Functional description

STLC2500A

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5

Functional description

5.1

Transmitter

The transmitter uses the serial transmit data from the Bluetooth Controller. The transmitter

modulator converts this data into GFSK modulated I and Q digital signals. These signals are

then converted to analogue signals that are low pass filtered before up-conversion. The

carrier frequency drift is limited by a closed loop PLL.

5.2

Receiver

The STLC2500A implements a low-IF receiver for Bluetooth modulated input signals. The

radio signal is taken from a balanced RF input and amplified by an LNA. The mixers are

driven by two quadrature LO signals, which are locally generated from a VCO signal running

at twice the frequency. The I and Q mixer output signals are band pass filtered by a poly-

phase filter for channel filtering and image rejection. The output of the band pass filter is

amplified by a VGA to the optimal input range for the A/D converter. Further channel filtering

is done in the digital part. The digital part demodulates the GFSK coded bit stream by

evaluating the phase information in the digital I and Q signals. RSSI data is extracted.

Overall automatic gain amplification in the receive path is controlled digitally. The RC time

constants for the analogue filters are automatically calibrated on chip.

5.3

PLL

The on-chip VCO is part of a PLL. The tank resonator circuitry for the VCO is completely

integrated without need of external components. Variations in the VCO centre frequency are

calibrated out automatically.