GC4114 Datasheet, PDF (32/45 Pages) Texas Instruments – QUAD TRANSMIT CHIP

GC4114 QUAD TRANSMIT CHIP
DATA SHEET REV 1.0
7.5 PULSE SHAPING AND MODULATING QPSK OR QAM DATA
The chip is designed to pulse shape and up-convert digital symbol data for BPSK, PSK, QPSK or QAM
modulation types. The symbol data is entered into the chip as I/Q pairs either at the symbol rate (baud rate) in the
1X mode, or at twice the symbol rate in the 2X mode. The PFIR is used to pulse shape the data using a root-raised
cosine (RRC) filter. A formula which calculates the RRC coefficients is:
taps(k) =
-_π---₍---1----3–---2--A-7---6₎---7-+-----4---A---
--------------------------1--------------------------
(1 – (4AR(31 – k))²)
-s--i--n---⁽---⁽--3---1--₍--3-–--1--k---–-⁾--R-k---^π-₎--R-⁽--1-----–----A-----⁾---⁾ + 4A cos ((31 – k)Rπ(1 + A))
where A is the desired excess bandwidth (typically 0.35), R is the ratio of the sample rate out of the PFIR to the baud
rate of the data (0.5 for the 1X mode and 0.25 for the 2X mode), and k is the tap number ranging from 0 to 62. Only
taps 0 through 31 are needed by the chip. The tap weights for an excess bandwidth of 0.35 are given in Section
3.4.1.
The choice between entering the data at the baud rate (1X mode) or entering the data at twice the baud rate
(2X mode), depends upon the amount of image rejection required by the application. If 70 dB image rejection is
adequate (see Figure 9b in Section 3.5), then entering the data at the baud rate will work. If more image rejection
is required, then the data should be entered at twice the baud rate by alternately entering symbol I/Q pairs and zeros.
The 2X input mode has greater than 90 dB image rejection as shown in Figure 9c.
The sample rate out of the chip, which is the same as the clock rate of the chip (F_CK), is equal to 4BN in the
1X mode and 8BN in the 2X mode. The maximum baud rate accepted by the chip is, therefore, F_CK/4N in the 1X
mode and is F_CK/8N in the 2X mode.
The following subsections give examples of how to configure and initialize the chip in both the 1X and 2X
modes for an example QPSK signal with a baud rate of 25KHz and an output sample rate of 60MHz. These
examples are for a configuration using four chips to modulate 16 channels of data. Figure 10 shows the suggested
interconnection between the four chips.
SIXTEEN SERIAL INPUTS
SI _GC4114 SO
IN ^U0 OUT ¹⁶
REQ
SI _GC4114 SO
IN ^U1 OUT ¹⁶
CK
60 MHZ
REQ to input circuitry or DSP chips
SI _GC4114 SO
IN ^U2 OUT ¹⁶
SI _GC4114 SO
IN
U3
14MSBs
OUT
DAC
ANALOG
MODULE ^OUTPUT
Figure 10. Sixteen Channel Modulator
Note that chip U0 provides the sync pulse that goes to all four chips. The REQ output from one of the chips
(it will be the same from all four chips) must be used to synchronize the input serial data (see Section 3.2).
Texas Instruments Inc.
- 28 -
MAY 22, 2000
This document contains information which may be changed at any time without notice