To implement a high-stability and high-frequency overtone temperature-compensated crystal oscillator (TCXO) conveniently, an improved design of the novel overtone TCXO is described in this paper. A 120-MHz TCXO based on mixing of dual crystal oscillators is implemented. It utilizes a 100-MHz AT-cut 5th-overtone crystal oscillator mixed with a 20-MHz AT-cut voltage-controlled crystal oscillator (VCXO). The 120-MHz mixed product is filtered to produce the output signal. The total frequency deviation of 20-MHz and 100-MHz crystal oscillators is compensated by adjusting the output frequency of the 20-MHz oscillator to produce the stable 120-MHz output frequency. In this work, verifying experimental results of the compensation are presented. The stability of the experimental 120-MHz overtone TCXO with microprocessor temperature compensation achieves plusmn2 times 10-7 over the temperature range from -30degC to +70degC. A phase noise level of -133 dBc/Hz at 1 kHz offset has been initially measured for the prototype TCXO. The experimental result demonstrates this approach can conveniently implement the high-frequency overtone temperature compensation with a relatively high stability, and it is available for a wider frequency range as well.

High-frequency overtone TCXO based on mixing of dual crystal oscillators

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 Deltaft=Deltaf100+Deltaf21.4 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.

Microcontroller compensated overtone TCXO with low phase noise characteristic

This paper presented an improved design on a 121.4MHz 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 100MHz 5th overtone crystal oscillator mixed with the frequency of a 21.4MHz fundamental mode voltage controlled crystal oscillator (VCXO). And then, the mixed frequency was filtered and amplified to produce a 121.4MHz 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 21.4MHz VCXO at every appointed temperature to generate a frequency correction Δf t =Δf 100 +Δf 21.4 and to implement the total compensation. Experimental results show that the frequency-temperature stability of the prototype 121.4MHz TCXO has achieved ±2×10 −7 within the temperature range from −40 to 85°. Besides, a phase noise level of −85dBc/Hz and −145dBc/Hz at 10Hz and 1 kHz offset has been realized.

An improved microcontroller compensated low phase noise overtone TCXO

This paper relates to a novel 121.4 MHz overtone TCXO with low phase noise. It differs from conventional approaches, and adding series inductances and frequency multiplication are not applied in it. It makes the frequency of a 100 MHz 5th overtone crystal oscillator mixed with that of a 21.4 MHz fundamental mode voltage controlled crystal oscillator(VCXO). The product is filtered and amplified to produce a 121.4 MHz signal. In this design, the microprocessor AT89S52 is used to control frequency of 21.4 MHz VCXO to implement the compensation. Experimental results are presented in this paper. The frequency-temperature stability of the prototype 121.4 MHz TCXO has achieved plusmn3times10-7 within the temperature range from -40 to 85degC. A phase noise level of -140 dBc/Hz at 1 kHz offset has been realized. The approach presented in this paper can provide a high stability and low phase noise level requirement of overtone TCXO. In fact, it is also available for a higher frequency.

Design For A Low Phase Noise Overtone TCXO

The design of the MCXO eliminates the three primary obstacles to stability of the conventional temperature compensated crystal oscillator (TCXO). These are: (1) errors in measuring the crystal temperature; (2) use of a fundamental mode AT instead of an overtone SC cut crystal; and, (3) hysteresis and aging associated with pulling the crystal. As a result, the MCXO achieves about 30 times better stability than the best TCXOs. In this paper we review the MCXO design technology, describe the Q-Tech MCXO, QT2001, and describe some future developments.

http://ieeexplore.ieee.org/iel3/4470/12663/00584857.pdf

H. Phillips

Papers

Development status of the microcomputer compensated crystal oscillator