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This lesson talks about the GSM 900 calling channel of Xiaomi mobile phone
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Take Mi 10 Pro as an example
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Open the circuit diagram of Mi 10 Pro
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Its antenna is generally on the first page of the drawing
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TX is transmit, RX is receive, TRX is transmit and receive
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The upper left foot tells us that the first page is the transceiver part of the main antenna
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In this circuit, we can use the method of elimination to find
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This is the antenna for WiFi, this is the antenna for 5G
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Then the remaining one is the antenna of ANT0
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This antenna has low band, mid band and high band
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Let's take a look at Xiaomi's GSM 900 channel
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J0204 is an antenna holder
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Antenna is mounted on the antenna
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Let's look at GSM 900 reception first
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The antenna converts the electromagnetic waves coming from the air into electrical signals,
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and then sends them to the seat
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The electrical signal is capacitively coupled and turned on by a 0 ohm resistor to form an antenna zero low, medium and high
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The low-band, mid-band, and high-band antennas are all here
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GSM 900 is in the low band so it's here too
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Let's see
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The receiving signal arrives at the seat J0304
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It is a coaxial switch, we also call normally closed contact
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This coaxial switch is factory tested
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Pin 2 and pin 1 of this coaxial switch are always on, in fact it is a wire inside
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After the received signal comes out from pin 1, it passes through the inductor and reaches the filter FL0301
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GSM 900 goes out from the 8-pin of the filter, and reaches the 8-pin of the S0308 through the inductor
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This is a Double Pole Double Throw switch, the same as Huawei's
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It forms a low-band antenna signal, LBANT
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This low band antenna signal contains GSM 900
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Let's look at the right
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Its RF3 pin and RF1 pin are turned on for a relatively long time
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Pin 2 is the signal from the diversity antenna
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This is a diversity reception, 2 and 4 are turned on
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We mainly look at the main set, which is the signal from pin 8 to pin 6
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Let's see where the signal goes
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The signal goes to pin 5 of the filter FL0600 and goes out from pin 2
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Then it reaches pin 2 of chip U0600
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The 2-pin is named as the antenna of the low frequency band, so GSM 900 must be here,
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because GSM 900 belongs to the low frequency band
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This is an antenna switch plus low-band power amplifier and 2G power amplifier chip
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After it comes in from pin 2, the internal switch is turned on, so that pin 2 is connected to pin 44
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Pin 44 is the receiver of B8, B8 receiver and GSM 900 take the same channel
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So here is the GSM 900 reception
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This is the GSM 900 line from antenna to power amplifier plus antenna switch
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That's all for this lesson
