TCM4400EPET datasheet(25/64 Pages) TI

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TCM4400E

GSM/DCS BASEBAND AND VOICE A/D

AND D/A RF INTERFACE CIRCUIT

SLWS082A – JULY 1999 – REVISED MARCH 2000

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

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).

NOTE A: Conformance with GSM Rec 05.05: simulated spectrum of an infinite modulation of random data with a Blackman

Harris analysis window.

Frequency – MHz

MAGNITUDE

FREQUENCY

–20

–40

–60

–80

– 100

– 120

0106

×106 3×106 4×106 5×106 6×106

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.

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