User Guide

NMM-3 Company Confidential
7 - System Module & UI CCS Technical Documentation
Page 7-38 Copyright  2003 Nokia Corporation Issue 1 (11/2003)
Company Confidential
After the pregain stages there are demodulator mixers at each signal path to convert the
RF signal directly down to baseband I- and Q-signals. Local oscillator signals for the
mixers are generated by an external VCO the frequency of which is divided by two in
GSM1800 and by four in GSM900. The frequency dividers are integrated in HELGO and
in addition to division they also provide accurate phase shifting by 90 degrees which is
needed for the demodulator mixers.
DtoS (differential to single ended) amplifiers are then used to combine the signals from
the three demodulators to a single common path so that from the output of the demod-
ulators to the baseband interface there are only two signal paths (I and Q) to both fre-
quency bands. The DtoS amplifiers also performs the first part of the channel filtering
and AGC (automatic gain control). They have two gain stages, the first one with a con-
stant gain of 12 dB and 85 kHz -3 dB bandwidth and the second one with a switchable
gain of 6 dB and -4 dB. The filters in the DtoS blocks are active RC filters. The rest of the
analog channel filtering is provided by blocks called BIQUAD, which include modified
Sallen-Key biquad filters.
The channel filters need large off-chip capacitors. They are needed because the direct
conversion receiver requires long RC time constants in the channel filters to be able to
operate properly.
Integrated resistors and capacitors of the channel filters are adjustable by a digital con-
trol word. The correct control words that compensate for the process variations of inte-
grated resistors and capacitors and for the tolerance of the off- chip capacitors is found
by a calibration circuit inside HELGO which is tuned in production.
After the DtoS and BIQUAD blocks there is another AGC-amplifier, which provides a gain
control range of 42 dB in 6 dB steps. The correlation between the gain steps and the
absolute received power levels is found by a calibration routine in production.
In addition to the AGC steps, the last AGC stage also performs the real time DC offset
compensation, which is needed in a direct conversion receiver to cancel out the effect of
the demodulators local oscillator leakage. DC offset compensation is performed during
an operation called DCN. DCN is carried out by charging integrated capacitors in the last
AGC stages to a voltage which cause a zero DC offset
After the last AGC and DC offset compensation stages the single ended and filtered I-
and Q-signals are finally fed to the RX ADCs. The maximum peak-to-peak voltage swing
for the ADCs is 1.45 V.
AGC strategy
The AGC-amplifier is used to maintain the voltage swing at the RF-BB interface at the
wanted level. The nominal voltage level is 240mVpp.
The AGC has to be set before each received burst. In idle mode this is done by an oper-
ation called pre-monitoring. The receiver is switched on roughly 240 us before the own
reception begins, the DSP measures the received signal level and then adjusts the AGC
stages by sending gain information through the RFBus. In call mode there is no pre-