Nokia NSE-3 SERIES, Service Manual

Page: 58 / 217

PAMS
Technical Documentation
NSE–3
System Module
Original 11/97
Page 3 – 52
Receiver
Receiver is a dual conversion linear receiver.
Received RF–signal from the antenna is fed via the duplex filter to LNA
(low noise amplifier ) in CRFU_1a. Active parts ( RF–transistor and bias-
ing and AGC–step circuitry ) are integrated into this chip. Input and output
matching networks are external. Gain selection is done with PDATA0 con-
trol. Gain step in LNA is activated when RF–level in antenna is about –45
dBm. After the LNA amplified signal ( with low noise level ) is fed to band-
pass filter, which is a SAW–filter ( SAW, surface acoustic wave ). Duplex
filter and RX interstage bandpass filters together define, how good are the
blocking characteristics against spurious signals outside receive band
and the protection against spurious responses, mainly the image of the
first mixer.
This bandpass filtered signal is then mixed down to 71 MHz, which is first
intermediate frequency. 1st mixer is located into CRFU_1a ASIC. This in-
tegrated mixer is a double balanced Gilbert cell. All active parts and bias-
ing are integrated and matching components are external. Because this is
an axtive mixer it also amplifies IF–frequency. Also local signal buffering
is integrated and upper side injection is used. First local signal is gener-
ated with UHF–synthesizer.
First IF–signal is then bandpass filtered with a selective SAW–filter. From
the mixer output to IF–circuit input signal path is balanced. IF–filter pro-
vides selectivity for channels greater than +/–200 kHz. Also it attenuates
image frequency of the second mixer and intermodulating signals. Selec-
tivity is required in this place, because of needed linearity and adjacent
channel interferers will be on too high signal level for the stages following.
Next stage in the receiver chain is AGC–amplifier. It is integrated into
PLUSSA–ASIC. AGC has analog gain control. Control voltage for the
AGC is generated with DA–converter in COBBA in baseband. AGC–stage
provides accurate gain control range ( min. 60 dB ) for the receiver. After
the AGC there is second mixer, which generates second intermediate fre-
quency, 13 MHz. Local signal is generated in PLUSSA by dividing VHF–
synthesizer output ( 232 MHz ) by four, so the 2nd LO–frequency is 58
MHz.
2nd IF–filter is a ceramic bandpass filter at 13 MHz. It attenuates adjacent
channels, except for +/– 200 kHz there is not much attenuation. Those
+/– 200 kHz interferers are filtered digitally by the baseband . So RX
DACs are so good, that there is enough dynamic range for the faded 200
kHz interferer. Also the whole RX has to be able to handle signal levels in
a linear way. After the 13 MHz filter there is a buffer for the IF–signal,
which also converts and amplifies single ended signal from filter to bal-
anced signal for the buffer and AD–converters in COBBA. Buffer in PLUS-
SA has got voltage gain of 36 dB and buffer gain setting in COBBA is 0
dB. It is possible to set gainstep ( 9.5 dB ) into COBBA via control bus, if
needed.