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This lesson talks about no service or no signal repairs

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First look at the failure phenomenon of no service

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Cell phone without service, the signal bar is grayed out

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When dialing a number, it prompts that the dialing failed, or prompts to open the antenna

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As shown in the picture below, after inserting the card,

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the mobile phone prompts that the number card can be recognized,

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but the signal strength bar in the upper right corner is gray

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This is no service or no signal

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Another is that when we dial a number, the mobile phone prompts that the antenna is being turned on

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It stays on this screen, can't make a call

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This is also no service or no signal

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Next, let's look at the causes of no service

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There are 4 reasons for no service

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As mentioned earlier, the failure of the baseband circuit will lead to no service,

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and no card reading will also lead to no service

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The next thing we need to look at is that the baseband is normal and the card reading is normal,

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and there is no service caused by abnormalities in the signal receiving channel or transmitting channel

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First look at the receiving channel of the mobile phone signal

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After the electromagnetic wave received from the air is converted into an electrical signal by the antenna,

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it is turned on to the filter by the coaxial switch

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After the filter filters out the clutter, it is given to the antenna switch to form a received signal

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The signal is amplified by the receiving amplifier and then sent to the RF transceiver

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The RF transceiver filters out the high-frequency carrier signal and demodulates the IQ signal

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This IQ signal contains the sound signal of the phone call

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The IQ signal is passed to the baseband, and the baseband demodulates it

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This is the receiving channel of the signal

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Signal transmission channel

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The sound signal from the sound circuit is given to the baseband CPU through the I2S bus

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After modulation by the baseband CPU, the transmitted IQ signal is formed

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This low-frequency signal cannot be propagated in the air,

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it has to be up-converted by radio frequency

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RF transceivers convert low-frequency signals into high-frequency transmit signals

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For example, China Mobile uses GSM900, which is a transmission signal of about 900 MHz

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This signal is very weak and needs to be amplified by electric power

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After amplification, the frequency is unchanged, and only the electric power is amplified

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This high-frequency signal is filtered out by a filter to remove clutter

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Continue through the conduction of the coaxial switch to the antenna

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The signal is radiated into the air through the antenna into high-frequency electromagnetic waves,

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and sent to the base station

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This is the transmission process of the signal

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Resulting in no service, either the receive channel is faulty or the transmit channel is faulty

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That's all for this lesson