Conversely, if the frequency of the FM input signal decreased below the unmodulated value, the phase shift due to capacitor C5 increases above 90° as the parallel tuned circuit becomes slightly inductive. This causes the voltage across diode D2 to increase and the final output from the demodulator becomes negative. The effect of noise is to change the amplitude of the incoming FM signal resulting in a proportional increase and decrease in the amplitudes of diode voltages VD1 and VD2 and the difference between them. And since this difference in voltage is the demodulated output, the circuit is susceptible to noise interference and should be preceded by a noise limiter circuit