Nokia Mobile Phones NSE-5 Series, Service Manual, Page: 71 / 327

PAMS
Technical Documentation
NSE–5
System Module
Page 2 – 57
Issue 1 07/99
external. Because it is an active mixer it also amplifies the IF-signal.
Buffering of the local signal is integrated too. The first local signal is
generated by the VHF-synthesizer.
There is a balanced discrete LC-bandpass filter in the output of the first
mixer which e.g. attenuates the critical spurious frequencies 161 MHz and
277 MHz and also the 151,5 MHz half-IF. It also matches the impedance
of 187MHz output to the input of the following stage. After this filter, the
187MHz IF-signal is mixed down to 71MHz IF, which is the second
GSM1800 IF. The VHF-mixer is also a double balanced Gilbert cell and is
located into the CRFU3. Lower side injection of the LO signal is used for
this down conversion.
The 116MHz LO signal comes from the SUMMA-, where it is derived by
dividing the 464MHz VHFLO signal by four. There is an external lowpass
filter for the 116MHz LO signal that attenuates the harmonics (especially
232MHz) so that the critical mixing spurious will be attenuated.
Common Receiver parts for GSM900 and GSM1800
After the down conversions in the CRFU3– the RX-signal path is common
for both systems. The 71MHz IF-signal is bandpass filtered with a
selective SAW-filter. From the output of to IF-circuit input of the SUMMA,
signal path is balanced. IF-filter provides selectivity for channels greater
than +/-200 kHz. Also it attenuates image frequency of the following mixer
and intermodulating signals. Selectivity is required in this place, because
of needed linearity and without filtering adjacent channel interferes would
be on too high signal level for the stages following.
Next stage in the receiver chain is an AGC-amplifier. It is integrated into
the SUMMA. The AGC gain control is analog. The control voltage for the
AGC is generated with a DA-converter in the COBBA in baseband. The
AGC-stage provides an accurate gain control range (min. 57 dB) for the
receiver. After the AGC-stage, the 71MHz IF-signal is mixed down to
13MHz. The needed 58MHz LO signal is generated in the SUMMA by
dividing the VHF-synthesizer output (464 MHz) by eight.
The following IF-filter is a ceramic bandpass filter. It attenuates the signals
in the adjacent channels, except for those sep/- 200 kHz relative
to the carrier. Very little attenuation is achieved for those signals in the
filter, but they are filtered digitally by the baseband. Because of this the
RX DACs has to be so good, that there is enough dynamic range for the
faded 200 kHz interferers. The whole RX has to be able to handle signal
levels in a linear way too. After the 13 MHz filter there is a buffer for the
IF-signal, which also converts and amplifies the single–ended signal from
filter to a balanced signal for the buffer and AD-converters in the COBBA.
The Buffer in the SUMMA has a voltage gain of 36 dB and the buffer gain
setting in the COBBA is 0 dB. It is possible to set the gainstep (95 dB) in
the COBBA via the control bus, if needed.