// 自动扫频测量幅频响应
void sweepTest() {
  for(float fc=100; fc<=10000; fc*=1.1) {
    setCutoffFrequency(fc);
    delay(50);  // 稳定时间

    float Vout = measureOutput();
    float gain = 20 * log10(Vout / Vin);

    Serial.print(fc); Serial.print(",");
    Serial.println(gain);
  }
}