An improved microcontroller compensated low phase noise overtone TCXO

This paper presented an improved design on a 121.4 MHz overtone TCXO which exhibited a low phase noise. The temperature compensation approach differs from conventional ones such as adding series inductances or frequency multiplication. It makes of a 100 MHz 5th overtone crystal oscillator mixed with the frequency of a 21.4 MHz fundamental mode voltage controlled crystal oscillator (VCXO). And then, the mixed frequency was filtered and amplified to produce a 121.4 MHz output. For a better performance, computer simulation tool as Agilent ADS was used to estimate the phase noise while designing these two oscillators. In this design, a microcontroller was used to control the compensating voltage of 21.4 MHz VCXO at every appointed temperature to generate a frequency correction Deltaf_t = Deltaf₁₀₀+Deltaf_21.4 and to implement the total compensation. Experimental results show that the frequency-temperature stability of the prototype 121.4 MHz TCXO has achieved plusmn2times10_-7 within the temperature range from -40 to 85deg. Besides, a phase noise level of -85 dBc/Hz and -145 dBc/Hz at 10 Hz and 1 kHz offset has been realized.