An way to change the frequency of the previous oscillator (CIR112E) is to add a variable capacitor in the feedback loop as shown here. This capacitor can typically be in a range from 2-20 to 5-100 pF. The upper frequency limit for this oscillator is about 4 MHz with a 12 V power supply. The upper limit of frequency depends on the times the signal passes across each gate.
Variable Frequency Ring Oscillator
