GC4014 datasheet(44/46 Pages) TI

44 / 46 page

GRAYCHIP, INC.

- 39 -

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

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:

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)(ejwt+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

. This means that the gain can be boosted by a factor of

within the

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.

CIC gain

SCALE

BIG_SCALE

–

()

Coarse gain

COARSE

CFIR gain

1.0

NARROW

0.97

()

PFIR gain

PFIR_SUM

65536

----------------------------

Final gain

------

---------

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