Samsung SGH-2488, Руководство по обслуживанию, Страница: 5 / 42

Samsung Electronics 2-1
2. Circuit Description
2-1 RF Part
2-1-1 Frequency Generator
The 13MHz reference clock (VCTCXO) drives the logic and RF part. The 13 MHz reference is controlled by the
logic (10bits DAC minimum) and is kept to a frequency error less than ±0.1 ppm after synchronization with the
GSM network.
The IF VCO generates 233~259.5MHz VHF LO used in the RX I,Q Demodulator and after
doubling(493~519MHz), it used in the TX I,Q modulator. The UHF LO for the first RX down conversion and
the TX offset mixing works in superheterodyne mode to reduce the relative bandwidth and to be able to work
at a frequency greater than 1 GHz.
2-1-2 Transmitter
The baseband GSM chipset (Kernel 5) generates I and Q baseband signals for the transmit modulator. The
modulator (U2893B) is the first stage which takes the baseband signal and upconverts it to a fixed intermediate
frequency. The modulator provides more than 35dBc of carrier and unwanted side-band refection and produces
GMSK modulated signal, the ÒreferenceÓ signal at 493~519 MHz which passes to the offset phase-locked loop
block (OPLL). The OPLL consists of a down-converter, phase detector, loop filter and transmit VCOs operation
at the final RF output frequency. The down converter mixes the UHF LO (eg. 1210MHz) with the transmit VCO
signal to generate a ÒfeedbackÓ signal at 493~519 MHz. The ÒfeedbackÓ signal passes via a limiter to one port
of the phase detector. The GMSK ÒreferenceÓ signal from the modulator passes via second limiter to the other
input port of the phase detector. The phase detector generates an error current proportional to the phase
difference between the ÒfeedbackÓ signal from the down-converter and the ÒreferenceÓ signal from the
modulator. This error current is filtered by a third order low-pass filter to generate an output voltage which
depends on the GMSK modulation and the desired channel frequency. This voltage controls the transmit VCO
such that the VCO output signal, centered on the correct RF channel, is frequency modulated with the original
GMSK data. The center frequency of the transmit VCO is offset from the UHF LO frequency by 493~519MHz.
The OPLL acts as a tracking narrowband band pass filter tuned to the desired channel frequency. This reduces
the wideband products. The OPLL architecture results in a low-noise GMSK modulated signal at
902MHz(1747MHz) with very low spurious content.
The RF GMSK output from the transmit VCO is fed to the RF power amplifier. The peak output power and the
profile of the transmitted burst are controlled by means of a closed feedback loop. The RF output from the PA is
sampled with a directional coupler. The sampled signal passes to an RF detector diode whose output voltage is
dependent on the incident RF level. This ÒfeedbackÓ voltage passes to the inverting input of the loop integrator.
A ÒreferenceÓ signal is generated within the baseband section under control of the layer 1 software. The loop
maintains zero difference between the ÒfeedbackÓ signal and the ÒreferenceÓ signal. In this way, the amplitude
and shape of the transmitted RF burst may be controlled by the baseband processor. In particular, the rise and
fall profiles can be controlled to meet the stringent power/time templates and switching transient requirements
of GSM 05.05.
The RF output passes to the antenna connector via an integrated TX/RX switch and lowpass filter to attenuate
the harmonics generated by the power amplifier.