Noise source of the cavity BPM electronics circuit was surveyed. Terminating the input of each stage of the electronics, the output signal of the last stage (phase detector) was measured by an oscilloscope, a spectram analyzer, and a charge-sensitive ADC. The dominant noise source was found to be in the converter module (and also in the first stage of the phase detector). The noise spectrum of the signal which was fed into the ADC was dominated by low frequency component.