TCM4400E Datasheet, PDF (25/64 Pages) Texas Instruments – GSM/DCS BASEBAND AND VOICE A/D AND D/A RF INTERFACE CIRCUIT

TCM4400E
GSM/DCS BASEBAND AND VOICE A/D
AND D/A RF INTERFACE CIRCUIT
SLWS082A – JULY 1999 – REVISED MARCH 2000
PRINCIPLE OF OPERATION
baseband uplink path
Traditional transmit and receive terms can be confusing when describing a cellular telephone two-way
communication. This document uses the terms uplink, meaning from a user device to a remote station, and
downlink meaning from a remote station, whether earthbound or satellite, 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 5.04. 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,
implemented with digital logic and a sin/cos look-up table in ROM, 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 third-order Bessel filters to reduce image frequencies
due to sampling and to obtain a spectrum consistent with GSM specification 05.05 (see Figure 6).
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 A: Conformance with GSM Rec 05.05: simulated spectrum of an infinite modulation of random data with a Blackman
Harris analysis window.
Figure 6. Typical GSM Modulation Spectrum
Full-differential buffered signals are available at ULIP, ULIN, ULQP, and ULQN. These signals are suitable for
use in the RF circuit for generating a phase-modulated signal of the form:
s(t) = A Cos (2 Pi fc t + Phi (t, alpha))
where fc is the RF carrier frequency, and Phi (t, alpha) is the phase component generated by the GMSK
modulator from the differential encoded data.
• POST OFFICE BOX 655303 DALLAS, TEXAS 75265
25