If a signal with the same frequency as the Broadcast Control Channel (BCH) downlink frequency is artificially transmitted in the area, and the signal power is greater than BCH, the mobile phone in the area cannot receive the BCH signal, so that the mobile phone in the area cannot talk, and the mobile phone signal jammer It is based on this principle.

    The mobile phone shield structure designed according to the shielding principle is generally composed of a power supply, a sawtooth wave generating circuit, a radio frequency module unit, a power amplifier unit, and a transmitting antenna unit. Its working principle is generally generated by a sawtooth wave signal generator, which enters the voltage controlled oscillator (VCO) to adjust to a specific frequency band of the mobile phone signal, and then is amplified by a power amplifier and controlled by a voltage regulator tube. The form is transmitted into the air, covering the BCH downlink frequency signal.

                      picture 1 Shielding Principle

    Taking the GSM signal shielding circuit as an example, firstly, the 10KHz band sawtooth wave generated by the NE555 timer and the peripheral circuit generates the high-frequency radio frequency signal through the GSMVCO to make the frequency reach about 900MHz. Through strict transmission line settings, it can reach the 50Ω matching the output of the radio frequency power amplifier. After that, through the radio frequency power amplifier device, the power reaches 40dB, and then it is transmitted through the antenna to achieve the purpose of shielding.