Astable multivibrator or oscillator circuit is based on positive feedback. We can design such circuit using op-amp, logic gates, or transistors. Here we can rely on single capacitor capacitor and resistor for the feedback inside op-amp like transistor circuit, which works over wide range of temperature, voltages and transistor gains. The stability is pretty good, for supply voltage changes between 6 and 12 V, the frequency will varies only by 0.05%. We can change the frequency/ timing  by changing  R1, R2 and C. The duty cycle  can be modified according to ratio of R3 to R, and the circuit gives 50% for values shown in the schematic diagram.

20 kHz Astable circuit schematic