Frequency Synthesizers; Receiver - Nokia NPL-4 Series Manual

NPL-4/5/RM-104
System Module and User Interface

Frequency synthesizers

The VCO frequency is locked by a phase locked loop (PLL) and VCTCXO which is running
at 26 MHz.
The frequency of the VCTCXO is in turn locked into the frequency of the base station with
the help of an AFC voltage which is generated in UEMEK by an 11 bit D/A (digital-to-
analog) converter.
The PLL is located in the Helgo and is controlled through the RFBus.
Loop filter filters out the comparison pulses of the phase detector and generates a DC
control voltage to the VCO.
The dividers are controlled via the RFBus. RFBusData is for the data, RFBusClk is a serial
clock for the bus and RFBusEna1X is a latch enable, which stores the new data into the
dividers.

Receiver

Each receiver path is a direct conversion linear receiver.
From the antenna the received RF-signal is fed to the front end module where a diplexer
first divides the signal to two separate paths according to the band of operation: either
lower, GSM850/EGSM900 or upper, GSM1800/GSM1900 path.
At each of the paths a pin-diode switch is used to select either receive or transmit mode.
At the upper band in receive mode either GSM1800 or GSM1900 path is further selected
by another pin-diode switch.
The selections are controlled by the Helgo which obtains the mode/band and timing
information through the RFBus. After the switches there is a bandpass filter at each of
the receiver paths. These filters are included in the front end module, except for
GSM1900 where it is external.
Then the signal is fed to the LNAs which are integrated in the Helgo in GSM850/
EGSM900 and GSM1800 while in GSM1900 the LNA is external.
In GSM1900 the amplified signal is fed to a balun and thereafter to a pregain stage of
the mixer while in GSM850/EGSM900 and GSM1800 the LNA's are directly connected to
the pregain stages without having SAW filters in between. The pregain stages as well as
all the following receiver blocks are integrated in the Helgo. The LNAs have three gain
levels. The first one is the maximum gain, the second one is about 30 dB below the max-
imum, and the last one is the off state.
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 of which is divided by two in
GSM1800 and GSM1900 and by four in GSM850/EGSM900. Those frequency dividers are
integrated in the Helgo and in addition to the division they also provide accurate phase
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