SLWS132 Datasheet, PDF (44/46 Pages) Texas Instruments – QUAD RECEIVER CHIP

GC4014 QUAD RECEIVER CHIP
DATA SHEET REV 0.6
7.9 OPTIMAL GAIN SETTINGS
The overall gain of the chip is the product of the CIC gain, the coarse gain, the CFIR gain, the PFIR gain
and the final gain. Each of these components are:
CIC gain = N⁴2^{(SCALE + 6 × BIG_SCALE – 56)}
Coarse gain = 2^COARSE
CFIR gain = (1.0 + NARROW × 0.97)
PFIR gain = ^{P----F---I6--R-5---_5---S3---6U-----M----}
Final gain = -G----
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The signal flows through these sections in the order given. The gain settings which optimize the dynamic
range of the chip, are the ones that maximize the signal amplitude without clipping at the output of each of these
processing stages. A conservative approach to gain would be to set each gain component so that the product of the
gains at each processing point in the flow is less than or equal to unity.
The conservative approach, described above, is usually less than optimal. The optimal gain takes the
following considerations into account.
7.9.1 Tuning Loss
The input to the chip can be described as a signal S(t) modulated to a center frequency of “w”. The input is,
therefore, S(t)cos(wt) = S(t)(e^jwt+e^-jwt)/2. If the downconverter tunes to the frequency “w”, then the tuner output will
be S(t)(1.0+e^-2wt)/2. The filters will then reject the component at “-2w”, leaving just the signal S(t)/2. This loss of
one-half amplitude can be compensated for by setting the overall gain equal to 2, not unity. The tuning gain loss
occurs after the CIC filters, so the optimum gain approach is to use gain settings that keep the gain product after
the coarse, CFIR, PFIR and final gain stages equal to 2.
7.9.2 Uniform Power Inputs
The gain can be further optimized if the user has control over the power levels of the signals in the input
bands. If all of the signals in the input are close to equal power, then the gain of the downconverted signal can be
boosted to maximize its dynamic range. For example, if there are “M” signals of equal power in the input band, then
the amplitude of each signal is ----1---- . This means that the gain can be boosted by a factor of M within the
M
downconverter. The coarse gain can be used to add the additional gain.
Examples of applications which can use this feature are FM-FDM systems, cellular systems which use
power control, and wireless local loop systems that fix the power level of each remote transmitter.
GRAYCHIP, INC.
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APRIL 27, 1999
This document contains information which may be changed at any time without notice