SLWS132 Datasheet, PDF (13/46 Pages) Texas Instruments – QUAD RECEIVER CHIP

GC4014 QUAD RECEIVER CHIP
DATA SHEET REV 0.6
The dashed vertical line in the plots shows the output Nyquist rate for the chip when the PFIR is in the
decimate by 2 mode. The narrow mode filter introduces a gain of 1.97 (5.9 dB).
3.3.5 The Programmable Final Filter (PFIR)
The second stage decimate by two or four filter uses either internal ROM based coefficients, or externally
downloaded filter coefficients. The internal 80% bandwidth filter has 80 dB of out of band image rejection and 0.03
dB peak to peak passband ripple. The internal filter is a 63 tap symmetric filter. The 32 unique coefficients are:
-14, -20, 19, 73, 43, -70, -82, 84, 171, -49, -269, -34, 374, 192, -449,
-430, 460, 751, -357, -1144, 81, 1581, 443, -2026, -1337, 2437, 2886,
-2770, -6127, 2987, 20544, 29647
Figure 9(a) shows the overall response for the internal PFIR when the CFIR is in the normal mode. Figure
9(b) shows the overall response when the CFIR is in the narrow mode. Note that the peaks in the stop band at 3.5
times the output sample rate will, after decimation, fold into the transition band from 0.4 to 0.5 of the output sample
rate. This out of band power, if necessary, can be filtered out by either using a custom PFIR filter with a narrower
passband, or by post-filtering.
An overall response using custom coefficients suitable to meet the stringent GSM Cellular requirements is
shown in Figure 9(c). See Sections 7.5 and 7.6 for more details on GSM and DAMPS configurations.
Peaks fold into
transition band
Peaks fold into
transition band
(a) Normal CFIR Mode
(b) Narrow CFIR Mode
(c) GSM Coefficients
Figure 9. Overall CIC-CFIR-PFIR Response
The externally downloaded coefficients can be used to tailor the spectral response to the user’s needs. For
example, it can be programmed as a Nyquist (typically a root-raised-cosine) filter for matched filtering digital data.
The user downloaded filter coefficients are 16 bit 2’s complement numbers. Unity gain will be achieved through the
filter if the sum of the 63 coefficients is equal to 65536. If the sum is not 65536, then the PFIR will introduce a gain
equal to (sum of coefficients)/65536.
The 63 coefficients are identified as coefficients h₀ through h₆₂, where h₃₁ is the center tap. The coefficients
are assumed to be symmetric, so only the first 32 coefficients (h₀ through h₃₁) are loaded into the chip. A
non-symmetric mode allows the user to download a 32 tap non-symmetric filter as taps h₀ through h₃₁. ERRATA:
The non-symmetry mode does not work properly for parts marked with mask code 55532B, Contact GRAYCHIP for
details.
GRAYCHIP, INC.
-8-
APRIL 27, 1999
This document contains information which may be changed at any time without notice