Transmitter - Signal Processing; Vcxo And Pll - Nokia RH-64 Service Manual

RH-64
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 located 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
(Renesas, Philips). 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 H3 filter will attenuate mainly the 2nd and 3rd harmonics of the modulated RF
signal and provide suitable termination impedance for the modulator on the fundamental and harmonic
frequencies. Owing to the low noise floor of the PMB3258 modulator a narrow-band filter for the TX noise is
not required on either band, although the margin to the specification limit is quite narrow in the GSM850/900
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 are built-in lowpass filters for the TX harmonics in the front-end module.
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.

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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