Nokia RM-57, Руководство по обслуживанию

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Table 34 Back-up battery electrical characteristics
Description Parameter Min Typ Max Unit
Back-Up
Battery Voltage
Vback 0 2.5 2.7 V
RF description
Introduction to receiver functionality
Receiver functions are implemented in RF ASIC N7500.
The receiver is a linear direct conversion receiver consisting of separate front ends (LNA and demodulator) for
each supported system.
After the demodulators, the signal paths are combined to one common BB path.
GSM receiver
As GSM Rx branches are functionally identical, the following description is applicable to all of them.
The received signal goes from the GSM antenna to the antenna switch module. The switch module contains PIN
diode switches for a band and Rx/Tx selection and also Rx SAW filters.
The antenna switch module is followed by integrated LNAs residing in N7501.
The LNAs are followed by demodulators which downconvert the signal to baseband I and Q signals. The channel
select filter is set to 115 kHz.
The DC compensation is carried out before the reception slot.
During an operation called DCN1, a sample of the DC level of the signal is stored in sufficiently large off chip
capacitors. During reception, information is in turn used for subtracting the DC information from the input signal
of the AGC amplifier.
DCN0 operation is carried out to discharge any charge from the capacitors before DCN1. This guarantees that
the starting point for the DC compensation is always the same.
Introduction to transmitter functionality
Transmitter functions are implemented in the RF ASIC N7501. It contains a BB frequency low pass filter, which
is tunable according to the signal bandwidth of the system in use.
In addition, N7501 contains separate RF paths comprising a final frequency IQ modulator and VGA amplifiers.
In order to eliminate the effect of process variations on the low pass filter characteristics, a tuning procedure
is carried out in production. The same tunings must be performed if the RF ASIC N7501 is changed.
GSM transmitter
An analogue IQ modulated signal is received to N7501 from digital BB.
The signal is first low pass filtered with filter corner frequency set to approximately 200 kHz. After the filter, the
signal is routed to the GSM modulator.
The appropriate routing after the modulator is selected by biasing either EGSM900 (/GSM850) or GSM1800/1900
variable gain amplifier. The amplifier gives 40 dB of power control dynamic range.
After the VGA stage the signal exits N7501. In case of GSM1800/1900 the signal goes directly to the GSM PA
module. In case of EGSM900 (and GSM850), the PA module is preceded by a SAW filter. After the filter, the signal
is fed to GSM PA module. Finally the signal is routed via antenna switch to the antenna.
RM-57/58
System Module
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