Transmitter - Signal Processing; Receiver - General Description - Nokia 2760 Service Manual

RM-258; RM-259
System Module
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.
The FEM is controlled with four digital control signals (TXP, Vc1, Vc2 and Vc3) to meet the TDMA frame timing
requirements.

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 1st order passive RC filter (discrete components on PWB) and a 3rd
order active filter (Legendre type) inside PMB3258.
The nominal output level of the modulator is +3.5 dBm in both bands. The modulator's output is an open-
collector type and need an external load and a DC supply feed. The load and the DC supply feed are
implemented as the part of the H3 filter.
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. The sensor is connected to one of the slow ADC channels in the baseband.

Receiver - general description

The receiver is a direct conversion linear receiver. It is a dual-band receiver with receiver paths for either
GSM850/1900 or GSM900/1800 configuration.
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), which reject
the out-of band blocking signals to low enough level to be handled by the RF ASIC.
There are two paths – one for each band. In each path, the signal is then fed to the low noise amplifier (LNA).
One LNA can handle both the GSM850 and GSM900 signals and another is used for GSM1800 and GSM1900.
The LNA inputs are matched to the SAW filter outputs by means of discrete LC matching networks. The SAW
filters and the matching networks are different for different band combinations, but the PWB layout is the
same for both 850/1900 and 900/1800 versions.
The RX front-end circuitry contains the LNA and the quadrature down converting mixers. The front-end gain
is programmable so that the gain can be reduced in strong-signal conditions. 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, which has programmable gain. One of the three poles is implemented
by an off-chip capacitor connected directly between the mixer outputs. There are a total of two off-chip
capacitors, one for I-channel and one for Q-channel, respectively.
After the blocking filter, the signal is fed into a buffer amplifier, which also has programmable gain. Around
the amplifier there is the first DC-offset compensation block, which removes most of the cumulated DC offset
so far. The DC offset compensation method is based on digital successive approximation technique.
The next block in the RX chain is a switched-capacitor (SC) channel filter, which provides the close-in selectivity
for the analog receiver. Because the SC-filter is insensitive to the IC process tolerances, no production
calibration of the filter is necessary. The SC-filter operates on 6.5 MHz clock, which is generated by dividing
the 26 MHz reference clock by four.
After the SC-filter there is a continuous-time smoothing filter which attenuates the alias signals generated
by the sampling process inherent in the SC-filter. The smoothing filter also has programmable gain.
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