Nokia Mobile Phones NSE-5 Series Service Manual page 70

NSE–5
System Module
diplexer that has one common antenna input/output is used. The selection
between GSM900 and GSM1800 operation modes in the CRFU3 is done
with the band selection signal (BAND_SEL) from the MAD in baseband.
GSM900 front–end
The GSM900 receiver is a dual conversion linear receiver. The front–end,
which is located in the CRFU3 RF-, is activated with the band-selection
signal (BAND_SEL) set to high-state. The received RF-signal from the
antenna is fed via the diplex filter and the duplex filter to the LNA (Low
Noise Amplifier) in the CRFU3. The active parts (RF-transistor and biasing
and AGC-step circuitry) are integrated into this chip. Input and output
matching networks are external. The Gain selection is done with the
FRACTRL signal. The gain step in the LNA is activated when the RF-level
at the antenna is about -47 dBm. After the LNA, the amplified signal (with
low noise level) is fed to the bandpass filter, which is a SAW-filter (Surface
Acoustic Wave). The duplex filter and the RX interstage bandpass filters
together define how good the blocking characteristics are.
The bandpass filtered signal is then mixed down to 71 MHz, which is the
first GSM900 intermediate frequency. The first mixer is located in the
CRFU3 and upper side injection is used for the down mixing. The
integrated mixer is a double balanced Gilbert cell. It is driven balanced. All
active parts and biasing are integrated. Matching components are
external. Because it is an active mixer it also amplifies the IF-signal.
Buffering of the local signal is integrated too. The first local signal is
generated by the UHF-synthesizer.
GSM1800 front–end
The GSM1800 receiver is a triple conversion linear receiver. The received
RF-signal from the antenna is fed via the diplex filter, the RX–TX switch
and the first RX SAW filter to the LNA in CRFU3. The RX–TX switch is
controlled by the band selection signal (BAND_SEL = low) and the supply
voltage for the transmitter part (VTX = low). VTX ensures that the switch
can not turn to transmit position when the transceiver is in receive mode.
The front–end in the CRFU3 is activated with band-selection signal
(BAND_SEL) set to low-state. The active parts (RF-transistor and biasing
and AGC-step circuitry) are integrated in this chip. The input and output
matching networks are external. The gain selection is done with the
FRACTRL signal. The gain step in the LNA is activated when the RF-level
at the antenna is about -47 dBm. After the LNA, the amplified signal (with
low noise level) is fed to the second RX–SAW bandpass filter. The two
RX–SAW bandpass filters together define how good the blocking
characteristics are.
The bandpass filtered signal is then mixed down to 187 MHz IF, which is
the first GSM1800 intermediate frequency. The first mixer is located in the
CRFU3 and upper side injection is used for the down mixing. The
integrated mixer is a double balanced Gilbert cell. It is driven balanced. All
active parts and biasing are integrated. Matching components are
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PAMS
Technical Documentation
Issue 1 07/99