高頻、高品質因素及低相位雜訊壓電振盪器之研究

Abstract

在高速數位通訊系統中，為解決不同傳輸速率設備間訊號之連結與訊號在長距離傳輸後之衰減，系統之寬頻收發器須具備時脈恢復電路對訊號作頻率轉換及波形重整以維持資料完整性。上述電路中均須使用高性能之壓控振盪器，因此具高穩定性之高品質因素壓控壓電振盪器成為最佳選擇。若以壓電共振器的製程限制及工作頻率來區分，當工作振盪頻率低於200 MHz時，石英晶體共振器為最佳選擇；當工作頻率低於3000 MHz時，表面聲波共振器(SAWR)為目前主流元件；而薄膜體波共振器(FBAR)則適用於微波頻段。 本論文共提出四種高品質因素壓電振盪器，分別為: 2488 MHz壓控聲橫波(STW)振盪器，2488 MHz壓控薄膜體波(FBAR)振盪器，622 MHz 修正式皮爾斯(Pierce)振盪器及433 MHz平衡式表面聲波(SAW)振盪器。 本文中2488 MHz壓控聲橫波振盪器之可調頻率範圍可達±200 ppm，其相位雜訊在100 kHz偏移時，相較於一般市售同類產品低8分貝；在振盪器設計階段藉由分析元件的殘餘相位雜訊(residual phase noise)，發現聲橫波共振器為本振盪器主要雜訊來源，而非主動元件。振盪器之相位雜訊可經由元件之殘餘相位雜訊及回路轉換函數預測且預測值與量測結果相當接近。本2488 MHz壓控薄膜體波振盪器之可調頻率範圍可達±6%，其振盪頻率之溫度係數大約為-34.5 ppm/oC。 我們藉由在傳統皮爾斯振盪器之回授π電路上增加一相移器來降低回授電路之插入損失，以改善其在高頻應用不易起振之缺點。本振盪器核心電路僅銷耗+1.2V, 10.2 mA，其相位雜訊在距中心載波10 kHz處可達-136 dBc/Hz。 我們藉由一般商用表面聲波共振器來製作平衡式表面聲波振盪器，其對偶特性經由量測獲得驗證，並增加一180度反相功率偶合器來完成push-push架構振盪器，其相位雜訊相對於單端輸出可獲6分貝之改善。

The increasing demand for high speed digital communication applications motivates a growing interest in the high data rate transceivers. In these transceivers, clock recovery circuits are used for data integrity. The clock is usually extracted from a phase-lock-loop circuit with low phase noise voltage-controlled oscillator (VCO). For the requirements on the high frequency and low phase noise voltage-controlled oscillators, the highly stable VCOs with piezoelectric resonators are the hopeful candidates. In the frequency below 200 MHz, the crystal resonators are the best choice. In the ultar-high-frequency (UHF) band, surface acoustic wave resonators (SAWR) play the important roles now. In the band of UHF and microwave frequency, the film bulk acoustic resonators (FBAR) are the future stars. There are four different types oscillators are presented in this thesis. They are VCO with STW resonator, VCO with FBAR, modified Pierce SAW oscillator, and balanced SAW oscillator. Both the surface transverse wave (STW) resonator and FBAR are applied for the low phase noise 2488 MHz voltage-controlled oscillators. The tuning ability of the 2488 MHz voltage-controlled oscillator with STW resonator achieves ±200 ppm and its phase noise performance is 8 dB better than the other commercial products at offset 100 kHz frequency. The white phase-noise floor is about -170 dBc/Hz. In the design phase of the oscillator with STW resonator, by examining the residual phase noises of the main components in the oscillator, we found that the STW resonator dominates the phase noise of the oscillator, instead of active devices. The behavior of the phase noise of the oscillator is shaped by the important factor of group delay. The tuning capability of the voltage-controlled FBAR oscillator achieves ±6%. The temperature coefficients factor is about -34.5 ppm/oC which is dominated by FBAR. An extra phase shifter is used to reduce the insertion loss of the π-feedback network in the conventional Pierce SAW oscillator. The modified Pierce oscillator provides 4.0dBm of output power and the core circuit consumes 10.2 mA from +1.2V DC power supply. The phase noise level of the oscillator is approximately –136 dBc/Hz at 10 kHz offset. With a popular commercial one-port SAW resonator, the behavior of the balanced SAW oscillator is presented. The balanced SAW oscillator provides pure antiphase signals without balun. By using an 180o power combiner, a push-push topology SAW oscillator is presented and results in 6dB phase noise improvement in comparison with the single-end output.