void setFilter4thOrder(float fc) {
  // 4阶巴特沃斯: 两个2阶级联
  // Stage1: Q=0.541
  // Stage2: Q=1.306

  float C = 10e-9;

  // Stage1 计算
  float R1 = calcResistor(fc, 0.541, C);
  uint8_t pot1 = (uint8_t)((R1/10000.0)*256);

  // Stage2 计算
  float R2 = calcResistor(fc, 1.306, C);
  uint8_t pot2 = (uint8_t)((R2/10000.0)*256);

  // 更新数字电位器
  MCP41010_Write(CS1_PIN, pot1);
  MCP41010_Write(CS2_PIN, pot2);

  // OLED显示
  OLED_ShowString(0, 0, "fc:");
  OLED_ShowNum(30, 0, (uint16_t)fc, 5);
  OLED_ShowString(80, 0, "Hz");
}