Wcdma Receiver; Gsm Receiver - Nokia RM-42 Service Manual

RM-42
System Module Nokia Customer Care

WCDMA receiver

In the WCDMA mode, the received signal is fed from the antenna to the duplex filter. After the duplex filter the
signal goes via balun to the integrated LNA residing in N7500. After the LNA, the signal goes trough an off chip
band pass SAW filter. The main task of the filter is to attenuate the Tx signal which is leaking trough the duplex
filter and amplified by LNA.
After filtering, the signal goes to the down conversion mixer, which converts the signal into baseband I and Q
signals (90 degrees phase shift). After the demodulator output there is a RC low pass filter with f0 of ca. 1.5
MHz. It is effectively part of the BB selectivity filtering.
At BB frequency the signal is amplified and fed to a low pass filter giving the selectivity of the receiver. The
filters need RC constants, which suffer of process variations. Therefore the integrated resistors are adjustable
by digital control word.
Rx channel filter must be calibrated with automatic routine whenever N7500 IC is changed to a phone.
In the WCDMA mode, the corner frequency of the filter is set to ca. 2.1MHz. The filter is followed by an AGC
amplifier with adjustable gain. Signal is further amplified before it is fed to balanced analogue IQ output pins.
Analogue output pins are accompanied by reference voltage output, which sets the DC level for the AD converter
in BB ASIC RAP3G.
The gain of the Rx chain can be adjusted in multiple phases. The first adjustable gain is in LNA which has low,
mid and high gain settings and isolation mode. After the mixer, there are adjustable gains (AGC) inside the
N7500 IC.
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 N7500 IC 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 N7500.
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 9–47)
9241872 (Issue 1) Company Confidential Page 9–47
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