Transmitter - Signal Processing; Receiver - General Description; Vcxo And Pll - Nokia RM-291 Service Manual

RM-291
Nokia Customer Care System module
Each path of the transmitter is composed of a baseband lowpass filter for the I/Q signals and a quadrature
direct modulator integrated in PMB3258. At the modulator's output there is a bandpass filter for each band
(so-called H3 filter) and a balun transformer to convert the differential output signal from the modulator into
a single-ended 50 ohm signal. This signal is fed into the input of the PA. The two power amplifiers and the
antenna switch are integrated in a single module with built-in power control loop.
The two control methods used are open-loop Vcc control (RFMD) and feedback control with current sensing.
The reference waveform (TXC) for the control loop comes from the baseband. The output of the PA goes into
a low pass filter located inside the FEM (Front End Module). Finally the transmit signal goes through the band
selection and TX/RX switches to the antenna port.

Transmitter - signal processing

The I/Q signals coming from the baseband section are fed into the modulator and converted up to the carrier
frequency. The I/Q are post filtered by a passive RC filter (discrete components on PWB) and a filter inside
PMB3258.
The nominal output level of the modulator is +3.5 dBm in both bands.
The filtered signal is fed into the input of the FEM, which amplifies it to the desired power level and provides
the signal at the antenna port.
There is also a temperature sensor close to the FEM to enable SW temperature compensation for e.g. the
power levels.

Receiver - general description

The receiver is a direct conversion linear receiver.
From the antenna the received RF-signal is fed into the front-end module, which routes the signal to the
appropriate RX path. After the FEM the RX signals are filtered by SAW filters (one for each band).
We now have two paths – one for each band. In each path the signal is then fed to the low noise amplifier
(LNA), which is integrated in PMB3258.
The RX front-end circuitry inside the PMB3258 contains the LNA and the quadrature down-converting mixers.
The mixers at each signal path convert the RF signal directly down to baseband I/Q signals. Local oscillator
signals for the mixers are generated by an on-chip VCO.
The output signals (I/Q) of each demodulator are all differential. They are combined to two differential signal
paths, one for I-channel and one for Q-channel, common for all bands. The baseband RX signals are then fed
into a 3rd order active blocking filter. One of the three poles is implemented by an off-chip capacitor
connected directly between the mixer outputs.
After the blocking filter the signal is fed into a buffer amplifier.
The next block in the RX chain is a switched-capacitor (SC) channel filter.
After the SC-filter there is a continuos-time smoothing filter.
The next block is a programmable gain amplifier (PGA), which has the second DC-offset compensation block
around it.
The last block in the analog receiver is an output buffer amplifier, which feeds the differential I/Q signals off-
chip to be A/D converted in the digital baseband.

VCXO and PLL

The VCO frequency is locked by a PLL (phase locked loop) into a stable frequency source given by a VCXO. The
frequency of the VCXO is in turn locked into the frequency of the base station with the help of an AFC (automatic
frequency control) voltage, which is generated in the UEM. The reference frequency is 26 MHz.
The VCXO also provides a 26 MHz system clock for the digital baseband.
The PLL is located in PMB3258 and it is controlled via the RFBUS.
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