Analysis and Optimization of the PGC Method in All Digital Demodulation Systems
Analysis and Optimization of the PGC Method in All Digital Demodulation Systems
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DOI：
10.1109/jlt.2008.928926
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发表时间：
2008-09
影响因子：
4.7
通讯作者：
Yang Liu;Liwei Wang;C. Tian;M. Zhang;Y. Liao
中科院分区：
文献类型：
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作者：
Yang Liu;Liwei Wang;C. Tian;M. Zhang;Y. Liao
本文对用于光学干涉仪的PGC解调算法进行了分析和优化，以扩大其在低成本全数字方案中的应用。在我们的低成本系统中，我们选择外差干涉仪和直接调制分布反馈（DFB）激光，并优化算法，以及一些系统参数，以获得最佳的线性和噪声性能。首先分析了低通滤波器（LPF）对PGC-DCM算法的影响。推导了LPF的详细理论模型，揭示了非理想LPF是数字系统线性度的主要极限。实验结果表明，PGC-atan算法的线性度比采用60阶数字FIR滤波器的PGC-DCM算法高40 db。然后分析了PGC-atan算法中强度调制的影响，这是PGC-atan算法中非线性的主要来源，并提出了一种简单而有效的改进方法。实验结果表明，该方法可将强度调制的影响降低约20 dB。进一步分析了光强噪声（LIN）和电路噪声对输出噪声级的影响。推导了由LIN或电路噪声引起的输出噪声的功率谱（PS）的计算公式。仿真结果表明，理论分析精度在0.5 dB以内。利用这些方程，定义了一个新的参数——噪声传递因子，以更好地描述PGC的噪声性能。结果表明，调制深度C和直流工作点Phi =平均值（Phi (t)）对输出噪声电平有很大影响。通过改变C和Phi，输出噪声可降低约20 dB。然后通过三个实例说明了如何根据LIN或电路噪声的特点来选择系统参数。同时对PGC-DCM的噪声性能进行了分析。结果表明，在输出噪声基处存在一个拐点，这使其低频噪声性能恶化。PGC-DCM在高于车削频率时具有相同的NTF。引入参数“频率转换系数”来计算拐点。
In this paper, we analyze and optimize the PGC demodulation algorithm for optical interferometers in order to extend its usage in low-cost all digital schemes. In our low-cost system, we choose heterodyne interferometer and direct modulation on Distributed FeedBack (DFB) laser and optimize the algorithm, as well as some of the system parameters in order to have the best linearity and noise performance. We first analyze the influence of low-pass filters (LPF) in the PGC-DCM algorithm. A detailed theoretical model of LPF is deduced and reveals that a nonideal LPF is the major limit of linearity in digital systems. Experimental results show that the PGC-atan algorithm will have a 40-dB higher linearity than PGC-DCM one with a 60-order digital FIR filter. Then we analyze the influence of the intensity modulation in PGC-atan algorithm, the main source of the nonlinearity in PGC-atan algorithm, and propose a simple but effective modification. Experiment results verified that it can reduce the influence of intensity modulation by about 20 dB. Further, the impact of Light Intensity Noise (LIN) and circuit noise on the output noise level is analyzed. Equations are derived to calculate the Power Spectrum (PS) of output noise caused by LIN or circuit noise. Simulations show that the theoretical analysis is of high accuracy-less than 0.5 dB. With these equations, a new parameter-Noise Transfer Factor-is defined for better discription of the noise performance of PGC. The results show that the modulation depth C and the DC work point Phi = mean(phi(t)) have great impact on the output noise level. By changing C and Phi , the output noise may decrease as much as about 20 dB. Then three examples show how to choose the system parameter according to the characteristics of LIN or circuit noise. As well, the noise performance of PGC-DCM is analyzed. The result reveals a turning point at the output noise base, which worsens its noise performance at lower frequency. PGC-DCM has the same NTF at frequency higher than the turning frequency. A parameter Frequency Conversion Coefficient is introduce to calculate the turning point.