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RM-36
System Module
Nokia Customer Care
The last stage of the RF Rx chain is an output buffer which feeds the signal and a reference voltage (VREFCM) to
BB ASIC. The AGC stages are used to maintain the voltage swing at the input of the AD converters at an adequate
level.
The gain of the Rx chain is measured in production at one RF frequency and power level, so that RSSI reporting
gets calibrated. If N7501IC is changed this calibration needs to be performed.
GSM receiver
As GSM900, GSM1800 and GSM1900 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.
After the down conversion mixer, the Rx chain is similar to WCDMA Rx. Channel select filter is set to 115 kHz in
the GSM mode.
In the GSM mode, 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.
See Also
•
WCDMA receiver (Page 9–46)
Transmitter
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.
WCDMA transmitter
In the transmitter side, an analogue I/Q modulated signal is received from the digital baseband into N7501 and
fed through the low pass filter.
The corner frequency of the filter is set to approximately 3 MHz.
After the filter the signal is fed to the IQ modulator, which converts the signal to final Tx frequency. There are
two separate I/Q modulators. One for WCDMA and another for EGSM900 and GSM1800/1900 signals.
The modulator is followed by two VGA stages giving 85 dB of gain control range. The signal then exits N7501
via a balanced line. In order to attenuate the out of band noise of the transmitter the signal is band pass filtered
by a SAW filter before it is fed to the WCDMA PA module.
After the PA the transmitted WCDMA signal is fed through an isolator and a duplex filter to the antenna.
Issue 1
Company Confidential
Page 9–47
Copyright ©2005 Nokia. All Rights Reserved.
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