GRAYCHIP, INC.

- 8 -

APRIL 27, 1999

GC4014 QUAD RECEIVER CHIP

DATA SHEET REV 0.6

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

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.

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 non-symmetry mode does not work properly for parts marked with mask code 55532B, Contact GRAYCHIP for

details.

(a) Normal CFIR Mode

(b) Narrow CFIR Mode

Peaks fold into

transition band

Peaks fold into

transition band

(c) GSM Coefficients