fir filter problems

[Altera_Forum]

also see “fir filter problems with ultrasonic signal” 

  I designed a filter with fir compilier to filtering of ultrasonic signals. The Filter is full parallel DA structure. AD convertor and the filter's sampling rate is 50Mhz. But after filtered , the sound path of ultrasonic signal has shorter . Sound path = velocity (constant) x echo time. So I guess the echo time has shorter .  however ，why the echo time has shorter ?  I infer the system clock driver is overloaded , resulting clock skew is too large, so the actual frequency of less than 50Mhz, and if so, how to solve it? ? ? thanks (system clock output by PLL ) )     我用fir compilier 做滤波器，对超声信号滤波。滤波器 采用的 全并行 DA结构。 AD和滤波器的采样率都是50Mhz的 。但滤波后，超声信号声程变短了。 声程= 声速（常量）x 回波时间 。所以是 回波时间 变小了。 回波时间变小的原因是什么呢？   我推测 是由于系统时钟CLK 驱动负载过重，导致时钟偏移过大，使得 实际频率 小于50Mhz，如果是这样，怎么解决呢？？？ （系统时钟CLK 是通过PLL稳定输出的）