The figure is composed by a NE555 and other withered frequency circuit by changing the self-excited multivibrator NE555 charging current to frequency modulation. The circuit is VT1 and VT2 constitute a current mirror circuit, the charging
current can be generated in the charging circuit, the current size is determined by R1 and RRP1. Low frequency modulation signal superimposed to the bias current IB after the oscillation frequency changes. Try to use the current Miller circuit on the tube.