GC5016 datasheet(24/88 Pages) TI

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GC5016

SLWS142G − JANUARY 2003 − REVISED NOVEMBER 2005

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ROUND

20 Data

Plus

8 Overflow

Data

7 Integer

Plus

12 Fractional

MAGNITUDE

agc_rnd

DELAY

Valid

Data Out

(Upper 20 − agc_rnd bits are valid,

lower bits are cleared)

COMPARE

agc_thresh

UNDER/OVER

DETECT

agc_sat_cnt

agc_zero_cnt

SHIFT SELECT

agc_Dblw

agc_Dadv

agc_Dzro

agc_Dsat

LIMIT

*agc_min < A(t) <

agc_max

agc_min

MS16

7 Integer

Plus

9 Fractional

ACCUMULATE

Sync

Sign Plus

7 Integer

Plus

16 Fractional

A(t+1)=A(t)+S 2−(D+3)G(t)

SHIFT

7 Integer

Plus

12 Fractional

±1, D=4-Bit Shift

Valid

agc_freeze

G(t)=Gain+A(t)

Gain

7 Integer

Plus

12 Fractional

A(t)=Gain Adjust

Under Limit

Over Limit

CLR

LS8

LS20

MS8

OVERFLOW

MS16

Figure 12. GC5016 AGC Circuit

The AGC portion of the circuit is used to change the adaptive gain so that the median magnitude of the output data

matches a target value. The magnitude of the gain-adjusted (manual + adaptive) output data is compared to a target

threshold. If the magnitude is greater than the threshold, the gain is decreased. If not, it is increased. The gain is

adjusted as:

G(t) = G + A(t)

A(t) = A(t) + G(t) x S x 2−(D+3)

where G is the default, user supplied gain value, and A(t) is the time varying adjustment, where S=1 if the magnitude

is less than the threshold and is −1 if the magnitude exceeds the threshold, and where D sets the adjustment step

size. Note that the adjustment is a fraction of the current gain. This is designed to set the AGC noise level to a known

and acceptable level, while keeping the AGC convergence and tracking rate constant, independent of the gain level.

Because the adjustment is a fraction of the current gain, one can show that the AGC noise is an amplitude jitter in

the data output equal to

±(data output) x 2−(D+3). This means that the AGC noise is always 6 x (D+3) dB below the

output signal’s power level. The AGC attack and decay rate is exponential with a time constant equal to 2(D+1.75)

complex samples. This means the AGC covers to within 63% of the required gain change in one time constant and

to within 98% of the change in the four time constants.

If one assumes the data is random with a Gaussian distribution, which is valid for UMTS if more than 12 users with

different codes have been overlaid, then the relationship between the RMS level and the median is MEDIAN = 0.6745

x RMS.

Hence the threshold should be set to 0.6745 times the desired RMS level.

GC5016 Datasheet(PDF) 24 Page - Texas Instruments

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