frequency that is substantially constant with temperature changes over the operating temperature range 0067 fig2 a and 2b show two further examples of the difference between a first and second frequencies over a range of temperatures 0068 fig3 shows an example of one possible oscillator circuit 100 using a sapphire loaded cavity u201c slc u201d resonator 102 designed as described above the oscillator circuit shown in fig3 uses the pound stabilisation technique as disclosed in r v pound 1946 u201c electronic frequency stabilization of microwave oscillators u201d rev sci instr vol 17 no 11 pp 490 505 the contents of which are incorporated herein by reference the oscillator circuit 100 comprises a first loop oscillator 104 and second loop oscillator 106 the first loop oscillator 104 is arranged to oscillate at a first frequency in a te mode the first loop oscillator 104 comprises the dielectrically loaded cavity resonator 102 as a frequency determining element a band pass filter 108 at the first frequency and a first microwave amplifier 110 provided in a loop the first loop oscillator 104 includes a first pound stabilisation circuit 112 the second loop oscillator 106 is arranged to oscillate at a second frequency in a tm mode the second loop oscillator 106 comprises the dielectrically loaded cavity resonator 102 as a frequency determining element a band pass filter 114 at the second frequency and a second microwave amplifier 116 operable at the second frequency provided in a loop the second loop oscillator 106 further comprises a first mixer 118 a third microwave amplifier 120 operable at the desired output.