User Guide

PAMS
Technical Documentation
NSE–8/9
System Module
Page 2– 78
Issue 1 07/99
Figure 23. Receiver Block Diagram
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.