Nokia NHM-7 series Service Manual page 47

NHM–7
PAMS Technical Documentation
System Module & UI
frequency. LO signal is divided by two or four in HAGAR (depending on
system mode).
Receiver
Receiver is a direct conversion, dual band linear receiver. Received RF–
signal from the antenna is fed via RF–antenna switch module to 1st RX
bandpass RF–SAW filters and MMIC LNAs (low noise amplifier). RF–an-
tenna switch module contains upperband and lowerband operation. The
nd
LNA amplified signal is fed to 2nd RX bandpass RF–SAW filters. Both 2
RX bandpass RF–SAW filters have un–bal/bal configuration to get the
balanced (balanced) feed for Hagar.
Discrete LNAs have three gain levels. The first one is max. gain, the se-
cond one is about –30dB(GSM1800) and –25dB(EGSM900) below max.
gain and the last one is off state. The gain selection control of LNAs
comes from HAGAR IC.
RX bandpass RF–SAW filters define how good are the blocking charac-
teristics against spurious signals outside passband and the protection
against spurious responses.
Differential RX signal is amplified and mixed directly down to BB frequen-
cy in HAGAR. Local signal is generated with external VCO. VCO signal is
divided by 2 (GSM1800) or by 4 (E–GSM900). PLL and dividers are in
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 differential signals to single ended. 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 sig-
nal 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 provides selectivity for channels
(–3dB @ +/–91 kHz typ.). Integrated base band filter is active–RC–filter
with two off–chip capacitors. Large RC–time constants needed in the
channel select filter of direct conversion receiver are produced with large
off–chip capacitors because the impedance levels could not be increased
due to the noise specifications. Baseband filter consists of two stages,
DTOS and BIQUAD. DTOS is differential to single–ended converter hav-
ing 8dB or 18dB gain. BIQUAD is modified Sallen–Key Biquad.
Integrated resistors and capacitors are tunable. These are controlled with
a digital control word. The correct control words that compensate for the
process variations of integrated resistors and capacitors and of tolerance
of off chip capacitors are found with the calibration circuit.
Next stage in the receiver chain is AGC–amplifier, also integrated into HA-
GAR. AGC has digital gain control via serial mode bus. AGC–stage pro-
vides gain control range (40 dB, 10 dB steps) for the receiver and also the
necessary DC compensation. Additional 10 dB AGC step is implemented
in DTOS stages.
E Nokia Corporation
Page 35
Issue 1 10/2001