A study on uncertainty and error of sampling clock in electromagnetic prospecting systems
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摘要:
电磁法观测系统中，采样时钟质量直接影响高精度ADC的信噪比，而高精度ADC芯片的信噪比作为电磁法观测系统信号检测非常关键的指标，决定了获取数据的质量.采样时钟的时间抖动是时钟孔径不确定度产生的原因，导致电磁法观测系统中ADC数据转换的信噪比变差，严重影响电磁系统观测质量，为此提出了ADC采样时钟不确定度最大时间抖动的确定方法.该方法通过对ADC采样过程进行时域、频域分析，借助正弦波导数给出采样时钟孔径不确定度引起的采样数据误差数学描述，结合极大值法与ADC信噪比计算方法获取系统信噪比需求范围内的最差时钟孔径不确定度，进而计算出相应采样时钟的最大时间抖动误差.以地球物理仪器中常用的ADC芯片AD7760为例进行了相应的时钟抖动误差计算，确定时钟的孔径不确定度抖动时间.根据信噪比指标确定了时钟孔径不确定度后，采用压控恒温晶振跟踪GPS信号中的秒脉冲的方法，保证时钟抖动误差能够满足AD7760的要求，根据测试表明，在GPS失锁4 h内，时钟输出能够满足技术要求.
Abstract:The sampling clock directly affects the high resolution ADC SNR (signal-to-noise ratio) that determines the quality of the sampled data in electromagnetic prospecting system.Time sampling clock jitter causes to the clock aperture uncertainty.This worsens the SNR of the ADC data transformation and weakens the prospecting quality in electromagnetic prospecting system.A method to determine the maximum time jitter for ADC sampling clock uncertainty is proposed.The analysis of ADC sampling process both in time domain and in frequency domain is done firstly.Using the derivative of sine wave, the mathematical description of sampling data error caused by the sampling clock uncertainty is given.By combining the maximum value method with the ADC SNR method, the worst clock aperture uncertainty within the scope of system SNR requirements is achieved.And then the maximum time jitter error of the corresponding sampling clock can be calculated.Taking the AD converter AD7760 as an example, which is commonly used in geophysical instruments, the corresponding clock jitter error is calculated and the clock aperture uncertainty is determined.The clock aperture uncertainty can be calculated according to SNR.The method using oven controlled crystal oscillator (OCXO) to track GPS pulse per second (PPS) is applied to ensure that the clock jitter error can satisfy the requirements of the AD7760.The actual tests show that the output of OCXO satisfies the technical requirements after GPS loss of lock within 4 hours.
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Key words:
- Clock jitter /
- FPGA /
- OCXO /
- PPS
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表 1
GPS失锁后1 Hz信号与PPS的跟踪误差统计
Table 1.
Statistical analysis of tracking error of 1 Hz signal and PPS after GPS lock off
失锁时间 30 min 1.5 h 2.5 h 4 h 跟踪误差 50 ns ＜380 ns ＜850 ns ＜1.3 μs 
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