SLWS132 Datasheet, PDF (12/46 Pages) Texas Instruments – QUAD RECEIVER CHIP

GC4014 QUAD RECEIVER CHIP
DATA SHEET REV 0.6
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: GAIN = 2^COARSE , 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
GAIN = N . 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:
(a) Normal Mode
(b) Narrow Mode
Figure 8. Combined CFIR and CIC Frequency Response
GRAYCHIP, INC.
-7-
APRIL 27, 1999
This document contains information which may be changed at any time without notice