Microcontroller compensated overtone TCXO with low phase noise characteristic

This paper presented a new 121.4 MHz overtone TCXO with low phase noise. The compensation approach differs from conventional ones such as adding series inductances or frequency multiplication. It can provide a very high stability and a low phase noise level for an overtone oscillator. It makes the frequency of a 100 MHz 5 th overtone crystal oscillator mixed with that 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. A microcontroller was used to control the compensating voltage of 21.4 MHz VCXO at given temperature to generate a frequency correction Deltaf_t=Deltaf₁₀₀+Deltaf_21.4 and implement the compensation. Experimental results show that the frequency-temperature stability of the prototype 121.4 MHz TCXO has achieved plusmn1.5times10^-7 within the temperature range from -40 to 85degC. Besides, a phase noise level of -85 dBc/Hz at 10 Hz offset and-140 dBc/Hz at 1 kHz offset has been realized.