應用於超寬頻和硬碟讀取系統之高速轉導電容式連續濾波器

Abstract

本論文提出二種應用於高速系統上的轉導電容式濾波器:超寬頻系統，硬碟讀取系統。使用轉導電容式濾波器的原因是因為轉導電容式濾波器比切換式電容濾波器更適合應用在高速的系統上。但是轉導電容式濾波器的線性度比較差，所以本論文提出二種改善線性度的電路而且又適合應用在高速的系統上。 第一種電路是改良傳統源極衰減電路，使電路在較小的電阻下而能達到所需要的線性度。因為電流回饋方式擁有比電壓回饋方式大的頻寬，所以電流回饋方式比電壓回饋方式較適合於高速的應用上。另外一種電路則是改良固定輸入對汲極到源極電壓的電路，使電路更適合應用在較低電壓及高速的系統上。另外，此電路需要共模前置回饋電路來增加抑制共模信號比例(CMRR)。最後利用轉導放大器組成4階濾波器，在此濾波器的輸出端接上輸出緩衝器，如此濾波器在量測的時候才不會受到儀器的負載影響。 量測的結果:電流回饋式濾波器的截止頻寬為250MHz，這是超頻寬的最低頻率。其群延遲變動約為5ns，而在輸入訊號80MHz時，其第三諧波失真(HD3)為-40dB。在輸入訊號252MHz及248MHz時，其第三內調變失真(IM3)為-35dB。量測的結果: 用前置電路固定輸入對汲極到源極電壓濾波器的截止頻寬為250MHz，這是超頻寬的最低頻率。其群延遲變動約為5ns，而在輸入訊號80MHz時，其第三諧波失真(HD3)為-40dB。在輸入訊號252MHz及248MHz時，其第三內調變失真(IM3)為-36dB。 此二顆低通濾波器的製程為台積電0.18μm CMOS製程。源極衰減電流負回饋電路的主動面積為0.386×0.264mm2，功率消耗約為42m瓦特。用前置電路固定輸入對汲極到源極電壓電路的主動面積為0.3515×0.3609mm2，功率消耗約為42m瓦特。

In this thesis, two transconductance-C filters are proposed for high frequency applications of ultra wideband and hard-disk driver systems. The transconductance-C filters are more suitable than switch capacitor filters in high frequency application. However, their main drawback is the linearity, so two linearity improvement techniques are proposed in this research. The first transconductor circuit is an improved source degeneration circuit. Source degeneration circuits usually use large values of resistors to improve linearity. Also, the current feedback works better at high frequency than voltage feedback because the former one has larger bandwidth. Hence, the proposed circuit utilizes negative current feedback to improve the linearity at high frequency, which can achieve the requirement without large resistor. The other transconductor circuit which achieves the constant drain-source voltage of the input pair is proposed. This circuit can be used in the lower power supply and high frequency applications. The transconductor circuit makes use of the common mode feedforward circuit to increase the common mode rejection ratio. Finally, two 4th-order linear phase filters by using these transconductor cells is presented.Output buffers are connected to avoid the influence of the loading in the measurement. From the measurement results, the cutoff frequency for the filter of the current feedback transconductor circuit is about 250MHz, which is the bandwidth of the ultra wideband system. The group delay variation is less than 5ns in the passband. The third harmonic distortion (HD3) is about -40dB at 80MHz input signal. The third intermodulation distortion (IM3) is about -35dB by the two-tone measurement of 248MHz and 252MHz. From the measurement results, the cutoff frequency for the filter of Constant-Vds with feedforward circuit is about 250MHz, which is the bandwidth of the ultra wideband system. The group delay variation is less than 5ns in the passband. The third harmonic distortion (HD3) is about -40dB at 80MHz input signal. The third intermodulation distortion (IM3) is about -36dB by the two-tone measurement of 248MHz and 252MHz. The low-pass filters were fabricated by the TSMC 180-nm CMOS process. Source degeneration with negative current feedback circuit occupies a small area of 0.386×0.264mm2 and the power consumption is 42mW under a 1.8V supply voltage. Constant-Vds with feedforward circuit occupies a small area of 0.3515×0.3609 mm2 and the power consumption is 42mW under a 1.8V supply voltage.