The output waveform of oscillator circuits shown in picture circuit below is square wave because capacitor C charges and discharges through the same resistor R.
In circuit below, the charge and discharge paths of capacitor C are different. Capacitor C charges quickly via RA and diode when output of gate 2 is HIGH and discharges slowly via RA and RB when output of gate 1 is LOW.
The output is, therefore, a series of pulses in which the pulse duration depends on RA while pulse spacing depends on (RA+RB). The duty ratio of the output pulse is equal to RA/(RA+RB). One of the free inputs of gate 1 is used as on and off control.