Although the oscillator using schmitt trigger gate is simple, it is hard to determine the accurate timing since its hysteresis thresholds depend on device characteristic.
Figure 1: Shumitt trigger Osc.
However, hysteresis thresholds are easily made by positive feedback. This is the hysteresis oscillator by normal inverter gates.
Figure 1: Shumitt trigger Osc.
LED attatched timing
Figure 2: Hysteresis Osc.
For ultra-low frequency oscillation, use tantalum condenser to reduce leak current, and use 40xxx instead of 74HCxx to reduce through current.
The most common gate circuit of CR oscillation is below.
Figure 1: Shumitt trigger Osc.
Figure 3: Typical CR Osc.
This is similar, in a sense. Assume the point (b) as a base voltage. You
will see two different thresholds (+Vdd/2, -Vdd/2), at the gate input. Non-polar capacitor is necessary because the C voltage swings from (+Vdd/2)
to (-Vdd/2).
Rp is a protection resistor for gate input.
You can omit a gate if inverters are buffered type.
Figure 4: Simplified Osc.
If the inverter is not an unbuffered type, it is a buffered type. ??
You can make a ring oscillator with single inverter.
Figure 5: Simplified Ring Osc.
Osamu Tamura @ Recursion Co.,
Ltd.
19 Jan. 2007