User Guide

PAMS
Technical Documentation
NSE–8/9
System Module
Page 2– 79
Issue 1 07/99
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
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 VHF-synthesizer.
There is a balanced discrete LC-bandpass filter in the output of the first
mixer which e.g. attenuates the critical spurious frequencies 161 MHz and
277 MHz and also the 151,5 MHz half-IF. It also matches the impedance
of 187MHz output to the input of the following stage. After this filter, the
187MHz IF-signal is mixed down to 71MHz IF, which is the second
GSM1800 IF. The VHF-mixer is also a double balanced Gilbert cell and is
located into the CRFU3. Lower side injection of the LO signal is used for
this down conversion.
The 116MHz LO signal comes from the SUMMA-, where it is derived by
dividing the 464MHz VHFLO signal by four. There is an external lowpass
filter for the 116MHz LO signal that attenuates the harmonics (especially
232MHz) so that the critical mixing spurious will be attenuated.
Common Receiver parts for GSM900 and GSM1800
After the down conversions in the CRFU3– the RX-signal path is common
for both systems. The 71MHz IF-signal is bandpass filtered with a
selective SAW-filter. From the output of to IF-circuit input of the SUMMA,
signal path is balanced. IF-filter provides selectivity for channels greater
than +/-200 kHz. Also it attenuates image frequency of the following mixer
and intermodulating signals. Selectivity is required in this place, because
of needed linearity and without filtering adjacent channel interferes would
be on too high signal level for the stages following.