User Guide

CCS Technical Documentation System Module LG4 and Grip Module LS4
Issue 2 11/02 ¤Nokia Corporation Page 43
Receiver
Figure 24: NHL-2NA Receiver chain
The receiver is a direct-conversion, dual-band linear receiver. RF signal energy gathered
by the antenna is fed via the antenna switch module to the 1
st
RX bandpass SAW filters
and MMIC LNAs. The RF antenna switch module provides for upper- and lower-band
operation. The signal having been amplified by the LNA is then fed to 2
nd
RX bandpass
SAW filters. Both of these 2
nd
RX bandpass SAW filters have UNBAL/BAL configuration to
achieve the balanced feed for HAGAR. The discrete LNAs have three gain levels. The first
one is maximum gain, the second one is about -30dB ( GSM1800 ) and –25dB ( E-
GSM900 ) below maximum gain and the last one is off state. The LNA gain selection is
controlled directly by HAGAR.
The performance of the RX bandpass SAW filters are mainly responsible for defining the
receiver's blocking characteristics against spurious signals outside passband and the pro-
tection against spurious responses.
The differential RX signal is amplified and mixed directly down to BB frequency in
HAGAR. The LO signal is generated with external VCO. This VCO signal is divided by 2 (
GSM1800 ) or by 4 ( E-GSM900 ). The PLL and dividers are internal to the HAGAR IC.
From the mixer output to ADC input RX signal is divided into I- and Q-signals. Accurate
phasing is generated in LO dividers. After the mixer DTOS amplifiers convert the differen-
tial signals to single ended.
The DTOS has two gain stages. The first one has constant gain of 12dB and 85kHz cut off
frequency. The gain of second stage is controlled with control signal g10. If g10 is high
(1) the gain is 6dB and if g10 is low (0) the gain of the stage is -4dB. The active channel
filters in HAGAR provide selectivity for channels (-3dB @ r 91 kHz typ.). The integrated
baseband filter inside HAGAR is an active-RC-filter with two off-chip capacitors. Large
RC-time constants are needed in the channel select filter of the direct-conversion
receiver and are achieved with large off-chip capacitors because the impedance levels
could not be increased due to the noise specifications.
The baseband filter consists of two stages, DTOS and BIQUAD. DTOS is differential to sin-
gle-ended converter having 8dB or 18dB gain. BIQUAD is modified Sallen-Key Biquad.
Integrated resistors and capacitors are tunable. These are controlled with a digital con-