User Guide

RH-26
2 - System Module CCS Technical Documentation
Page 46 ©Nokia Corporation Issue 1 02/04
Receiver
Each receiver path is a direct conversion linear receiver. From the antenna, the received
RF-signal is fed to a front end module where a diplexer first divides the signal into two
separate paths according to the band of operation: either lower, GSM850 or upper,
GSM1800/GSM1900 path. Each of the paths follows a pin-diode switch which is used to
select either a receive or transmit mode. At the upper band in the receive mode, either
GSM1800 or GSM1900 path is further selected by another pin-diode switch. The selec-
tions are controlled by Helgo, which obtains the mode/band and timing information
through the RFBus. After the front end module there is a SAW filter at each of the
receiver paths. Then, the signal is fed to the LNAs which are integrated in Helgo in
GSM850 and GSM1800 while in GSM1900 the LNA is external. In GSM1900 the ampli-
fied signal is fed to balun and after that to a pregain stage of the mixer, while in
GSM850 and GSM1800 the signals are connected directly to the LNA’s and after that to
the pregain stages. The pregain stages as well as all the following receiver blocks are
integrated in Helgo. The LNAs have three gain levels. The first one is the maximum gain,
the second one is about 30 dB below the maximum, and the last one is the off state
level.
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 mix-
ers are generated by an external VCO. The frequency is divided by two in GSM1800 and
GSM1900 and by four in GSM850. Those frequency dividers are integrated in Helgo. They
provide accurate phase shifting by 90 degrees which is needed for the demodulator mix-
ers.
The demodulator output signals are all differential. After the demodulators, there are
amplifiers called DtoS (differential to single ended) which convert those differential sig-
nals to single ended. Before that, they combine the signals from the three demodulators
to a single path which means that from the output of the demodulators to the baseband
interface there are just two signal paths (I and Q) which are common to all the frequency
bands of operation. In addition, the DtoS amplifiers perform the first part of the channel
filtering and AGC (automatic gain control): they have two gain stages, the first one with
a constant gain of 12 dB and 85 kHz -3 dB bandwidth and the second one with a switch-
able gain of 6 dB and -4 dB. The filters in the DtoS blocks are active RC filters. The rest of
the analogue channel filtering is provided by blocks called BIQUAD which include modi-
fied Sallen-Key biquad filters.
After the DtoS and BIQUAD blocks there is another AGC-amplifier which provides a gain
control range of 42 dB in 6 dB steps.
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 local oscillator leakage. DC offset compensation is performed during the operations
called DCN1. DCN1 is carried out by charging large off-chip capacitors in the last AGC
stages to a voltage which causes 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