Motorola TALKABOUT T2688, Руководство по обслуживанию

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Topaz T2688/T200, T2988/T205 L3 Circuit Description
12
Motorola Proprietary Information
VEGA IC and has a range of approximately 1.2V – 1.5V. The Oscillator supply is
generated from the Base-band Voltage Regulator U27 Pin 24 (VTCXO 2.8V). The
13Mhz is buffered by IC402 before being O/P as 13MHz to VEGA for use as the
system clock.
68. At the same time it is input to the Synthesiser IC on Pin 1 OSCin, from here the IF
tuning voltage IF_TUNE as spoken about in points 12 (RECEIVE) and 46
(TRANSMIT) to drive the 540 / 135MHz Varactor diode.
69. All the controlling signals for the IC are generated from Gemini and enter on Pin 14
DATA, the signals are clocked in ( Pin 15 CLK ) and the IC is enabled using the
control logic SYNPRG on Pin 13 ( LE )
70. The IF Support voltage for the IC is SYN2V8 , a 2.8V support, generated in the RF
Voltage Regulator IC301 Pin 2 ( OUTC ). A sample of this voltage is taken to ensure
the synthesiser IC is switched off during idle time. This sample is fed into Pins 6 and
7 .
71. The Feedback from the 540 / 135MHz Varactor diode is returned to the IC as the
signal
LO2 and is fed in on Pin 3 ( finif ). To complete the phase lock loop.
72. On the RF side, the supporting voltage is again SYN2V8 and enters on Pin 11
( vccrf ). The charge pump output for the TXVCO is fed out on Pin 8 ( Dorf ) to the
RXVCO U401 Pin 3 . U401 is supported by SYN2V8 .
73. The band select signal BS enters U401 on Pin 4 and will select the appropriate output
to either GSM or DCS on Pin 1 . The output will then be fed out to the TX / RX IC as
LO1 to create the 1
st
IF, and also a sample will be fed back into the Synthesiser IC to
provide Phase Lock Loop feedback.
Power up Sequence
74. The main core of the power up sequence next to the operating software is the Power
Management IC U27 . U27 contains 4 Voltage regulators.
?
DVCC – 2.765V @ 100mA and provides the Base-band logic supplies. Pin 2
?
AVCC – 2.765V @ 130mA and provides for analogue voltage supplies . Pin
27
?
VTCXO – 2.765V @ 5mA – for use to drive the system 13Mhz clock. Pin 24
?
VRTC – To charge to the Real Time Clock battery U32 (when battery voltage
falls below 1.2V). Pin 8
75. The battery voltage VBAT on Pin 1 supplies U27 .
76. Once power is supplied to the unit, there are a number of ways that the unit can be
powered up. The input pulse is a 40mS +ve going pulse through
U53 and U54 and
onto U27 inputs Pins 4&5 ,