Functional Description; Receiver - Nokia 9000i Service Manual

RAE/RAK–1N
RF

Functional Description

Receiver

The GSM receiver is a double conversion receiver. The PCN receiver has
three conversions.
The received RF signal from the antenna is fed via a duplex filter to the
receiver unit. The signal is amplified by a discrete low noise preamplifier.
The gain of the amplifier is controlled by the AGC control line (PDATA0).
The nominal gain of 10 dB in PCN is reduced in the strong field condition
about 24 dB and in GSM the nominal gain of 16.5 dB is reduced about 36
dB. After the preamplifier the signal is filtered by ceramic (PCN) or SAW
(GSM) RF filter. The filter rejects spurious signals coming from the
antenna and spurious emissions coming from the receiver unit.
In PCN the filtered RF–signal is down converted by a passive diode
mixer. The frequency of the first IF is 313 MHz. The first local signal is
generated by the UHF synthesizer. The IF signal is amplified and then
filtered by a microstripline filter. The filtered 1st IF is down converted by
the second mixer, which is also a passive diode mixer. The 2nd IF
frequency is 87 MHz. The 2nd local signal is generated by the VHF
synthesizer.
In the GSM system the filtered RF–signal is also down converted by the
passive diode mixer. The first IF frequency is 71 MHz and the first local
signal is generated by the UHF synthesizer.
All the IF signals 71 and 87 MHz are amplified and filtered by SAW filter
in GSM and PCN . The filter rejects adjacent channel signal,
intermodulating signals and the last IF image signal.
The filtered IF signal is fed to the receiver part of the integrated RF
circuit CRFRT. In CRFRT the filtered IF signal is amplified by an AGC
amplifier which has gain control range of 57 dB. The gain is controlled by
an analog signal via TXC–line. The amplified IF signal is down converted
to the last IF in the mixer of CRFRT. The last local signal is generated
from VHF VCO by dividing the original signal by 4 in the dividers of
CRFRT.
The last IF frequency is 13 MHz in GSM and PCN. The last IF is filtered
by a ceramic filter. The filter rejects signals of the adjacent channels. The
filtered last IF is fed back to CRFRT where it is amplified. Finally the IF
signal is split to +45 and –45 signals and then fed to RFI.
The block diagram overleaf, Figure 4., is common for GSM and PCN.
Blocks in parenthesis refer to PCN only. In GSM these parts do not exist
and in the signal path are replaced by direct connection to the next block.
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