low temperatures more specifically as temperature varies the series resistance to the thermistor th 1 changes to thereby compensate for the reactance or capacitance of the colpitts oscillator circuit 3 as a result the frequency is maintained at a constant value the control voltage generating circuit 2 provides temperature compensation at high temperatures as temperature varies the series resistance to the thermistor th 2 changes producing a control voltage at the emitter of the transistor tr 2 this voltage is applied to the variable capacitance diode vd 1 so that the capacitance of the colpitts oscillator circuit 3 is compensated as a result the frequency is retained at a constant value the capacitors c 3 c 5 are used to provide temperature compensation at middle temperatures they have a negative coefficient that is their capacitance decreases as temperature increases the colpitts oscillator circuit 3 is temperature compensated by these capacitors the temperature compensations provided for for low middle and high temperature ranges are indicated by curves a b and c respectively in fig2 where frequency variation u03b4f f is plotted against temperature u00b0 c the cubic curve shown in fig7 is obtained by combining together these curves a b c this temperature compensating curve will be described in detail later this curve has the waveform that compensates the cubic curve which represents the relation of the frequency variation u03b4f f of an at cut crystal oscillator to temperature as shown in fig3 this at cut crystal oscillator will also be described in detail later as a result the oscillation frequency varies little over a broad frequency range for example.