GC4114 datasheet(13/45 Pages) TI

13 / 45 page

Texas Instruments Inc.

- 9 -

MAY 22, 2000

GC4114 QUAD TRANSMIT CHIP

DATA SHEET REV 1.0

This document contains information which may be changed at any time without notice

Figure 8. Example NCO Output

3.5

THE OVERALL INTERPOLATION FILTER RESPONSE

The image rejection of the up-convert channel is equal to the stop band rejection of the overall interpolation

filter response. The overall response is obtained by superimposing the interpolated responses of the PFIR and CFIR

filters onto the CIC filter response. The overall response is shown in Figure 9. Figure 9a shows the overall response

using the default PFIR coefficients. Figure 9b shows the response using a root-raised-cosine PFIR coefficient set.

Figure 9c shows the response when using the 2x mode root-raised cosine PFIR coefficient set.

Figure 9. Overall Filter Response

3.6

THE SUM TREE

The four up-convert channel outputs are summed together, scaled down by powers of two and then added

to an external sum input. The sum tree output is rounded to 8, 10, 12, 14 or 16 bits and output from the chip. The

sum of the four channels within the chip can be scaled down by powers of two in order to prevent saturation when

channels from multiple chips are summed together. The scale factor is equal to 2-SUM_SCALE, where SUM_SCALE

is 0,1,2 or 3 (See Section 5.9). Overflows in the sum tree are saturated to plus or minus full scale.

The latency from IN[0:15] to OUT[[0:15] is seven clock cycles. The chip will optionally invert the MSB output

(OUT15) from the chip in order to drive offset binary format digital to analog converters (DACs)

With Dithering

No Dithering

(a) Internal Coefficients

(b) RRC (Alpha=0.35) Filter

GC4114 Datasheet(PDF) 13 Page - Texas Instruments