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This lesson talks about the GSM call channel of Xiaomi mobile phone

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Take Mi 10 Pro as an example

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In this circuit, we first look at its receiving channel

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The antenna shrapnel in the mobile phone is connected to the antenna

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The high-frequency electromagnetic wave signal of GSM900 is sensed by the antenna,

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and finally becomes a high-frequency electrical signal, reaching the antenna shrapnel J0204

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On the circuit diagram it is referred to as LMH

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There are not only low-frequency signals, but also mid-frequency and high-frequency signals induced on this line

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The received signal of GSM900 reaches the coaxial switch J0304,

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and the two pins of the coaxial switch are always on

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After it is turned on, it reaches the three pins of the filter FL0301

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After filtering out the clutter, output from the eight pins, and then reach a small antenna switch

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The eight-pin and six-pin of the small antenna switch are turned on,

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so that the signal reaches a low-frequency band filter

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It only passes low frequency band signals

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The received signal enters from pin 5 and exits from pin 2,

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and then reaches the low-frequency antenna switch and power amplifier U0600

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The signal passes through the antenna switch and comes out from pin 44 to form the receiving signal of B8

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Because the reception of GSM900 and B8 take the same channel

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So B8 reception is GSM 900 reception

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The signal continues on to the receiving amplifier, which is identified as an LNA in the drawing

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After internal power amplification, it comes out from the fourth pin and reaches the A8 pin of the RF chip U1301

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After the high-frequency carrier is removed by radio frequency,

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preliminary demodulation is carried out, and it is sent to the Qlink bus

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This Qlink bus contains voice signals, which are given to the baseband CPU

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The baseband CPU demodulates the received signal,

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and sends the demodulated sound signal to the large CPU through the bus

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This is its receiving channel

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Its transmission channel starts from the baseband CPU

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The baseband CPU modulates the sound signal sent by the large CPU,

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and finally sends it to the RF transceiver through the Qlink bus

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This is a low frequency signal containing speech that cannot travel long distances and quickly over the air

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The radio frequency chip generates a high-frequency carrier signal,

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and carries the low-frequency voice signal on the high-frequency carrier to form a transmission signal

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The transmission signal of this GSM 900 is already a high-frequency signal, and it has to continue

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It reaches the low-frequency power amplifier through capacitive coupling for power amplification

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After internal power amplification, it is output from the second pin of U0600,

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and then reaches the antenna end

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After the filter is filtered to reach the six-pin and eight-pin output of the small antenna switch,

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and then reach the eight-pin of the total filter

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The three-pin output of the filter, through the conduction of the coaxial switch, finally reaches the antenna shrapnel

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The antenna shrapnel is in contact with the antenna

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The antenna converts this high-frequency transmission signal into a high-frequency electromagnetic wave signal

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and transmits it to the base station

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This is its launch channel

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It should be noted here that the line from the second pin of the low-frequency power amplifier to the antenna end

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is shared by receiving and transmitting

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The separation of receive and transmit begins at the rear end of the low-band power amplifier

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The reception is from the 44 feet to the receiving channel, and finally to the radio frequency,

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and the transmission is from the radio frequency to the low-frequency power amplifier

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This is the channel of Xiaomi's GSM 900 call

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GSM 900 is actually the calling channel of China Mobile

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That's all for this lesson