input: def generate_voltage_signal(freq: int, amplitude: float, duration: float, sample_rate: int) -> np.ndarray:
      """Generate a voltage signal with specified frequency and amplitude, and add random noise to it.
      >>> generate_voltage_signal(250, 1, 1, 1000)
      array([ 0.628939,  0.067517, -0.373613, ...,  0.484274,  0.814403,  0.342900])
      """
      t = np.linspace(0, duration, int(duration * sample_rate))
      voltage_signal = amplitude * np.sin(2 * np.pi * freq * t) + np.random.normal(0, 0.1, len(t))
      return voltage_signal
output:     t = np.linspace(0, duration, int(duration * sample_rate))
      voltage_signal = amplitude * np.sin(2 * np.pi * freq * t) + np.random.normal(0, 0.1, len(t))
      return voltage_signal