TCM4400 Datasheet, PDF (37/72 Pages) Texas Instruments – GSM/DCS BASEBAND AND VOICE A/D AND D/A RF INTERFACE CIRCUIT

4 Principles of Operation
4.1 Baseband Uplink Path
Instead of the traditional transmit and receive terms, which can be confusing when describing a cellular
telephone two-way communication, the terms uplink, which means from a user device to a remote station,
and downlink, which means from a remote station, whether earthbound or satellite, are used to indicate the
signal flow direction.
The modulator circuit in the baseband uplink path performs the Gaussian minimum shift keying (GMSK) in
accordance with the GSM Specification, Section 05.04, Digital Cellular Telecommunication System;
Modulation. The data to be modulated flows from the DSP through the serial interface. It is differentially
encoded, and it is applied to the input of the GMSK modulator. The GMSK modulator is implemented with
digital logic and a sin/cos look-up table in ROM, and it generates the I and Q components (words) with an
interpolation ratio of 16.
These digital I and Q words are sampled at a 4.33-MHz rate and applied to the inputs of a pair of high-speed
8-bit DACs. The analog outputs are then processed by second-order Bessel filters to reduce image
frequencies due to sampling and to obtain a spectrum consistent with GSM Specification, Section 05.05,
Digital Cellular Telecommunications System (Phase 2+); Radio Transmission and Reception (see
Figure 4–1).
MAGNITUDE
vs
FREQUENCY
0
– 20
– 40
– 60
– 80
– 100
– 120
0
106 2×106 3×106 4×106 5×106 6×106
Frequency – MHz
NOTE: Conformance with GSM Specification, Section 05.5:
simulated spectrum of an infinite modulation of random
data with a Blackman Harris analysis window
Figure 4–1. Typical GSM Modulation Spectrum
4–1