitor reaches the low level that you need then desactivate the current sink.
To activate and desactivate the current sink you need a schmitt trigger, that you can implement with transistors.
This is a relaxation oscillator, we already simulated and implemented and worked very well.