GRAYCHIP, INC.

- 7 -

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

should be less than or equal to 7 and 2 respectively. The BIG_SCALE and SCALE settings are common to all

channels.

3.3.3

Coarse Channel Gain

The gain of each channel can be boosted up to 42 dB by shifting the output of the CIC filter up by 0 to 7 bits

prior to rounding it to 16 bits. The coarse gain is:

, where COARSE ranges from 0 to 7. Overflows

in the coarse gain circuit are saturated to plus or minus full scale. The coarse gain is used to increase the gain of

an individual signal after the input bandwidth of the downconverter has been reduced by a factor of N in the CIC

filter. If the signal power across the input bandwidth is relatively flat, as is the case in most frequency division

multiplexed (FDM) systems, then one would want to boost the signal power out of the CIC filter by a factor of

. Each channel can be given its own coarse gain setting. Note that the final gain stage described in

Section 3.4 can boost the overall gain by up to 24 more dB.

3.3.4

The Compensating Decimate By Two Filter (CFIR)

The CIC/Coarse gain outputs are filtered by two stages of filtering. The first stage is a 21 tap decimate by

2 filter with two sets of fixed coefficients. The first set of coefficients is used in the normal mode to give a passband

which is flat (0.01 dB ripple) over 100% of the final output bandwidth and which has 85 dB of out of band rejection.

The filter also compensates for the droop associated with the CIC programmable decimation filter. The filter is

symmetric with the following taps:

29, -85, -308, -56, 1068, 1405, -2056, -6009, 1303, 21121, 32703

The narrow set of coefficients are intended for applications that need deeper stop bands or need

oversampled outputs. These requirement are common in cellular systems where out of band rejection requirements

can exceed 100 dB. The filter coefficients for the narrow mode are:

-98, -679, -2016, -3234, -2537, 850, 6053, 12060, 18230, 23239, 25212

The combined frequency response of the CIC and CFIR filter for both modes is shown below:

Figure 8. Combined CFIR and CIC Frequency Response

GAIN

2

COARSE

=

GAIN

N

=

(a) Normal Mode

(b) Narrow Mode